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I have a co worker who need a method to be available to two classes.
He decided to create a new interface to be implemented by those classes.
The interface has one method
default doThis(String parameter)
It does not have any other interface methods, there is no indication that other methods would be added to this interface.
I feel this is an incorrect usage of the interface and it should be done in a different way. I.e perhaps a class which has the method allowing other classes to consume this by using the object.
Does anyone with experience on this have any opinions to share?
I can update with more clarification based on your comments.
Update:
Here is the code and the question remains:
is this a valid use of the default method or should this common logic have been done in another way like a Utilities class which does the saving to preferences ?
Interface:
public interface LogInCookie {
default void mapCookiesToPreferences(String cookie) {
if (cookie.contains(MiscConstants.HEADER_KEY_REFRESH)) {
String refreshToken = cookie.replace(MiscConstants.HEADER_KEY_REFRESH, StringUtils.EMPTY);
SharedPrefUtils.addPreference(SharedPrefConstants.REFRESH_TOKEN, refreshToken);
}
}
}
public class HDAccountActivity extends AbstractActivity implements LogInCookie {
private void mapCookies(List<String> mValue) {
LogInCookie.super.mapCookiesToPreferences(mValue); //ekh!
}
}
public class BaseSplashPage extends AppCompatActivity implements DialogClickedCallBack, LogInCookie {
//method which uses this
private void mapCookiesToPreferences(List<String> headers) {
int firstItemInHeader = 0;
for (String header : headers) {
String mValue = header.substring(firstItemInHeader,header.indexOf(MiscConstants.SEMICOLON));
LogInCookie.super.mapCookiesToPreferences(mValue); //ekh!
}
}
}
A default method in an interface, which doesn’t define other methods, can’t do much useful things with the instance of the implementing class. It can only use methods inherited from java.lang.Object, which are unlikely to carry semantics associated with the interface.
If the code doesn’t use instance methods on this at all, in other words, is entirely independent from the this instance, you should make it static, change the containing class to a non-instantiable class type, i.e.
final class SomeUtilClass {
static void doThis(String parameter) {
// ...
}
private SomeUtilClass() {} //no instances
}
and use import static packageof.SomeUtilClass.doThis; in the classes using this method.
That way, all these classes can invoke the method like doThis(…) without a qualifying type name, without needing a misleading type hierarchy.
When the method actually uses the this instance, which, as said, can only be in terms of methods inherited from java.lang.Object, the type inheritance might be justified. Since this is rather unlikely, you might still consider the type hierarchy to be misleading and rewrite the code to
final class SomeUtilClass {
static void doThis(Object firstParameter, String parameter) {
// ...
}
private SomeUtilClass() {} //no instances
}
using firstParameter instead of this, which can be invoke like doThis(this, …).
Ideally you would put that method doThis() in an abstract class that both classes extend. However if you need to achieve multiple inheritance then using an interface here is fine.
A class with a static method doThis() that you can call staticly would also work.
It all depends on how you have your project organized imo.
In java 8 , default keyword in interface was introduced for those cases where if any set of apis had long inheritance hierarchy and we wanted to introduce a method that should be available in all of the lower lying classes.
So for ex. in Java 8 stream() method was introduced in the Collection interface as a default method and it ended up being available in all of the underlying classes.
As far as your case in considered , if I go by your words then if yours is a new development then you should be using interface -> abstract class -> actual implementing class.
Only if yours was an older development setup and you already had classes implementing from an interface , that could have been an ideal scenario for using default method in your interface.
A default method in an interface
*)can have a default implementation
*)which can overridden by the implementing class
yes its a correct usage since JAVA8.
we can have default method in an interface as well as a abstract method
Given the following class:
class Example implements Interface1, Interface2 {
...
}
When I instantiate the class using Interface1:
Interface1 example = new Example();
...then I can call only the Interface1 methods, and not the Interface2 methods, unless I cast:
((Interface2) example).someInterface2Method();
Of course, to make this runtime safe, I should also wrap this with an instanceof check:
if (example instanceof Interface2) {
((Interface2) example).someInterface2Method();
}
I'm aware that I could have a wrapper interface that extends both interfaces, but then I could end up with multiple interfaces to cater for all the possible permutations of interfaces that can be implemented by the same class. The Interfaces in question do not naturally extend one another so inheritance also seems wrong.
Does the instanceof/cast approach break LSP as I am interrogating the runtime instance to determine its implementations?
Whichever implementation I use seems to have some side-effect either in bad design or usage.
I'm aware that I could have a wrapper interface that extends both
interfaces, but then I could end up with multiple interfaces to cater
for all the possible permutations of interfaces that can be
implemented by the same class
I suspect that if you're finding that lots of your classes implement different combinations of interfaces then either: your concrete classes are doing too much; or (less likely) your interfaces are too small and too specialised, to the point of being useless individually.
If you have good reason for some code to require something that is both a Interface1 and a Interface2 then absolutely go ahead and make a combined version that extends both. If you struggle to think of an appropriate name for this (no, not FooAndBar) then that's an indicator that your design is wrong.
Absolutely do not rely on casting anything. It should only be used as a last resort and usually only for very specific problems (e.g. serialization).
My favourite and most-used design pattern is the decorator pattern. As such most of my classes will only ever implement one interface (except for more generic interfaces such as Comparable). I would say that if your classes are frequently/always implementing more than one interface then that's a code smell.
If you're instantiating the object and using it within the same scope then you should just be writing
Example example = new Example();
Just so it's clear (I'm not sure if this is what you were suggesting), under no circumstances should you ever be writing anything like this:
Interface1 example = new Example();
if (example instanceof Interface2) {
((Interface2) example).someInterface2Method();
}
Your class can implement multiple interfaces fine, and it is not breaking any OOP principles. On the contrary, it is following the interface segregation principle.
It is confusing why would you have a situation where something of type Interface1 is expected to provide someInterface2Method(). That is where your design is wrong.
Think about it in a slightly different way: Imagine you have another method, void method1(Interface1 interface1). It can't expect interface1 to also be an instance of Interface2. If it was the case, the type of the argument should have been different. The example you have shown is precisely this, having a variable of type Interface1 but expecting it to also be of type Interface2.
If you want to be able to call both methods, you should have the type of your variable example set to Example. That way you avoid the instanceof and type casting altogether.
If your two interfaces Interface1 and Interface2 are not that loosely coupled, and you will often need to call methods from both, maybe separating the interfaces wasn't such a good idea, or maybe you want to have another interface which extends both.
In general (although not always), instanceof checks and type casts often indicate some OO design flaw. Sometimes the design would fit for the rest of the program, but you would have a small case where it is simpler to type cast rather than refactor everything. But if possible you should always strive to avoid it at first, as part of your design.
You have two different options (I bet there are a lot more).
The first is to create your own interface which extends the other two:
interface Interface3 extends Interface1, Interface2 {}
And then use that throughout your code:
public void doSomething(Interface3 interface3){
...
}
The other way (and in my opinion the better one) is to use generics per method:
public <T extends Interface1 & Interface2> void doSomething(T t){
...
}
The latter option is in fact less restricted than the former, because the generic type T gets dynamically inferred and thus leads to less coupling (a class doesn't have to implement a specific grouping interface, like the first example).
The core issue
Slightly tweaking your example so I can address the core issue:
public void DoTheThing(Interface1 example)
{
if (example instanceof Interface2)
{
((Interface2) example).someInterface2Method();
}
}
So you defined the method DoTheThing(Interface1 example). This is basically saying "to do the thing, I need an Interface1 object".
But then, in your method body, it appears that you actually need an Interface2 object. Then why didn't you ask for one in your method parameters? Quite obviously, you should've been asking for an Interface2
What you're doing here is assuming that whatever Interface1 object you get will also be an Interface2 object. This is not something you can rely on. You might have some classes which implement both interfaces, but you might as well have some classes which only implement one and not the other.
There is no inherent requirement whereby Interface1 and Interface2 need to both be implemented on the same object. You can't know (nor rely on the assumption) that this is the case.
Unless you define the inherent requirement and apply it.
interface InterfaceBoth extends Interface1, Interface2 {}
public void DoTheThing(InterfaceBoth example)
{
example.someInterface2Method();
}
In this case, you've required InterfaceBoth object to both implement Interface1 and Interface2. So whenever you ask for an InterfaceBoth object, you can be sure to get an object which implements both Interface1 and Interface2, and thus you can use methods from either interface without even needing to cast or check the type.
You (and the compiler) know that this method will always be available, and there's no chance of this not working.
Note: You could've used Example instead of creating the InterfaceBoth interface, but then you would only be able to use objects of type Example and not any other class which would implement both interfaces. I assume you're interested in handling any class which implements both interfaces, not just Example.
Deconstructing the issue further.
Look at this code:
ICarrot myObject = new Superman();
If you assume this code compiles, what can you tell me about the Superman class? That it clearly implements the ICarrot interface. That is all you can tell me. You have no idea whether Superman implements the IShovel interface or not.
So if I try to do this:
myObject.SomeMethodThatIsFromSupermanButNotFromICarrot();
or this:
myObject.SomeMethodThatIsFromIShovelButNotFromICarrot();
Should you be surprised if I told you this code compiles? You should, because this code doesn't compile.
You may say "but I know that it's a Superman object which has this method!". But then you'd be forgetting that you only told the compiler it was an ICarrot variable, not a Superman variable.
You may say "but I know that it's a Superman object which implements the IShovel interface!". But then you'd be forgetting that you only told the compiler it was an ICarrot variable, not a Superman or IShovel variable.
Knowing this, let's look back at your code.
Interface1 example = new Example();
All you've said is that you have an Interface1 variable.
if (example instanceof Interface2) {
((Interface2) example).someInterface2Method();
}
It makes no sense for you to assume that this Interface1 object also happens to implement a second unrelated interface. Even if this code works on a technical level, it is a sign of bad design, the developer is expecting some inherent correlation between two interfaces without actually having created this correlation.
You may say "but I know I'm putting an Example object in, the compiler should know that too!" but you'd be missing the point that if this were a method parameter, you would have no way of knowing what the callers of your method are sending.
public void DoTheThing(Interface1 example)
{
if (example instanceof Interface2)
{
((Interface2) example).someInterface2Method();
}
}
When other callers call this method, the compiler is only going to stop them if the passed object does not implement Interface1. The compiler is not going to stop someone from passing an object of a class which implements Interface1 but does not implement Interface2.
Your example does not break LSP, but it seems to break SRP. If you encounter such case where you need to cast an object to its 2nd interface, the method that contains such code can be considered busy.
Implementing 2 (or more) interfaces in a class is fine. In deciding which interface to use as its data type depends entirely on the context of the code that will use it.
Casting is fine, especially when changing context.
class Payment implements Expirable, Limited {
/* ... */
}
class PaymentProcessor {
// Using payment here because i'm working with payments.
public void process(Payment payment) {
boolean expired = expirationChecker.check(payment);
boolean pastLimit = limitChecker.check(payment);
if (!expired && !pastLimit) {
acceptPayment(payment);
}
}
}
class ExpirationChecker {
// This the `Expirable` world, so i'm using Expirable here
public boolean check(Expirable expirable) {
// code
}
}
class LimitChecker {
// This class is about checking limits, thats why im using `Limited` here
public boolean check(Limited limited) {
// code
}
}
Usually, many, client-specific interfaces are fine, and somewhat part of the Interface segregation principle (the "I" in SOLID). Some more specific points, on a technical level, have already been mentioned in other answers.
Particularly that you can go too far with this segregation, by having a class like
class Person implements FirstNameProvider, LastNameProvider, AgeProvider ... {
#Override String getFirstName() {...}
#Override String getLastName() {...}
#Override int getAge() {...}
...
}
Or, conversely, that you have an implementing class that is too powerful, as in
class Application implements DatabaseReader, DataProcessor, UserInteraction, Visualizer {
...
}
I think that the main point in the Interface Segregation Principle is that the interfaces should be client-specific. They should basically "summarize" the functions that are required by a certain client, for a certain task.
To put it that way: The issue is to strike the right balance between the extremes that I sketched above. When I'm trying to figure out interfaces and their relationships (mutually, and in terms of the classes that implement them), I always try to take a step back and ask myself, in an intentionally naïve way: Who is going to receive what, and what is he going to do with it?
Regarding your example: When all your clients always need the functionality of Interface1 and Interface2 at the same time, then you should consider either defining an
interface Combined extends Interface1, Interface2 { }
or not have different interfaces in the first place. On the other hand, when the functionalities are completely distinct and unrelated and never used together, then you should wonder why the single class is implementing them at the same time.
At this point, one could refer to another principle, namely Composition over inheritance. Although it is not classically related to implementing multiple interfaces, composition can also be favorable in this case. For example, you could change your class to not implement the interfaces directly, but only provide instances that implement them:
class Example {
Interface1 getInterface1() { ... }
Interface2 getInterface2() { ... }
}
It looks a bit odd in this Example (sic!), but depending on the complexity of the implementation of Interface1 and Interface2, it can really make sense to keep them separated.
Edited in response to the comment:
The intention here is not to pass the concrete class Example to methods that need both interfaces. A case where this could make sense is rather when a class combines the functionalities of both interfaces, but does not do so by directly implementing them at the same time. It's hard to make up an example that does not look too contrived, but something like this might bring the idea across:
interface DatabaseReader { String read(); }
interface DatabaseWriter { void write(String s); }
class Database {
DatabaseConnection connection = create();
DatabaseReader reader = createReader(connection);
DatabaseReader writer = createWriter(connection);
DatabaseReader getReader() { return reader; }
DatabaseReader getWriter() { return writer; }
}
The client will still rely on the interfaces. Methods like
void create(DatabaseWriter writer) { ... }
void read (DatabaseReader reader) { ... }
void update(DatabaseReader reader, DatabaseWriter writer) { ... }
could then be called with
create(database.getWriter());
read (database.getReader());
update(database.getReader(), database.getWriter());
respectively.
With the help of various posts and comments on this page, a solution has been produced, which I feel is correct for my scenario.
The following shows the iterative changes to the solution to meet SOLID principles.
Requirement
To produce the response for a web service, key + object pairs are added to a response object. There are lots of different key + object pairs that need to be added, each of which may have unique processing required to transform the data from the source to the format required in the response.
From this it is clear that whilst the different key / value pairs may have different processing requirements to transform the source data to the target response object, they all have a common goal of adding an object to the response object.
Therefore, the following interface was produced in solution iteration 1:
Solution Iteration 1
ResponseObjectProvider<T, S> {
void addObject(T targetObject, S sourceObject, String targetKey);
}
Any developer that needs to add an object to the response can now do so using an existing implementation that matches their requirement, or add a new implementation given a new scenario
This is great as we have a common interface which acts as a contract for this common practise of adding response objects
However, one scenario requires that the target object should be taken from the source object given a particular key, "identifier".
There are options here, the first is to add an implementation of the existing interface as follows:
public class GetIdentifierResponseObjectProvider<T extends Map, S extends Map> implements ResponseObjectProvider<T, S> {
public void addObject(final T targetObject, final S sourceObject, final String targetKey) {
targetObject.put(targetKey, sourceObject.get("identifier"));
}
}
This works, however this scenario could be required for other source object keys ("startDate", "endDate" etc...) so this implementation should be made more generic to allow for reuse in this scenario.
Additionally, other implementations may require more context information to perform the addObject operation... So a new generic type should be added to cater for this
Solution Iteration 2
ResponseObjectProvider<T, S, U> {
void addObject(T targetObject, S sourceObject, String targetKey);
void setParams(U params);
U getParams();
}
This interface caters for both usage scenarios; the implementations that require additional params to perform the addObject operation and the implementations that do not
However, considering the latter of the usage scenarios, the implementations that do not require additional parameters will break the SOLID Interface Segregation Principle as these implementations will override getParams and setParams methods but not implement them. e.g:
public class GetObjectBySourceKeyResponseObjectProvider<T extends Map, S extends Map, U extends String> implements ResponseObjectProvider<T, S, U> {
public void addObject(final T targetObject, final S sourceObject, final String targetKey) {
targetObject.put(targetKey, sourceObject.get(U));
}
public void setParams(U params) {
//unimplemented method
}
U getParams() {
//unimplemented method
}
}
Solution Iteration 3
To fix the Interface Segregation issue, the getParams and setParams interface methods were moved into a new Interface:
public interface ParametersProvider<T> {
void setParams(T params);
T getParams();
}
The implementations that require parameters can now implement the ParametersProvider interface:
public class GetObjectBySourceKeyResponseObjectProvider<T extends Map, S extends Map, U extends String> implements ResponseObjectProvider<T, S>, ParametersProvider<U>
private String params;
public void setParams(U params) {
this.params = params;
}
public U getParams() {
return this.params;
}
public void addObject(final T targetObject, final S sourceObject, final String targetKey) {
targetObject.put(targetKey, sourceObject.get(params));
}
}
This solves the Interface Segregation issue but causes two more issues... If the calling client wants to program to an interface, i.e:
ResponseObjectProvider responseObjectProvider = new GetObjectBySourceKeyResponseObjectProvider<>();
Then the addObject method will be available to the instance, but NOT the getParams and setParams methods of the ParametersProvider interface... To call these a cast is required, and to be safe an instanceof check should also be performed:
if(responseObjectProvider instanceof ParametersProvider) {
((ParametersProvider)responseObjectProvider).setParams("identifier");
}
Not only is this undesirable it also breaks the Liskov Substitution Principle - "if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program"
i.e. if we replaced an implementation of ResponseObjectProvider that also implements ParametersProvider, with an implementation that does not implement ParametersProvider then this could alter the some of the desirable properties of the program... Additionally, the client needs to be aware of which implementation is in use to call the correct methods
An additional problem is the usage for calling clients. If the calling client wanted to use an instance that implements both interfaces to perform addObject multiple times, the setParams method would need to be called before addObject... This could cause avoidable bugs if care is not taken when calling.
Solution Iteration 4 - Final Solution
The interfaces produced from Solution Iteration 3 solve all of the currently known usage requirements, with some flexibility provided by generics for implementation using different types. However, this solution breaks the Liskov Substitution Principle and has a non-obvious usage of setParams for the calling client
The solution is to have two separate interfaces, ParameterisedResponseObjectProvider and ResponseObjectProvider.
This allows the client to program to an interface, and would select the appropriate interface depending on whether the objects being added to the response require additional parameters or not
The new interface was first implemented as an extension of ResponseObjectProvider:
public interface ParameterisedResponseObjectProvider<T,S,U> extends ResponseObjectProvider<T, S> {
void setParams(U params);
U getParams();
}
However, this still had the usage issue, where the calling client would first need to call setParams before calling addObject and also make the code less readable.
So the final solution has two separate interfaces defined as follows:
public interface ResponseObjectProvider<T, S> {
void addObject(T targetObject, S sourceObject, String targetKey);
}
public interface ParameterisedResponseObjectProvider<T,S,U> {
void addObject(T targetObject, S sourceObject, String targetKey, U params);
}
This solution solves the breaches of Interface Segregation and Liskov Substitution principles and also improves the usage for calling clients and improves the readability of the code.
It does mean that the client needs to be aware of the different interfaces, but since the contracts are different this seems to be a justified decision especially when considering all the issues that the solution has avoided.
The problem you describe often comes about through over-zealous application of the Interface Segregation Principle, encouraged by languages' inability to specify that members of one interface should, by default, be chained to static methods which could implement sensible behaviors.
Consider, for example, a basic sequence/enumeration interface and the following behaviors:
Produce an enumerator which can read out the objects if no other iterator has yet been created.
Produce an enumerator which can read out the objects even if another iterator has already been created and used.
Report how many items are in the sequence
Report the value of the Nth item in the sequence
Copy a range of items from the object into an array of that type.
Yield a reference to an immutable object that can accommodate the above operations efficiently with contents that are guaranteed never to change.
I would suggest that such abilities should be part of the basic sequence/enumeration interface, along with a method/property to indicate which of the above operations are meaningfully supported. Some kinds of single-shot on-demand enumerators (e.g. an infinite truly-random sequence generator) might not be able to support any of those functions, but segregating such functions into separate interfaces will make it much harder to produce efficient wrappers for many kinds of operations.
One could produce a wrapper class that would accommodate all of the above operations, though not necessarily efficiently, on any finite sequence which supports the first ability. If, however, the class is being used to wrap an object that already supports some of those abilities (e.g. access the Nth item), having the wrapper use the underlying behaviors could be much more efficient than having it do everything via the second function above (e.g. creating a new enumerator, and using that to iteratively read and ignore items from the sequence until the desired one is reached).
Having all objects that produce any kind of sequence support an interface that includes all of the above, along with an indication of what abilities are supported, would be cleaner than trying to have different interfaces for different subsets of abilities, and requiring that wrapper classes make explicit provision for any combinations they want to expose to their clients.
First of all... Sorry for this post. I know that there are many many posts on stackoverflow which are discussing multiple inheritance. But I already know that Java does not support multiple inheritance and I know that using interfaces should be an alternative. But I don't get it and see my dilemma:
I have to make changes on a very very large and complex tool written in Java. In this tool there is a data structure built with many different class objects with a linked member hierarchy. Anyway...
I have one class Tagged which has multiple methods and returns an object tag depending on the object's class. It needs members and static variables.
And a second class called XMLElement allows to link objects and in the end generate a XML file. I also need member and static variables here.
Finally, I have these many many data classes which nearly all should extend XMLElement and some of them Tagged.
Ok ok, this won't work since it's only possible to extend just one class. I read very often that everything with Java is ok and there is no need for having multiple inheritance. I believe, but I don't see how an interface should replace inheritance.
It makes no sense to put the real implementation in all data classes since it is the same every time but this would be necessary with interfaces (I think).
I don't see how I could change one of my inheritance classes to an interface. I have variables in here and they have to be exactly there.
I really don't get it so please can somebody explain me how to handle this?
Actually, I have no good answer other than Java SHOULD have Multiple Inheritance. The whole point that interfaces should be able to replace the need for Multiple Inheritance is like the big lie that when repeated enough times becomes true.
The argument is that Multiple Inheritance causes all these problems (la-di-dah), yet I keep hearing those arguments from Java developers who have never used C++. I also don't EVER remember C++ programmers saying "Gee, I love C++, but if they would only get rid of Multiple Inheritance, it would become a great language". People used it when it was practical and didn't when it wasn't.
Your problem is a classic case of where Multiple Inheritance would be appropriate. Any suggestion to refactor the code is really telling you how to work around the PROBLEM that Java has no Multiple Inheritance.
Also all the discussion that "oh, delegation is better, la-di-dah" is confusing religion with design. There is no right way. Things are either more useful or less useful and that is all.
In your case Multiple Inheritance would be more useful and a more elegant solution.
As far as refactoring your code into a less useful form to satisfy all the religious people who have never used Multiple Inheritance and believe "Multiple Inheritance is bad", I guess you will have to downgrade your code because I don't see Java "improving" in that way any time soon. There are too many people repeating the religious mantra to the point of stupidity that I can't see it ever being added to the language.
Actually, my solution for you would be "x extends Tagged, XMLElement" and that would be all.
...but as you can see from the solutions provided above, most people think that such a solution would be WAY TOO COMPLEX AND CONFUSING!
I would prefer to venture into the "x extends a,b" territory myself, even if it is a very frightening solution that might overwhelm the abilities of most Java programmers.
What is even more amazing about the solutions suggested above is that everyone here who suggested that you refactor your code into "delegation" because Multiple Inheritance is bad, would, if they were confronted with the very same problem, would solve the problem by simply doing: "x extends a,b" and be done with it, and all their religious arguments about "delegation vs inheritance" would disappear. The whole debate is stupid, and it only being advanced by clueless programmers who only demonstrate how well they can recite out of a book and how little they can think for themselves.
You are 100% correct that Multiple Inheritance would help, and no, you are doing anything wrong in your code if you think Java should have it.
You should probably favor composition (and delegation) over inheritance :
public interface TaggedInterface {
void foo();
}
public interface XMLElementInterface {
void bar();
}
public class Tagged implements TaggedInterface {
// ...
}
public class XMLElement implements XMLElementInterface {
// ...
}
public class TaggedXmlElement implements TaggedInterface, XMLElementInterface {
private TaggedInterface tagged;
private XMLElementInterface xmlElement;
public TaggedXmlElement(TaggedInterface tagged, XMLElementInterface xmlElement) {
this.tagged = tagged;
this.xmlElement = xmlElement;
}
public void foo() {
this.tagged.foo();
}
public void bar() {
this.xmlElement.bar();
}
public static void main(String[] args) {
TaggedXmlElement t = new TaggedXmlElement(new Tagged(), new XMLElement());
t.foo();
t.bar();
}
}
Similar to what Andreas_D suggested but with the use of inner classes. This way you indeed extend each class and can override it in your own code if desired.
interface IBird {
public void layEgg();
}
interface IMammal {
public void giveMilk();
}
class Bird implements IBird {
public void layEgg() {
System.out.println("Laying eggs...");
}
}
class Mammal implements IMammal {
public void giveMilk() {
System.out.println("Giving milk...");
}
}
class Platypus implements IMammal, IBird {
private class LayingEggAnimal extends Bird {}
private class GivingMilkAnimal extends Mammal {}
private LayingEggAnimal layingEggAnimal = new LayingEggAnimal();
private GivingMilkAnimal givingMilkAnimal = new GivingMilkAnimal();
#Override
public void layEgg() {
layingEggAnimal.layEgg();
}
#Override
public void giveMilk() {
givingMilkAnimal.giveMilk();
}
}
First it makes no sense to put the real implementation in all data classes since it is the same every time but this would be necessary with interfaces (I think).
How about using aggregation for the tags?
Rename your Tagged class to Tags.
Create a Tagged interface:
interface Tagged {
Tags getTags();
}
Let each class that needs to be "tagged", implement Tagged and let it have a tags field, which is returned from getTags.
Second I don't see how I could change one of my inheritance classes to an interface. I have variables in here and they have to be exactly there.
That's right, interfaces can't have instance variables. The data structures storing the tags however, shouldn't necessarily IMO be part of the classes that are tagged. Factor out the tags in a separate data structure.
I'd solve it that way: extract interfaces for the Tagged and XMLElement class (maybe you don't need all methods in the public interface). Then, implement both interfaces and the implementing class has a Tagged (your actual concrete Tagged class) and an XMLElement (your actual concrete XMLElement class):
public class MyClass implements Tagged, XMLElement {
private Tagged tagged;
private XMLElement xmlElement;
public MyClass(/*...*/) {
tagged = new TaggedImpl();
xmlElement = new XMLElementImpl();
}
#Override
public void someTaggedMethod() {
tagged.someTaggedMethod();
}
}
public class TaggedImpl implements Tagged {
#Override
public void someTaggedMethod() {
// so what has to be done
}
}
public interface Tagged {
public void someTaggedMethod();
}
(and the same for XMLElement)
one possible way;
1- You can create base class(es) for common functionality, make it abstract if you dont need to instantiate it.
2- Create interfaces and implement those interfaces in those base class(es). If specific implementation is needed, make the method abstract. each concrete class can have its own impl.
3- extend the abstract base class for in concrete class(es) and implement specific interfaces at this level as well
Just wondering if one could not simply use inner (member) classes (LRM 5.3.7)?
E.g. like this (based on the first answer above):
// original classes:
public class Tagged {
// ...
}
public class XMLElement {
// ...
}
public class TaggedXmlElement {
public/protected/private (static?) class InnerTagged extends Tagged {
// ...
}
public/protected/private (static?) class InnerXmlElement extends XMLElement {
// ...
}
}
This way you have a class TaggedXmlElement which actually contains all elements from the two original classes and within TaggedXmlElement you have access to non-private members of the member classes. Of course one would not use "super", but call member class methods.
Alternatively one could extend one of the classes and make the other a member class.
There are some restrictions, but I think they can all be worked around.
Well using Interface and single base class you are simply stating:
A) One object can be of only one type (Which is true in real life if you think ,
A pigeon is a bird, a toyota is a car , etc .. A pigeon is also an animal but every bird is animal anyway, so its hierarchically above the bird type -And in your OOP design Animal class should be base of Bird class in case you need to represent it -)
and
B) can do many different things (A bird can sing, can fly . A car can run , can stop ,etc..) which also fits the real life objects.
In a world where objects can be of multiple types (horizontally)
Let's say a a dolphin is a mammal and also a sea animal, in this case multiple inheritance would make more sense. It would be easier to represent it using multiple inheritance.
Using composition would be the way to go as another developer suggested. The main argument against multiple inheritance is the ambiguity created when you're extending from two classes with the same method declaration (same method name & parameters). Personally, however, I think that's a load of crap. A compilation error could easily be thrown in this situation, which wouldn't be much different from defining multiple methods of the same name in a single class. Something like the following code snippet could easily solve this dilema:
public MyExtendedClass extends ClassA, ClassB {
public duplicateMethodName() {
return ClassA.duplicateMethodName();
}
}
Another argument against multiple inheritance is that Java was trying to keep things simple so that amateur developers don't create a web of interdependent classes that could create a messy, confusing software system. But as you see in your case, it also complicates and confuses things when it's not available. Plus, that argument could be used for a 100 other things in coding, which is why development teams have code reviews, style checking software, and nightly builds.
In your particular situation though, you'll have to settle with composition (see Shojaei Baghini's answer). It adds a bit of boiler plate code, but it emulates the same behavior as multiple inheritance.
I run in a similar problem on Android. I needed to extend a Button and a TextView (both inheriting from View) with additional functions. Due to not having access to their super class, I needed to find another solution. I´ve written a new class which encapsulates all the implementations:
class YourButton extends Button implements YourFunctionSet {
private Modifier modifier;
public YourButton(Context context) {
super(context);
modifier = new Modifier(this);
}
public YourButton(Context context, AttributeSet attrs) {
super(context, attrs);
modifier = new Modifier(this);
}
public YourButton(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
modifier = new Modifier(this);
}
#Override
public void generateRandomBackgroundColor() {
modifier.generateRandomBackgroundColor();
}
}
class Modifier implements YourFunctionSet {
private View view;
public Modifier(View view) {
this.view = view;
}
#Override
public void generateRandomBackgroundColor() {
/**
* Your shared code
*
* ......
*
* view.setBackgroundColor(randomColor);
*/
}
}
interface YourFunctionSet {
void generateRandomBackgroundColor();
}
The problem here is, your classes need the same super class. You can also try to use different classes, but check which type it is from, for example
public class Modifier{
private View view;
private AnotherClass anotherClass;
public Modifier(Object object) {
if (object instanceof View) {
this.view = (View) object;
} else if (object instanceof AnotherClass) {
this.anotherClass = (AnotherClass) object;
}
}
public void generateRandomBackgroundColor(){
if(view!=null){
//...do
}else if(anotherClass!=null){
//...do
}
}
}
So here is basically my Modifier class the class which encapsulates all implementations.
Hope this helps someone.
EDIT: As of Java 8, static methods are now allowed in interfaces.
Here's the example:
public interface IXMLizable<T>
{
static T newInstanceFromXML(Element e);
Element toXMLElement();
}
Of course this won't work. But why not?
One of the possible issues would be, what happens when you call:
IXMLizable.newInstanceFromXML(e);
In this case, I think it should just call an empty method (i.e. {}). All subclasses would be forced to implement the static method, so they'd all be fine when calling the static method. So why isn't this possible?
EDIT: I guess I'm looking for answer that's deeper than "because that's the way Java is".
Is there a particular technological reason why static methods can't be overwritten? That is, why did the designers of Java decide to make instance methods overrideable but not static methods?
EDIT: The problem with my design is I'm trying to use interfaces to enforce a coding convention.
That is, the goal of the interface is twofold:
I want the IXMLizable interface to allow me to convert classes that implement it to XML elements (using polymorphism, works fine).
If someone wants to make a new instance of a class that implements the IXMLizable interface, they will always know that there will be a newInstanceFromXML(Element e) static constructor.
Is there any other way to ensure this, other than just putting a comment in the interface?
Java 8 permits static interface methods
With Java 8, interfaces can have static methods. They can also have concrete instance methods, but not instance fields.
There are really two questions here:
Why, in the bad old days, couldn't interfaces contain static methods?
Why can't static methods be overridden?
Static methods in interfaces
There was no strong technical reason why interfaces couldn't have had static methods in previous versions. This is summed up nicely by the poster of a duplicate question. Static interface methods were initially considered as a small language change, and then there was an official proposal to add them in Java 7, but it was later dropped due to unforeseen complications.
Finally, Java 8 introduced static interface methods, as well as override-able instance methods with a default implementation. They still can't have instance fields though. These features are part of the lambda expression support, and you can read more about them in Part H of JSR 335.
Overriding static methods
The answer to the second question is a little more complicated.
Static methods are resolvable at compile time. Dynamic dispatch makes sense for instance methods, where the compiler can't determine the concrete type of the object, and, thus, can't resolve the method to invoke. But invoking a static method requires a class, and since that class is known statically—at compile time—dynamic dispatch is unnecessary.
A little background on how instance methods work is necessary to understand what's going on here. I'm sure the actual implementation is quite different, but let me explain my notion of method dispatch, which models observed behavior accurately.
Pretend that each class has a hash table that maps method signatures (name and parameter types) to an actual chunk of code to implement the method. When the virtual machine attempts to invoke a method on an instance, it queries the object for its class and looks up the requested signature in the class's table. If a method body is found, it is invoked. Otherwise, the parent class of the class is obtained, and the lookup is repeated there. This proceeds until the method is found, or there are no more parent classes—which results in a NoSuchMethodError.
If a superclass and a subclass both have an entry in their tables for the same method signature, the sub class's version is encountered first, and the superclass's version is never used—this is an "override".
Now, suppose we skip the object instance and just start with a subclass. The resolution could proceed as above, giving you a sort of "overridable" static method. The resolution can all happen at compile-time, however, since the compiler is starting from a known class, rather than waiting until runtime to query an object of an unspecified type for its class. There is no point in "overriding" a static method since one can always specify the class that contains the desired version.
Constructor "interfaces"
Here's a little more material to address the recent edit to the question.
It sounds like you want to effectively mandate a constructor-like method for each implementation of IXMLizable. Forget about trying to enforce this with an interface for a minute, and pretend that you have some classes that meet this requirement. How would you use it?
class Foo implements IXMLizable<Foo> {
public static Foo newInstanceFromXML(Element e) { ... }
}
Foo obj = Foo.newInstanceFromXML(e);
Since you have to explicitly name the concrete type Foo when "constructing" the new object, the compiler can verify that it does indeed have the necessary factory method. And if it doesn't, so what? If I can implement an IXMLizable that lacks the "constructor", and I create an instance and pass it to your code, it is an IXMLizable with all the necessary interface.
Construction is part of the implementation, not the interface. Any code that works successfully with the interface doesn't care about the constructor. Any code that cares about the constructor needs to know the concrete type anyway, and the interface can be ignored.
This was already asked and answered, here
To duplicate my answer:
There is never a point to declaring a static method in an interface. They cannot be executed by the normal call MyInterface.staticMethod(). If you call them by specifying the implementing class MyImplementor.staticMethod() then you must know the actual class, so it is irrelevant whether the interface contains it or not.
More importantly, static methods are never overridden, and if you try to do:
MyInterface var = new MyImplementingClass();
var.staticMethod();
the rules for static say that the method defined in the declared type of var must be executed. Since this is an interface, this is impossible.
The reason you can't execute "result=MyInterface.staticMethod()" is that it would have to execute the version of the method defined in MyInterface. But there can't be a version defined in MyInterface, because it's an interface. It doesn't have code by definition.
While you can say that this amounts to "because Java does it that way", in reality the decision is a logical consequence of other design decisions, also made for very good reason.
With the advent of Java 8 it is possible now to write default and static methods in interface.
docs.oracle/staticMethod
For example:
public interface Arithmetic {
public int add(int a, int b);
public static int multiply(int a, int b) {
return a * b;
}
}
public class ArithmaticImplementation implements Arithmetic {
#Override
public int add(int a, int b) {
return a + b;
}
public static void main(String[] args) {
int result = Arithmetic.multiply(2, 3);
System.out.println(result);
}
}
Result : 6
TIP : Calling an static interface method doesn't require to be implemented by any class. Surely, this happens because the same rules for static methods in superclasses applies for static methods on interfaces.
Normally this is done using a Factory pattern
public interface IXMLizableFactory<T extends IXMLizable> {
public T newInstanceFromXML(Element e);
}
public interface IXMLizable {
public Element toXMLElement();
}
Because static methods cannot be overridden in subclasses, and hence they cannot be abstract. And all methods in an interface are, de facto, abstract.
Why can't I define a static method in a Java interface?
Actually you can in Java 8.
As per Java doc:
A static method is a method that is associated with the class in which
it is defined rather than with any object. Every instance of the class
shares its static methods
In Java 8 an interface can have default methods and static methods. This makes it easier for us to organize helper methods in our libraries. We can keep static methods specific to an interface in the same interface rather than in a separate class.
Example of default method:
list.sort(ordering);
instead of
Collections.sort(list, ordering);
Example of static method (from doc itself):
public interface TimeClient {
// ...
static public ZoneId getZoneId (String zoneString) {
try {
return ZoneId.of(zoneString);
} catch (DateTimeException e) {
System.err.println("Invalid time zone: " + zoneString +
"; using default time zone instead.");
return ZoneId.systemDefault();
}
}
default public ZonedDateTime getZonedDateTime(String zoneString) {
return ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString));
}
}
Interfaces are concerned with polymorphism which is inherently tied to object instances, not classes. Therefore static doesn't make sense in the context of an interface.
First, all language decisions are decisions made by the language creators. There is nothing in the world of software engineering or language defining or compiler / interpreter writing which says that a static method cannot be part of an interface. I've created a couple of languages and written compilers for them -- it's all just sitting down and defining meaningful semantics. I'd argue that the semantics of a static method in an interface are remarkably clear -- even if the compiler has to defer resolution of the method to run-time.
Secondly, that we use static methods at all means there is a valid reason for having an interface pattern which includes static methods -- I can't speak for any of you, but I use static methods on a regular basis.
The most likely correct answer is that there was no perceived need, at the time the language was defined, for static methods in interfaces. Java has grown a lot over the years and this is an item that has apparently gained some interest. That it was looked at for Java 7 indicates that its risen to a level of interest that might result in a language change. I, for one, will be happy when I no longer have to instantiate an object just so I can call my non-static getter method to access a static variable in a subclass instance ...
"Is there a particular reason that static methods cannot be overridden".
Let me re-word that question for your by filling in the definitions.
"Is there a particular reason that methods resolved at compile time cannot be resolved at runtime."
Or, to put in more completely, If I want to call a method without an instance, but knowing the class, how can I have it resolved based upon the instance that I don't have.
Static methods aren't virtual like instance methods so I suppose the Java designers decided they didn't want them in interfaces.
But you can put classes containing static methods inside interfaces. You could try that!
public interface Test {
static class Inner {
public static Object get() {
return 0;
}
}
}
Commenting EDIT: As of Java 8, static methods are now allowed in interfaces.
It is right, static methods since Java 8 are allowed in interfaces, but your example still won't work. You cannot just define a static method: you have to implement it or you will obtain a compilation error.
Several answers have discussed the problems with the concept of overridable static methods. However sometimes you come across a pattern where it seems like that's just what you want to use.
For example, I work with an object-relational layer that has value objects, but also has commands for manipulating the value objects. For various reasons, each value object class has to define some static methods that let the framework find the command instance. For example, to create a Person you'd do:
cmd = createCmd(Person.getCreateCmdId());
Person p = cmd.execute();
and to load a Person by ID you'd do
cmd = createCmd(Person.getGetCmdId());
cmd.set(ID, id);
Person p = cmd.execute();
This is fairly convenient, however it has its problems; notably the existence of the static methods can not be enforced in the interface. An overridable static method in the interface would be exactly what we'd need, if only it could work somehow.
EJBs solve this problem by having a Home interface; each object knows how to find its Home and the Home contains the "static" methods. This way the "static" methods can be overridden as needed, and you don't clutter up the normal (it's called "Remote") interface with methods that don't apply to an instance of your bean. Just make the normal interface specify a "getHome()" method. Return an instance of the Home object (which could be a singleton, I suppose) and the caller can perform operations that affect all Person objects.
Why can't I define a static method in a Java interface?
All methods in an interface are explicitly abstract and hence you cannot define them as static because static methods cannot be abstract.
Well, without generics, static interfaces are useless because all static method calls are resolved at compile time. So, there's no real use for them.
With generics, they have use -- with or without a default implementation. Obviously there would need to be overriding and so on. However, my guess is that such usage wasn't very OO (as the other answers point out obtusely) and hence wasn't considered worth the effort they'd require to implement usefully.
An interface can never be dereferenced statically, e.g. ISomething.member. An interface is always dereferenced via a variable that refers to an instance of a subclass of the interface. Thus, an interface reference can never know which subclass it refers to without an instance of its subclass.
Thus the closest approximation to a static method in an interface would be a non-static method that ignores "this", i.e. does not access any non-static members of the instance. At the low-level abstraction, every non-static method (after lookup in any vtable) is really just a function with class scope that takes "this" as an implicit formal parameter. See Scala's singleton object and interoperability with Java as evidence of that concept.
And thus every static method is a function with class scope that does not take a "this" parameter. Thus normally a static method can be called statically, but as previously stated, an interface has no implementation (is abstract).
Thus to get closest approximation to a static method in an interface, is to use a non-static method, then don't access any of the non-static instance members. There would be no possible performance benefit any other way, because there is no way to statically link (at compile-time) a ISomething.member(). The only benefit I see of a static method in an interface is that it would not input (i.e. ignore) an implicit "this" and thus disallow access to any of the non-static instance members. This would declare implicitly that the function that doesn't access "this", is immutate and not even readonly with respect to its containing class. But a declaration of "static" in an interface ISomething would also confuse people who tried to access it with ISomething.member() which would cause a compiler error. I suppose if the compiler error was sufficiently explanatory, it would be better than trying to educate people about using a non-static method to accomplish what they want (apparently mostly factory methods), as we are doing here (and has been repeated for 3 Q&A times on this site), so it is obviously an issue that is not intuitive for many people. I had to think about it for a while to get the correct understanding.
The way to get a mutable static field in an interface is use non-static getter and setter methods in an interface, to access that static field that in the subclass. Sidenote, apparently immutable statics can be declared in a Java interface with static final.
Interfaces just provide a list of things a class will provide, not an actual implementation of those things, which is what your static item is.
If you want statics, use an abstract class and inherit it, otherwise, remove the static.
Hope that helps!
You can't define static methods in an interface because static methods belongs to a class not to an instance of class, and interfaces are not Classes. Read more here.
However, If you want you can do this:
public class A {
public static void methodX() {
}
}
public class B extends A {
public static void methodX() {
}
}
In this case what you have is two classes with 2 distinct static methods called methodX().
Suppose you could do it; consider this example:
interface Iface {
public static void thisIsTheMethod();
}
class A implements Iface {
public static void thisIsTheMethod(){
system.out.print("I'm class A");
}
}
class B extends Class A {
public static void thisIsTheMethod(){
System.out.print("I'm class B");
}
}
SomeClass {
void doStuff(Iface face) {
IFace.thisIsTheMethod();
// now what would/could/should happen here.
}
}
Something that could be implemented is static interface (instead of static method in an interface). All classes implementing a given static interface should implement the corresponding static methods. You could get static interface SI from any Class clazz using
SI si = clazz.getStatic(SI.class); // null if clazz doesn't implement SI
// alternatively if the class is known at compile time
SI si = Someclass.static.SI; // either compiler errror or not null
then you can call si.method(params).
This would be useful (for factory design pattern for example) because you can get (or check the implementation of) SI static methods implementation from a compile time unknown class !
A dynamic dispatch is necessary and you can override the static methods (if not final) of a class by extending it (when called through the static interface).
Obviously, these methods can only access static variables of their class.
While I realize that Java 8 resolves this issue, I thought I'd chime in with a scenario I am currently working on (locked into using Java 7) where being able to specify static methods in an interface would be helpful.
I have several enum definitions where I've defined "id" and "displayName" fields along with helper methods evaluating the values for various reasons. Implementing an interface allows me to ensure that the getter methods are in place but not the static helper methods. Being an enum, there really isn't a clean way to offload the helper methods into an inherited abstract class or something of the like so the methods have to be defined in the enum itself. Also because it is an enum, you wouldn't ever be able to actually pass it as an instanced object and treat it as the interface type, but being able to require the existence of the static helper methods through an interface is what I like about it being supported in Java 8.
Here's code illustrating my point.
Interface definition:
public interface IGenericEnum <T extends Enum<T>> {
String getId();
String getDisplayName();
//If I was using Java 8 static helper methods would go here
}
Example of one enum definition:
public enum ExecutionModeType implements IGenericEnum<ExecutionModeType> {
STANDARD ("Standard", "Standard Mode"),
DEBUG ("Debug", "Debug Mode");
String id;
String displayName;
//Getter methods
public String getId() {
return id;
}
public String getDisplayName() {
return displayName;
}
//Constructor
private ExecutionModeType(String id, String displayName) {
this.id = id;
this.displayName = displayName;
}
//Helper methods - not enforced by Interface
public static boolean isValidId(String id) {
return GenericEnumUtility.isValidId(ExecutionModeType.class, id);
}
public static String printIdOptions(String delimiter){
return GenericEnumUtility.printIdOptions(ExecutionModeType.class, delimiter);
}
public static String[] getIdArray(){
return GenericEnumUtility.getIdArray(ExecutionModeType.class);
}
public static ExecutionModeType getById(String id) throws NoSuchObjectException {
return GenericEnumUtility.getById(ExecutionModeType.class, id);
}
}
Generic enum utility definition:
public class GenericEnumUtility {
public static <T extends Enum<T> & IGenericEnum<T>> boolean isValidId(Class<T> enumType, String id) {
for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
if(enumOption.getId().equals(id)) {
return true;
}
}
return false;
}
public static <T extends Enum<T> & IGenericEnum<T>> String printIdOptions(Class<T> enumType, String delimiter){
String ret = "";
delimiter = delimiter == null ? " " : delimiter;
int i = 0;
for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
if(i == 0) {
ret = enumOption.getId();
} else {
ret += delimiter + enumOption.getId();
}
i++;
}
return ret;
}
public static <T extends Enum<T> & IGenericEnum<T>> String[] getIdArray(Class<T> enumType){
List<String> idValues = new ArrayList<String>();
for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
idValues.add(enumOption.getId());
}
return idValues.toArray(new String[idValues.size()]);
}
#SuppressWarnings("unchecked")
public static <T extends Enum<T> & IGenericEnum<T>> T getById(Class<T> enumType, String id) throws NoSuchObjectException {
id = id == null ? "" : id;
for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
if(id.equals(enumOption.getId())) {
return (T)enumOption;
}
}
throw new NoSuchObjectException(String.format("ERROR: \"%s\" is not a valid ID. Valid IDs are: %s.", id, printIdOptions(enumType, " , ")));
}
}
Let's suppose static methods were allowed in interfaces:
* They would force all implementing classes to declare that method.
* Interfaces would usually be used through objects, so the only effective methods on those would be the non-static ones.
* Any class which knows a particular interface could invoke its static methods. Hence a implementing class' static method would be called underneath, but the invoker class does not know which. How to know it? It has no instantiation to guess that!
Interfaces were thought to be used when working with objects. This way, an object is instantiated from a particular class, so this last matter is solved. The invoking class need not know which particular class is because the instantiation may be done by a third class. So the invoking class knows only the interface.
If we want this to be extended to static methods, we should have the possibility to especify an implementing class before, then pass a reference to the invoking class. This could use the class through the static methods in the interface. But what is the differente between this reference and an object? We just need an object representing what it was the class. Now, the object represents the old class, and could implement a new interface including the old static methods - those are now non-static.
Metaclasses serve for this purpose. You may try the class Class of Java. But the problem is that Java is not flexible enough for this. You can not declare a method in the class object of an interface.
This is a meta issue - when you need to do ass
..blah blah
anyway you have an easy workaround - making the method non-static with the same logic. But then you would have to first create an object to call the method.
To solve this :
error: missing method body, or declare abstract
static void main(String[] args);
interface I
{
int x=20;
void getValue();
static void main(String[] args){};//Put curly braces
}
class InterDemo implements I
{
public void getValue()
{
System.out.println(x);
}
public static void main(String[] args)
{
InterDemo i=new InterDemo();
i.getValue();
}
}
output :
20
Now we can use static method in interface
I think java does not have static interface methods because you do not need them. You may think you do, but...
How would you use them? If you want to call them like
MyImplClass.myMethod()
then you do not need to declare it in the interface. If you want to call them like
myInstance.myMethod()
then it should not be static.
If you are actually going to use first way, but just want to enforce each implementation to have such static method, then it is really a coding convention, not a contract between instance that implements an interface and calling code.
Interfaces allow you to define contract between instance of class that implement the interface and calling code. And java helps you to be sure that this contract is not violated, so you can rely on it and don't worry what class implements this contract, just "someone who signed a contract" is enough. In case of static interfaces your code
MyImplClass.myMethod()
does not rely on the fact that each interface implementation has this method, so you do not need java to help you to be sure with it.
What is the need of static method in interface, static methods are used basically when you don't have to create an instance of object whole idea of interface is to bring in OOP concepts with introduction of static method you're diverting from concept.
Does the Java language have delegate features, similar to how C# has support for delegates?
Not really, no.
You may be able to achieve the same effect by using reflection to get Method objects you can then invoke, and the other way is to create an interface with a single 'invoke' or 'execute' method, and then instantiate them to call the method your interested in (i.e. using an anonymous inner class).
You might also find this article interesting / useful : A Java Programmer Looks at C# Delegates (#blueskyprojects.com)
Depending precisely what you mean, you can achieve a similar effect (passing around a method) using the Strategy Pattern.
Instead of a line like this declaring a named method signature:
// C#
public delegate void SomeFunction();
declare an interface:
// Java
public interface ISomeBehaviour {
void SomeFunction();
}
For concrete implementations of the method, define a class that implements the behaviour:
// Java
public class TypeABehaviour implements ISomeBehaviour {
public void SomeFunction() {
// TypeA behaviour
}
}
public class TypeBBehaviour implements ISomeBehaviour {
public void SomeFunction() {
// TypeB behaviour
}
}
Then wherever you would have had a SomeFunction delegate in C#, use an ISomeBehaviour reference instead:
// C#
SomeFunction doSomething = SomeMethod;
doSomething();
doSomething = SomeOtherMethod;
doSomething();
// Java
ISomeBehaviour someBehaviour = new TypeABehaviour();
someBehaviour.SomeFunction();
someBehaviour = new TypeBBehaviour();
someBehaviour.SomeFunction();
With anonymous inner classes, you can even avoid declaring separate named classes and almost treat them like real delegate functions.
// Java
public void SomeMethod(ISomeBehaviour pSomeBehaviour) {
...
}
...
SomeMethod(new ISomeBehaviour() {
#Override
public void SomeFunction() {
// your implementation
}
});
This should probably only be used when the implementation is very specific to the current context and wouldn't benefit from being reused.
And then of course in Java 8, these do become basically lambda expressions:
// Java 8
SomeMethod(() -> { /* your implementation */ });
Short story: no.
Introduction
The newest version of the Microsoft Visual J++ development environment
supports a language construct called delegates or bound method
references. This construct, and the new keywords delegate and
multicast introduced to support it, are not a part of the JavaTM
programming language, which is specified by the Java Language
Specification and amended by the Inner Classes Specification included
in the documentation for the JDKTM 1.1 software.
It is unlikely that the Java programming language will ever include
this construct. Sun already carefully considered adopting it in 1996,
to the extent of building and discarding working prototypes. Our
conclusion was that bound method references are unnecessary and
detrimental to the language. This decision was made in consultation
with Borland International, who had previous experience with bound
method references in Delphi Object Pascal.
We believe bound method references are unnecessary because another
design alternative, inner classes, provides equal or superior
functionality. In particular, inner classes fully support the
requirements of user-interface event handling, and have been used to
implement a user-interface API at least as comprehensive as the
Windows Foundation Classes.
We believe bound method references are harmful because they detract
from the simplicity of the Java programming language and the
pervasively object-oriented character of the APIs. Bound method
references also introduce irregularity into the language syntax and
scoping rules. Finally, they dilute the investment in VM technologies
because VMs are required to handle additional and disparate types of
references and method linkage efficiently.
Have you read this :
Delegates are a useful construct in event-based systems. Essentially
Delegates are objects that encode a method dispatch on a specified
object. This document shows how java inner classes provide a more
generic solution to such problems.
What is a Delegate? Really it is very similar to a pointer to member
function as used in C++. But a delegate contains the target object
alongwith the method to be invoked. Ideally it would be nice to be
able to say:
obj.registerHandler(ano.methodOne);
..and that the method methodOne would be called on ano when some specific event was received.
This is what the Delegate structure achieves.
Java Inner Classes
It has been argued that Java provides this
functionality via anonymous inner classes and thus does not need the additional
Delegate construct.
obj.registerHandler(new Handler() {
public void handleIt(Event ev) {
methodOne(ev);
}
} );
At first glance this seems correct but at the same time a nuisance.
Because for many event processing examples the simplicity of the
Delegates syntax is very attractive.
General Handler
However, if event-based programming is used in a more
pervasive manner, say, for example, as a part of a general
asynchronous programming environment, there is more at stake.
In such a general situation, it is not sufficient to include only the
target method and target object instance. In general there may be
other parameters required, that are determined within the context when
the event handler is registered.
In this more general situation, the java approach can provide a very
elegant solution, particularly when combined with use of final
variables:
void processState(final T1 p1, final T2 dispatch) {
final int a1 = someCalculation();
m_obj.registerHandler(new Handler() {
public void handleIt(Event ev) {
dispatch.methodOne(a1, ev, p1);
}
} );
}
final * final * final
Got your attention?
Note that the final variables are accessible from within the anonymous
class method definitions. Be sure to study this code carefully to
understand the ramifications. This is potentially a very powerful
technique. For example, it can be used to good effect when registering
handlers in MiniDOM and in more general situations.
By contrast, the Delegate construct does not provide a solution for
this more general requirement, and as such should be rejected as an
idiom on which designs can be based.
I know this post is old, but Java 8 has added lambdas, and the concept of a functional interface, which is any interface with only one method. Together these offer similar functionality to C# delegates. See here for more info, or just google Java Lambdas.
http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html
No, but they're fakeable using proxies and reflection:
public static class TestClass {
public String knockKnock() {
return "who's there?";
}
}
private final TestClass testInstance = new TestClass();
#Test public void
can_delegate_a_single_method_interface_to_an_instance() throws Exception {
Delegator<TestClass, Callable<String>> knockKnockDelegator = Delegator.ofMethod("knockKnock")
.of(TestClass.class)
.to(Callable.class);
Callable<String> callable = knockKnockDelegator.delegateTo(testInstance);
assertThat(callable.call(), is("who's there?"));
}
The nice thing about this idiom is that you can verify that the delegated-to method exists, and has the required signature, at the point where you create the delegator (although not at compile-time, unfortunately, although a FindBugs plug-in might help here), then use it safely to delegate to various instances.
See the karg code on github for more tests and implementation.
Yes & No, but delegate pattern in Java could be thought of this way. This video tutorial is about data exchange between activity - fragments, and it has great essence of delegate sorta pattern using interfaces.
I have implemented callback/delegate support in Java using reflection. Details and working source are available on my website.
How It Works
There is a principle class named Callback with a nested class named WithParms. The API which needs the callback will take a Callback object as a parameter and, if neccessary, create a Callback.WithParms as a method variable. Since a great many of the applications of this object will be recursive, this works very cleanly.
With performance still a high priority to me, I didn't want to be required to create a throwaway object array to hold the parameters for every invocation - after all in a large data structure there could be thousands of elements, and in a message processing scenario we could end up processing thousands of data structures a second.
In order to be threadsafe the parameter array needs to exist uniquely for each invocation of the API method, and for efficiency the same one should be used for every invocation of the callback; I needed a second object which would be cheap to create in order to bind the callback with a parameter array for invocation. But, in some scenarios, the invoker would already have a the parameter array for other reasons. For these two reasons, the parameter array does not belong in the Callback object. Also the choice of invocation (passing the parameters as an array or as individual objects) belongs in the hands of the API using the callback enabling it to use whichever invocation is best suited to its inner workings.
The WithParms nested class, then, is optional and serves two purposes, it contains the parameter object array needed for the callback invocations, and it provides 10 overloaded invoke() methods (with from 1 to 10 parameters) which load the parameter array and then invoke the callback target.
What follows is an example using a callback to process the files in a directory tree. This is an initial validation pass which just counts the files to process and ensure none exceed a predetermined maximum size. In this case we just create the callback inline with the API invocation. However, we reflect the target method out as a static value so that the reflection is not done every time.
static private final Method COUNT =Callback.getMethod(Xxx.class,"callback_count",true,File.class,File.class);
...
IoUtil.processDirectory(root,new Callback(this,COUNT),selector);
...
private void callback_count(File dir, File fil) {
if(fil!=null) { // file is null for processing a directory
fileTotal++;
if(fil.length()>fileSizeLimit) {
throw new Abort("Failed","File size exceeds maximum of "+TextUtil.formatNumber(fileSizeLimit)+" bytes: "+fil);
}
}
progress("Counting",dir,fileTotal);
}
IoUtil.processDirectory():
/**
* Process a directory using callbacks. To interrupt, the callback must throw an (unchecked) exception.
* Subdirectories are processed only if the selector is null or selects the directories, and are done
* after the files in any given directory. When the callback is invoked for a directory, the file
* argument is null;
* <p>
* The callback signature is:
* <pre> void callback(File dir, File ent);</pre>
* <p>
* #return The number of files processed.
*/
static public int processDirectory(File dir, Callback cbk, FileSelector sel) {
return _processDirectory(dir,new Callback.WithParms(cbk,2),sel);
}
static private int _processDirectory(File dir, Callback.WithParms cbk, FileSelector sel) {
int cnt=0;
if(!dir.isDirectory()) {
if(sel==null || sel.accept(dir)) { cbk.invoke(dir.getParent(),dir); cnt++; }
}
else {
cbk.invoke(dir,(Object[])null);
File[] lst=(sel==null ? dir.listFiles() : dir.listFiles(sel));
if(lst!=null) {
for(int xa=0; xa<lst.length; xa++) {
File ent=lst[xa];
if(!ent.isDirectory()) {
cbk.invoke(dir,ent);
lst[xa]=null;
cnt++;
}
}
for(int xa=0; xa<lst.length; xa++) {
File ent=lst[xa];
if(ent!=null) { cnt+=_processDirectory(ent,cbk,sel); }
}
}
}
return cnt;
}
This example illustrates the beauty of this approach - the application specific logic is abstracted into the callback, and the drudgery of recursively walking a directory tree is tucked nicely away in a completely reusable static utility method. And we don't have to repeatedly pay the price of defining and implementing an interface for every new use. Of course, the argument for an interface is that it is far more explicit about what to implement (it's enforced, not simply documented) - but in practice I have not found it to be a problem to get the callback definition right.
Defining and implementing an interface is not really so bad (unless you're distributing applets, as I am, where avoiding creating extra classes actually matters), but where this really shines is when you have multiple callbacks in a single class. Not only is being forced to push them each into a separate inner class added overhead in the deployed application, but it's downright tedious to program and all that boiler-plate code is really just "noise".
It doesn't have an explicit delegate keyword as C#, but you can achieve similar in Java 8 by using a functional interface (i.e. any interface with exactly one method) and lambda:
private interface SingleFunc {
void printMe();
}
public static void main(String[] args) {
SingleFunc sf = () -> {
System.out.println("Hello, I am a simple single func.");
};
SingleFunc sfComplex = () -> {
System.out.println("Hello, I am a COMPLEX single func.");
};
delegate(sf);
delegate(sfComplex);
}
private static void delegate(SingleFunc f) {
f.printMe();
}
Every new object of type SingleFunc must implement printMe(), so it is safe to pass it to another method (e.g. delegate(SingleFunc)) to call the printMe() method.
With safety-mirror on the classpath you get something similar to C#'s delegates and events.
Examples from the project's README:
Delegates in Java!
Delegate.With1Param<String, String> greetingsDelegate = new Delegate.With1Param<>();
greetingsDelegate.add(str -> "Hello " + str);
greetingsDelegate.add(str -> "Goodbye " + str);
DelegateInvocationResult<String> invocationResult =
greetingsDelegate.invokeAndAggregateExceptions("Sir");
invocationResult.getFunctionInvocationResults().forEach(funInvRes ->
System.out.println(funInvRes.getResult()));
//prints: "Hello sir" and "Goodbye Sir"
Events
//Create a private Delegate. Make sure it is private so only *you* can invoke it.
private static Delegate.With0Params<String> trimDelegate = new Delegate.With0Params<>();
//Create a public Event using the delegate you just created.
public static Event.With0Params<String> trimEvent= new Event.With0Params<>(trimDelegate)
See also this SO answer.
While it is nowhere nearly as clean, but you could implement something like C# delegates using a Java Proxy.
No, but it has similar behavior, internally.
In C# delegates are used to creates a separate entry point and they work much like a function pointer.
In java there is no thing as function pointer (on a upper look) but internally Java needs to do the same thing in order to achieve these objectives.
For example, creating threads in Java requires a class extending Thread or implementing Runnable, because a class object variable can be used a memory location pointer.
No, Java doesn't have that amazing feature. But you could create it manually using the observer pattern. Here is an example:
Write C# delegate in java
The code described offers many of the advantages of C# delegates. Methods, either static or dynamic, can be treated in a uniform manner. The complexity in calling methods through reflection is reduced and the code is reusable, in the sense of requiring no additional classes in the user code. Note we are calling an alternate convenience version of invoke, where a method with one parameter can be called without creating an object array.Java code below:
class Class1 {
public void show(String s) { System.out.println(s); }
}
class Class2 {
public void display(String s) { System.out.println(s); }
}
// allows static method as well
class Class3 {
public static void staticDisplay(String s) { System.out.println(s); }
}
public class TestDelegate {
public static final Class[] OUTPUT_ARGS = { String.class };
public final Delegator DO_SHOW = new Delegator(OUTPUT_ARGS,Void.TYPE);
public void main(String[] args) {
Delegate[] items = new Delegate[3];
items[0] = DO_SHOW .build(new Class1(),"show,);
items[1] = DO_SHOW.build (new Class2(),"display");
items[2] = DO_SHOW.build(Class3.class, "staticDisplay");
for(int i = 0; i < items.length; i++) {
items[i].invoke("Hello World");
}
}
}
Java doesn't have delegates and is proud of it :). From what I read here I found in essence 2 ways to fake delegates:
1. reflection;
2. inner class
Reflections are slooooow! Inner class does not cover the simplest use-case: sort function. Do not want to go into details, but the solution with inner class basically is to create a wrapper class for an array of integers to be sorted in ascending order and an class for an array of integers to be sorted in descending order.