Greetings,
I am trying to set up a pattern where a class definition must include an internal interface definition. So for instance if someone on my team were to define a class is it possible via an interface or base class to ensure that they have to declare and define an internal interface like below? I am not concerned with the specific definition of the interface, like what methods they declare, but that an interface named Display was defined at all.
public class Foo {
public interface Display {
void Bar();
}
public void SomeMethod() {
}
}
I appreciate any insight or pointers to documentation on this subject.
No way you can do this kind of static check.
Sometimes when you just hear a question you can literally smell bad design. That is exactly that type of question.
Related
I have just found a static nested interface in our code-base.
class Foo {
public static interface Bar {
/* snip */
}
/* snip */
}
I have never seen this before. The original developer is out of reach. Therefore I have to ask SO:
What are the semantics behind a static interface? What would change, if I remove the static? Why would anyone do this?
The static keyword in the above example is redundant (a nested interface is automatically "static") and can be removed with no effect on semantics; I would recommend it be removed. The same goes for "public" on interface methods and "public final" on interface fields - the modifiers are redundant and just add clutter to the source code.
Either way, the developer is simply declaring an interface named Foo.Bar. There is no further association with the enclosing class, except that code which cannot access Foo will not be able to access Foo.Bar either. (From source code - bytecode or reflection can access Foo.Bar even if Foo is package-private!)
It is acceptable style to create a nested interface this way if you expect it to be used only from the outer class, so that you do not create a new top-level name. For example:
public class Foo {
public interface Bar {
void callback();
}
public static void registerCallback(Bar bar) {...}
}
// ...elsewhere...
Foo.registerCallback(new Foo.Bar() {
public void callback() {...}
});
The question has been answered, but one good reason to use a nested interface is if its function is directly related to the class it is in. A good example of this is a Listener. If you had a class Foo and you wanted other classes to be able to listen for events on it, you could declare an interface named FooListener, which is ok, but it would probably be more clear to declare a nested interface and have those other classes implement Foo.Listener (a nested class Foo.Event isn't bad along with this).
Member interfaces are implicitly static. The static modifier in your example can be removed without changing the semantics of the code. See also the the Java Language Specification 8.5.1. Static Member Type Declarations
An inner interface has to be static in order to be accessed. The interface isn't associated with instances of the class, but with the class itself, so it would be accessed with Foo.Bar, like so:
public class Baz implements Foo.Bar {
...
}
In most ways, this isn't different from a static inner class.
Jesse's answer is close, but I think that there is a better code to demonstrate why an inner interface may be useful. Look at the code below before you read on. Can you find why the inner interface is useful? The answer is that class DoSomethingAlready can be instantiated with any class that implements A and C; not just the concrete class Zoo. Of course, this can be achieved even if AC is not inner, but imagine concatenating longer names (not just A and C), and doing this for other combinations (say, A and B, C and B, etc.) and you easily see how things go out of control. Not to mention that people reviewing your source tree will be overwhelmed by interfaces that are meaningful only in one class.So to summarize, an inner interface enables the construction of custom types and improves their encapsulation.
class ConcreteA implements A {
:
}
class ConcreteB implements B {
:
}
class ConcreteC implements C {
:
}
class Zoo implements A, C {
:
}
class DoSomethingAlready {
interface AC extends A, C { }
private final AC ac;
DoSomethingAlready(AC ac) {
this.ac = ac;
}
}
To answer your question very directly, look at Map.Entry.
Map.Entry
also this may be useful
Static Nested Inerfaces blog Entry
Typically I see static inner classes. Static inner classes cannot reference the containing classes wherease non-static classes can. Unless you're running into some package collisions (there already is an interface called Bar in the same package as Foo) I think I'd make it it's own file. It could also be a design decision to enforce the logical connection between Foo and Bar. Perhaps the author intended Bar to only be used with Foo (though a static inner interface won't enforce this, just a logical connection)
If you will change class Foo into interface Foo the "public" keyword in the above example will be also redundant as well because
interface defined inside another interface will implicitly public
static.
In 1998, Philip Wadler suggested a difference between static interfaces and non-static interfaces.
So far as I can see, the only difference in making an
interface non-static is that it can now include non-static inner
classes; so the change would not render invalid any existing Java
programs.
For example, he proposed a solution to the Expression Problem, which is the mismatch between expression as "how much can your language express" on the one hand and expression as "the terms you are trying to represent in your language" on the other hand.
An example of the difference between static and non-static nested interfaces can be seen in his sample code:
// This code does NOT compile
class LangF<This extends LangF<This>> {
interface Visitor<R> {
public R forNum(int n);
}
interface Exp {
// since Exp is non-static, it can refer to the type bound to This
public <R> R visit(This.Visitor<R> v);
}
}
His suggestion never made it in Java 1.5.0. Hence, all other answers are correct: there is no difference to static and non-static nested interfaces.
In Java, the static interface/class allows the interface/class to be used like a top-level class, that is, it can be declared by other classes. So, you can do:
class Bob
{
void FuncA ()
{
Foo.Bar foobar;
}
}
Without the static, the above would fail to compile. The advantage to this is that you don't need a new source file just to declare the interface. It also visually associates the interface Bar to the class Foo since you have to write Foo.Bar and implies that the Foo class does something with instances of Foo.Bar.
A description of class types in Java.
Static means that any class part of the package(project) can acces it without using a pointer. This can be usefull or hindering depending on the situation.
The perfect example of the usefullnes of "static" methods is the Math class. All methods in Math are static. This means you don't have to go out of your way, make a new instance, declare variables and store them in even more variables, you can just enter your data and get a result.
Static isn't always that usefull. If you're doing case-comparison for instance, you might want to store data in several different ways. You can't create three static methods with identical signatures. You need 3 different instances, non-static, and then you can and compare, caus if it's static, the data won't change along with the input.
Static methods are good for one-time returns and quick calculations or easy obtained data.
As I looked at many of the interface answers from questions here, and on Google and on this video class tutorial I am looking at I have a question. I am asking here because I can't comment if my reputation is not high so hopefully this is not to redundant. I am understanding that interfaces is like psuedocode but with more of an actual way to implement your psuedocode into the program. I undertsand
public Interface someInterface{
public void doSomething();
}
is like saying we need that function in our program so lets make this interface so when we do this
public class imDoingSomething implements someInterface{ // looking at the implements someInterface
#Override // optional
public void doSomething(){
System.out.println("Doing Something");
}
}
it makes sure as I write my program I don't forget to write this function for it is vital to my program. Is this correct?
In your example you have correctly implemented an interface. An interface can be viewed as a contract that a class must fulfill. Knowing that the class has met the requirements specified by an interface allows the object to used as the interfaces type by client code and guarantees particular methods will exist with a specified signature. This can make code more abstract and reusable for a variety of types.
So if we have an interface Playable:
public interface Play{
public void play();
}
And two classes implementing Playable:
public class Record implements Playable{
public void play(){
System.out.println("Playing Record");
}
}
public class MP3 implements Playable{
public void play(){
System.out.println("Playing MP3");
}
}
They can be used in an abstract manner by a client because it knows all classes implementing Playable have a play method:
public class Application{
List<Playable> audioFiles = new ArrayList<Playable>();
public static void main(String[] args){
audioFiles.add(new Record());
audioFiles.add(new MP3());
for(Playable p: audioFiles){
play(p);
}
}
public static void play(Playable playable){
playable.play();
}
}
On a side note
Follow Java naming standards when creating classes or interfaces. In Java these types use a capital letter for each word in the name. So your example would have a SomeInterface interface and a ImDoingSomething class.
It's more easy if you see interfaces from a consumer perspective - when you have a class which uses other objects and does not care about how these objects are concretely defined but only how these objects should behave, one creates an interface providing the methods and using it internally - and everyone which wants to use this certain class has to provide access to his data through implementing the interface so that the class knows how access everything on a code level.
An interface is a collection of abstract methods[No defination]. A class implements an interface, thereby inheriting the abstract methods of the interface.With interfaces, all fields are automatically public, static, and final, and all methods that you declare or define (as default methods) are public.
You can not instantiate an interface - you can instantiate one of their subclasses/implementers.
Examples of such a thing are typical in the use of Java Collections.
List<String> stringList = new ArrayList<String>();
List is interface but the instance itself is an ArrayList
Interfaces are a way of enforcing design restrictions. By declaring the type of a variable or parameter as an interface, you're sure that the instance referenced by that variable or parameter is going to have an implementation for every method of the interface. That's the basis of polymorphism.
I know few differences between abstract class and concrete class. I know that you can't create an instance with abstract class unlike concrete class, abstract class can have 'abstract' methods.
But i have an example like the following. A lot of times, we see the following examples at work. I will just skip some common methods that can be defined in the Parent class.
public abstract class Parent {
public void init() {
doInit();
}
public abstract void doInit();
}
public class Child extends Parent {
public void doInit() {
// implementation
}
}
I think that we can do the same thing with a concrete class like the following.
public class Parent {
public void init() {
doInit();
}
public void doInit() {
// Empty
}
}
I am curious to see if there is any unique situation that we have to use abstract class. Is there any significant difference during runtime with the above example?
Thank you.
The reason to use abstract class in this situation is to force everyone inheriting your base class to override the abstract doInit method. Without the class and the method being abstract, they may forget to do so, and the compiler would not catch them.
In addition to this pragmatic purpose, abstract classes provide a powerful way to communicate your design idea to the readers of your code. An abstract class tells the reader that the methods inside provide some common implementation for a group of related classes, rather than implementing a single concept that you are modeling. Very often communicating your intent to your readers is as important as it is to write correct code, because otherwise they might break something while maintaining your code.
It is customary in Java to call abstract classes Abstract...; in your example that would be AbstractParent.
Of course you can do it that way, but it all depends on the right business logic.There might arise a situation where you'd want to enforce a policy on people extending your code.
For example, I write an Employee class and you extend my class for writing a ProjectManager class. But suppose the business does not allow direct instantiation of Employee (like I said, just an example). So I declare my Employee class as abstract, thereby enforcing upon all extenders (read:you) of my class the rule that they can't instantiate Employee directly. (It will happen indirectly through the inheritance chain, of course, i.e. parent objects are created before child objects.)
Used properly, a person at place A controls how another person at place B will code.
A concrete class is one which has implementation (code inside) for all the methods. It does not matter whether it is derived from some other class.
public abstract class IAmAbstract{
public void writeMe(){
System.out.println("I am done with writing");
}
}
public class IAmConcrete extends IAmAbstract{
public void writeMe(){
System.out.println("I am still writing");
}
}
Abstract classes have a variety of useful properties in use with software design.
Other than the obvious differences, such as being unable to be instantiated and being able to hold abstract methods. They are useful for defining common, yet overridable, functions, holding static methods that deal with it's children in a logical manner.
My favorite is the abstract factory pattern though.
By making a factory that is the parent of all the classes it may create, it can force functionality required for creation, this actually causes an odd artefact where technically tighter-coupled code is actually easier to maintain.
It's been a few years since I've been heavily into Java. Coming back to it I'm seeing this pattern all over the place:
ratingBar.setOnRatingBarChangeListener(new OnRatingBarChangeListener() {
public void onRatingChanged(RatingBar ratingBar, float rating, boolean fromUser) {
// do work
}
});
This looks more like Functional programming to me. It's a nice pattern but how is it possible to pass a method like this? In the old days a class was a class, and once compiled there was little you could do to it.
My questions are:
Can anyone let me know what this pattern is called?
How can I write a class that can be instantiated in this way.
Are there any other useful examples of functional patterns that have made their way into Java?
What do I need to Google to read more about this?
Thanks.
This passes an anonymous class, not a method.
This is a typical pattern, not just in Swing programming, but anywhere you need (hopefully) short, "throw-away" implementations of an interface or class that doesn't need to be re-used, instead of creating a full-blown implementation.
Any class/interface can be instantiated like this, there's nothing special about it:
public interface Foo {
String foo();
}
...
public class Main {
public static void main(String[] args) {
System.out.println(new Foo() {
public String foo() {
return "plugh";
}
});
}
}
Anonymous inner classes get their own class files, too, even though their source is embedded.
In this example, a Main$1.class file will be generated for the anonymous inner class, in addition to the expected Main.class file.
The statement: new OnRatingBarChangeListener() creates a new instance of a class. The following part inside the curly braces is the definition of the class.
In this case that class in an anonymous class that implements the named interface.
Anonymous classes are classes, that are declared without a name, and thus, can not be used like regular named classes.
This pattern is very common when using listeners, that often contain only a single to a few methods that do an almost trivial task.
This is the Listener pattern. Rating bar takes an implementation of OnRatingBarChangeListener and calls its onRatingChanged method on the appropriate event.
You can use instance of any class which implements OnRatingBarChangeListener. So you can use either a named class of your own or you can pass it an anonymous class like in the example. The anonymous class in the example is effectively a unnamed class which extends Object and implements OnRatingBarChangeListener. Since the class isn't named it cannot be referenced and so the instance passed is the only instance existing.
This is called "Observer pattern". A good example for this is adding action listeners for java button or other component. For example,
myButton.addActionListener(
new java.awt.event.ActionListener()
{
public void actionPerformed(ActionEvent e)
{
//Work here
}
});
In here "myButton" is the subject and ActionListener is the observer.
Is it possible to have an inner class inside the interface in java ???
You can. But here's what O'Reilly says about it:
Nested Classes in Interfaces?
Java supports the concept of nested classes in interfaces. The syntax and dynamics work just like nested classes declared in a class. However, declaring a class nested inside an interface would be extremely bad programming. An interface is an abstraction of a concept, not an implementation of one. Therefore, implementation details should be left out of interfaces. Remember, just because you can cut off your hand with a saw doesn't mean that it's a particularly good idea.
That said, I could see an argument for a static utility class nested into an interface. Though why it would need to be nested into the interface instead of being a stand-alone class is completely subjective.
I agree that this should be generally rare, but I do like to use inner classes in interfaces for services when the interface method needs to return multiple pieces of information, as it's really part of the contract and not the implementation. For example:
public interface ComplexOperationService {
ComplexOperationResponse doComplexOperation( String param1, Object param2 );
public static class ComplexOperationResponse {
public int completionCode;
public String completionMessage;
public List<Object> data;
// Or use private members & getters if you like...
}
}
Obviously this could be done in a separate class as well, but to me it feels like I'm keeping the whole API defined by the interface in one spot, rather than spread out.
Yes, it is possible but it is not common practice.
interface Test
{
class Inner
{ }
}
class TestImpl implements Test
{
public static void main(String[] arg)
{
Inner inner = new Inner();
}
}
Doesn't answer your question directly, but on a related note you can also nest an interface inside another interface. This is acceptable, especially if you want to provide views. Java's collection classes do this, for example Map.java in the case of the Map.Entry view:
public interface Map<K,V> {
...
public static interface Entry<K,V> {
....
}
}
This is acceptable because you're not mixing implementation details into your interface. You're only specifying another contract.
Yes. Straight from the language spec:
An inner class is a nested class that is not explicitly or implicitly declared static.
And (boldface mine):
A nested class is any class whose declaration occurs within the body of another class or interface.
One use case for this that I find quite useful is if you have a builder that creates an instance of the Interface. If the builder is a static member of the Interface, you can create an instance like this:
DigitalObject o = new DigitalObject.Builder(content).title(name).build();
It is legal, but I only really do it with nested interfaces (as already mentioned) or nested enums. For example:
public interface MyInterface {
public enum Type { ONE, TWO, THREE }
public Type getType();
public enum Status { GOOD, BAD, UNKNOWN }
public Status getStatus();
}