I have been studying abstract methods lately and I can't understand why do we need them?
I mean, after all, we are just overriding them. Do you know its just a declaration? Why do we need them?
Also, I tried understanding this from the internet and everywhere there's an explanation like imagine there's an abstract class human then there're its subclasses disabled and not disabled then the abstract function in human class walking() will contain different body or code. Now what I am saying is why don't we just create a function in the disabled and not disabled subclasses instead of overriding. Thus again back to the question in the first paragraph. Please explain it.
One of the most obvious uses of abstract methods is letting the abstract class call them from an implementation of other methods.
Here is an example:
class AbstractToy {
protected abstract String getName();
protected abstract String getSize();
public String getDescription() {
return "This is a really "+getSize()+" "+getName();
}
}
class ToyBear extends AbstractToy {
protected override String getName() { return "bear"; }
protected override String getSize() { return "big"; }
}
class ToyPenguin extends AbstractToy {
protected override String getName() { return "penguin"; }
protected override String getSize() { return "tiny"; }
}
Note how AbstractToy's implementation of getDescription is able to call getName and getSize, even though the definitions are in the subclasses. This is an instance of a well-known design pattern called Template Method.
The abstract method definition in a base type is a contract that guarantees that every concrete implementation of that type will have an implementation of that method.
Without it, the compiler wouldn't allow you to call that method on a reference of the base-type, because it couldn't guarantee that such a method will always be there.
So if you have
MyBaseClass x = getAnInstance();
x.doTheThing();
and MyBaseClass doesn't have a doTheThing method, then the compiler will tell you that it can't let you do that. By adding an abstract doTheThing method you guarantee that every concrete implementation that getAnInstance() can return has an implementation, which is good enough for the compiler, so it'll let you call that method.
Basically a more fundamental truth, that needs to be groked first is this:
You will have instances where the type of the variable is more general than the type of the value it holds. In simple cases you can just make the variable be the specific type:
MyDerivedClassA a = new MyDerivcedClassA();
In that case you could obviously call any method of MyDerivedClassA and wouldn't need any abstract methods in the base class.
But sometimes you want to do a thing with any MyBaseClass instance and you don't know what specific type it is:
public void doTheThingsForAll(Collection<? extends MyBaseClass> baseClassReferences) {
for (MyBaseClass myBaseReference : baseClassReferences) {
myBaseReference.doTheThing();
}
}
If your MyBaseClass didn't have the doTheThing abstract method, then the compiler wouldn't let you do that.
To continue with your example, at some point you might have a List of humans, and you don't really care whether they are disabled or not, all you care about is that you want to call the walking() method on them. In order to do that, the Human class needs to define a walking() method. However, you might not know how to implement that without knowing whether the human is or isn't disabled. So you leave the implementation to the inheriting classes.
There are some examples of how you'd use this in the other answers, so let me give some explanation of why you might do this.
First, one common rule of Object Oriented Design is that you should, in general, try to program to interfaces rather than specific implementations. This tends to improve the program's flexibility and maintainability if you need to change some behavior later. For example, in one program I wrote, we were writing data to CSV files. We later decided to switch to writing to Excel files instead. Programming to interfaces (rather than a specific implementation) made it a lot easier for us to make this change because we could just "drop in" a new class to write to Excel files in place of the class to write to CSV files.
You probably haven't studied this yet, but this is actually important for certain design patterns. A few notable examples of where this is potentially helpful are the Factory Pattern, the Strategy Pattern, and the State Pattern.
For context, a Design Pattern is a standard way of describing and documenting a solution to a known problem. If, for example, someone says "you should use the strategy pattern to solve this problem," this makes the general idea of how you should approach the problem clear.
Because sometimes we need a method that should behave differently in its instances.
For example, imagine a class Animal which contains a method Shout.
We are going to have different instances of this Animal class but we need to implement the method differently in some cases like below:
class Animal:
/**
different properties and methods
which are shared between all animals here
*/
...
method shout():
pass
class Dog extends Animal:
method shout():
makeDogVoice()
class Cat extends Animal:
method shout():
makeCatVoice()
dog = new Animal
cat = new Animal
dog.shout()
cat.shout()
So dog shouts like dogs, and cat shouts like cats! Without implementing the shared behaviors twice
There is a different behavior of shouting in these instances. So we need abstract classes.
Suppose you don't know about implementation and still want to declare a method then we can do that with the help of abstract modifier and making it an abstract method. For abstract method only declaration is available but not the implementation. Hence they should end with ;
Example:
public abstract void m1(); // this is correct
public abstract void m1(){ ... } // this is wrong
Advantage: By declaring abstract method in parent class we can provide guideline to child classes such that which methods are compulsory to implement.
Example:
abstract class Vehicle{
abstract int getNoOfWheels();
}
Class Bus extends Car{
public int getNoOfWheels(){
return 4;
}
}
If you want the short answer, think of this:
You have an abstract class Car.
You implement 2 classes that extend it, Ferrari and Mercedes.
Now:
What if you did one of the following, for the method drive(), common to all cars:
1) changed the visibility of the method,
2) changed the name of the method from driving to Driving,
3) changed the return type, from a boolean to an int
Think about it. It might not seem to make any difference right, because they are different implementations?
Wrong!
If I am iterating through an array of cars, I would have to call a different method for each type of car, thereby making this implementation of abstract useless.
Abstract classes are there to group classes with a common template, that share common properties. One way this helps would be the looping over the array:
Abstract methods ensure that all cars declare the same method,
and therefore, any object of a subclass of Car will have the method drive(), as defined in the abstract class, making the for loop mentioned easy to implement.
Hope this helps.
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.
I am studying Design Patters and I have a situation where I am not sure what would be a better practice:
I have a class "Category" which has several fields: name, 2 kinds of urls, list of related objects. There is a method 'toHtml()' which basically generates some HTML from instances of that class.
There are 4 different types of 'Categories' which have exactly the same fields but 'toHtml()' method should give different result for each one.
I am not sure if I should pass a parameter "type" and series of ifs/switch statement to generate different html or should I make a Category class abstract and create several sub-classes that override the toHtml() method and then use CategoryFactory class to generate them? In both cases I need to pass 'type' parameter.
I tried to think about 'Close for modification, open for extension' OOP rule. But in this case if I want to add 'fifth' category type, that generates different html - for first solution I need to modify only toHtml method (adding one more if), going for second solution I need to create additional sub-class AND modify CategoryFactory class.
What would be better practice? Is there any extra rule I should follow when I have similar kind of dilemma?
First, I believe you are referring to the Factory Method, and not the Abstract Factory Pattern.
The main difference being, in the former you define a common template for a single product, whereas in the latter you define a template for a family of products. For more information, you could look here.
In your case, you wish to define a template for Category. With this assumption, here is what your group of classes would look like:
abstract class Category {
public void doSomething() {
Foo f = makeFoo();
f.whatever();
}
abstract void toHtml();
}
class Category1 extends Category {
public override void toHtml() {
... // do something here
}
}
class Category2 extends Category {
public override void toHtml() {
... // do something else here
}
}
It is true that this certainly a lot of code, and could easily be represented like this:
class Category {
public void toHtml(Integer param) {
if(param == 1) { // do something for Category1
}
else { // do something for Category2
}
}
At the end of the day, it really is a design decision. There are some factors you can consider. Is this going to be a constantly changing class? Is this going to be declared global for the customer to use? How do you want the customer to be able to use this?
The easier thing at this point would be to take the path of least resistance. Having one class to service all categories certainly results in lesser code and in Salesforce, less code is always a better thing. But consider this: Abstracting your functionality into separate classes makes for more maintainable code. You may find it easier to write a class and a wall of if statements, but tomorrow when you're not around and there's a critical failure and someone has to look through your code to figure out exactly which if caused the problem, they'll curse you for it.
Keep in mind that inheritance is an all or nothing mechanism. You may find it particularly useful to use if you have some common functionality, in which case you can choose to abstract that out into the parent class and have your children take care of the specifics.
If you create a subclass of Category and override the toHtml () method, why do you need to have a factory pattern. The toHtml () method of the runtime resolved class will be called if you are calling it using the reference. This implies that if you add a new Category subclass then you override the toHtml () method and it should work fine.
I am reading a Java book and stuck again this time thinking about what this whole paragraph actually means:
Interfaces are designed to support dynamic method resolution at run time. Normally, in order for a method to be called from one class to another, both classes need to be present at compile time so the Java compiler can check to ensure that the method signatures are compatible. This requirement by itself makes for a static and nonextensible classing environment. Inevitably in a system like this, functionality gets pushed up higher and higher in the class hierarchy so that the mechanisms will be available to more and more subclasses. Interfaces are designed to avoid this problem. They disconnect the definition of a method or set of methods from the inheritance hierarchy. Since interfaces are in a different hierarchy from classes, it is possible for classes that are unrelated in terms of the class hierarchy to implement the same interface. This is where the real power of interfaces is realized.
First question: what does the author mean by saying from one class to another? Does he mean that those classes are related in terms of the hierarchy? I mean, assigning subclass object reference to its superclass type variable and then calling a method?
Second question: what does the author again mean by saying This requirement by itself makes for a static and nonextensible classing environment? I don't understand the makes for meaning (english is not my main language) and why the environment is called static and nonextensible.
Third question: what does he mean by saying functionality gets pushed up higher and higher? Why does it get pushed up higher and higher? What functionality? Also, mechanisms will be available to more and more subclasses. What mechanisms? Methods?
Fourth question: Interfaces are designed to avoid this problem. What problem???
I know the answers must be obvious but I don't know them. Maybe mainly because I don't undestand some magic english phrases. Please help me to understand what is this whole paragraph telling.
Between any two classes. If your code contains a call to String.substring() for example, the String class and its substring() method must be available at compile time.
As said, "makes for" means the same as "creates". The environment is non-extensible because everything you may want to use must be available at compile time. (This isn't 100% true though. Abstract classes and methods provide extension points even when no interfaces are present, but they aren't very flexible as we're going to see.)
Imagine that you have two classes: Foo and Bar. Both classes extend the class Thingy. But then you want to add a new functionality, let's say you want to display both in HTML on a web page. So you add a method to both that does that.
The basic problem
abstract class Thingy { ... }
class Foo extends Thingy {
...
public String toHTMLString() {
...
}
}
class Bar extends Thingy {
...
public String toHTMLString() {
...
}
}
This is great but how do you call this method?
public String createWebPage( Thingy th ) {
...
if (th instanceof Foo)
return ((Foo)th).toHTMLString();
if (th instanceof Bar)
return ((Bar)th).toHTMLString();
...
}
Clearly this way isn't flexible at all. So what can you do? Well, you can push toHTMLString() up into their common ancestor, Thingy. (And this is what the book is talking about.)
A naive attempt to resolve it
abstract class Thingy {
...
public abstract String toHTMLString();
}
class Foo extends Thingy {
...
public String toHTMLString() {
...
}
}
class Bar extends Thingy {
...
public String toHTMLString() {
...
}
}
And then you can call it like this:
public String createWebPage( Thingy th ) {
...
return th.toHTMLString();
}
Success! Except now you've forced every class extending Thingy to implement a toHTMLString() method, even if it doesn't make sense for some of them. Even worse, what if the two objects do not extend anything explicitly, they're completely unrelated? You'd have to push the method up all the way into their common ancestor, which is java.lang.Object. And you can't do that.
Solution with interfaces
So what can we do with interfaces?
abstract class Thingy { ... }
interface HTMLPrintable {
public String toHTMLString();
}
class Foo extends Thingy implements HTMLPrintable {
...
public String toHTMLString() {
...
}
}
class Bar extends Thingy implements HTMLPrintable {
...
public String toHTMLString() {
...
}
}
//We've added another class that isn't related to all of the above but is still HTMLPrintable,
//with interfaces we can do this.
class NotEvenAThingy implements HTMLPrintable {
public String toHTMLString() {
...
}
}
And the calling code will be simply
public String createWebPage( HTMLPrintable th ) {
...
return th.toHTMLString(); // "implements HTMLPrintable" guarantees that this method exists
}
What are interfaces then?
There are many metaphors used to understand interfaces, the most popular is probably the idea of a contract. What it says to the caller is this: "If you need X done, we'll get it done. Don't worry about how, that's not your problem." (Although the word "contract" is often used in a more general sense, so be careful.)
Or in another way: if you want to buy a newspaper, you don't care if it's sold in a supermarket, a newsagents or a small stall in the street, you just want to buy a newspaper. So NewspaperVendor in this case is an interface with one method: sellNewsPaper(). And if someone later decides to sell newspaper online or door-to-door, all they need to do is implement the interface and people will buy from them.
But my favourite example is the little sticker in shop windows that says "we accept X,Y and Z credit cards". That's the purest real-world example of an interface. The shops could sell anything (they may not even be shops, some might be restaurants), the card readers they use are different too. But you don't care about all of that, you look at the sign and you know you can pay with your card there.
The Key to paragraph is "classes need to be present at compile time" in line 2. Classes are more concrete. While interfaces are abstract.
As classes are concrete so Designer and programmer needs to know all about class structure and how the methods are implemented. Where as interfaces are more abstract. (They contain abstract methods only). So programmer needs to know only what methods an interface has to have and signature of those methods. He does not need to know detail how these are implemented.
Thus using interfaces is easier and better while making subclasses. You only need to know method signatures of interface.
Using concrete class we have to implement functionality of a method high in class hierarchy while using interface avoids this problem. (There is a related concept of polymorphism that you would probably learn later)
I was thinking about programming to interfaces and not to concrete classes, but I had a doubt: should any interface method be able to hold references to concrete classes?
Suppose the following scenarios:
1)
public interface AbsType1 {
public boolean method1(int a); // it's ok, only primitive types here
}
2)
public interface AbsType2 {
public boolean method2(MyClass a); // I think I have some coupling here
}
Should I choose a different design here in order to avoid the latter? e.g.
public interface MyInterface {} // yes, this is empty
public classe MyClass implements MyInterface {
// basically identical to the previous "MyClass"
}
public interface AbsType2 {
public boolean method2(MyInterface a); // this is better (as long as the
// interface is really stable)
}
But there's still something that doesn't convince me... I feel uncomfortable with declaring an empty interface, though I saw someone else doing so.
Maybe and Abstract Class would work better here?
I am a little bit confused.
EDIT:
Ok, I'll try to be more specific by making an example. Let's say I'm desining a ShopCart and I want of course to add items to the cart:
public interface ShopCart {
public void addArticle(Article a);
}
Now, if Article were a concrete class, what if its implementation changes over time? This is why I could think of making it an Interface, but then again, it's probably not suitable at least at a semantic level because interfaces should specify behaviours and an Article has none (or almost none... I guess it's a sort of entity class).
So, probably I'm ending up right now to the conclusion that making Article an abstract class in this case would be the best thing... what do you think about it?
I would use interfaces because composition is much better than inheritance. "Should any interface method be able to hold references to concrete classes ?", why it shouldn't? Some classes within package are coupled, it's a fact and common use technique. When you marked this relation in interface then you see on which classes is dependent your implementation. Dependency or composition relations are not inheritance so a i would avoid abstract class.
In my opinion Interfaces are fine for all types where the implementation may vary. But if you define a module which introduces a new type, that isn't intended to have alternative implementations then there is no need to define it as an Interface in the first place. Often this would be over-design in my opinion. It depends on the problem domain and often on the way how support testing or AOP-weaving.
For example consider a 2D problem domain where you need to model a Location as a type. If it is clear that a Location is always represented by a x and y coordinate, you may provide it as a Class. But if you do not know which properties a Location could have (GPS data, x, y, z coordinates, etc.) but you rely on some behavior like distance(), you should model it as an Interface instead.
If there are no public methods which AbsType would access in MyClass then the empty interface is probably not a good way to go.
There is no interface declaration (contract) for static methods, which otherwise might make sense here.
So, if AbsType is not going to use any methods from MyClass/MyInterface, then I assume it's basically only storing the class object for some other purpose. In this case, consider using generics to make clear how you want AbsType to be used without coupling closely to the client's code, like
public class AbsType3<C extends Class<?>> {
public boolean method3(T classType) {...}
}
Then you can restrict the types of classes to allow if needed by exchanging the <C extends Class<?>> type parameter for something else which may also be an interface, like
<C extends Class<Collection<?>>>.
Empty interfaces are somewhat like boolean flags for classes: Either a class implements the interface (true) or it doesn't (false). If at all, these marker interfaces should be used to convey an significant statement about how a class is meant to be (or not to be) used, see Serializable for example.