There are several classes (which I can't edit) which implement an interface, is it possible to add a method that only calls methods defined in the interface to each class that implements that interface?
(without java 8)
No, I don't believe so. Such a method would have to realized in the concrete class as a real method, but without access to the classes themselves you can't do that.
You might create subclasses of each, and all those subclasses could implement you new interface with the new method.
I'm not sure if I get you right but if classes A, B, C, ... implement an interface I and you can not edit A, B, C, ... and want to add a method you propably could just extend each class with a new subclass and each subA, subB, subC and so on could implement the new method...
If you are using Java 8 and can modify the interface, then yes:
interface Face {
Object existing();
default Object additional() {
return "Hello " + existing() + "!";
}
}
Otherwise, no, you cannot. Java does not have a feature like Javascript prototypes, C# extension methods, etc at this time.
Just write a static method that takes in the interface type and calls the methods:
interface Fooable
{
void foo();
}
public class FooableExtensions
{
public static void doSomethingToFooable( Fooable f /*, other parameters*/ )
{
f.foo();
}
}
It doesn't have the fancy syntactic-sugar that C# extension methods provide, but it should do the trick.
I don't think there is a way to do what you are asking, not without resorting to byte code manipulation anyway.
Perhaps AOP would help, you could add a point cut to the target methods of the interface.
Related
Lets say I have class like:
class A
{
}
by any means I can make the class implements an interface on the runtime?
This is what I'm trying to achieve, when some one creates object of class A, I need to intercept the calls to that object.
Very new to Java, thanks.
A class cannot be made to implement an interface at runtime. The best that can be done at runtime is creating a dynamic subclass of your class, which additionally implements an interface.
By the Liskov Substitution Principle this solution will work quite well because any code written against your type A will also work against its subtypes. Also, any code written against the interface you are implementing will also work and be able to access the behavior implemented in your class A, to the extent to which this behavior is reflected through the behavior of the interface's methods.
You can do this with Instrumentation, however I wouldn't do this unless you know Java AND Byte Code very well and there really isn't another option.
A better option is to use composition or inheritance
class A {
}
class B extend A implement I {
// B is an A and implements I
}
A a = new B();
class C implement I {
A a;
}
Either B or C implement your interface without having to change A.
You can use a Proxy object to adapt any object at run time to make it appear as if it implements an interface.
There is a nice article here that discusses a factory method for doing this to objects.
Essentially, a Proxy object can be used to wrap an object by intercepting all method calls to the object and redirecting them dynamically.
This should be solvable with an adapter. Have an other class defined that implements your interface and delegates to the real object:
class YourAdapter implements YourInterface {
private final YourClass realObject;
public YourAdapter(YourClass realObject) {
this.realObject = realObject;
}
#Override
public methodFromInterface() {
realObject.methodFromInterface();
}
// .......
}
Now, given a method that expects YourInterface and an object of type YourClass:
void someMethod(YourInterface param) {}
void test() {
YourClass object = getFromSomewhere();
someMethod( YourAdapter(object) );
}
2nd way :
Using Proxy class :
Refer this link.
Example of dynamically implement an interface using Dynamic Proxy
Can Someone Explain how the methods of interface used in classes?
Note: My Doubt is "Methods are already defined in Class then why we should implement it ? "
For Example :
interface printable{
void print();
}
class A implements printable{
public void print(){System.out.println("Hello");}
public static void main(String args[]){
A obj = new A();
obj.print();
}
}
why print() is declared in interface??
You define a method by giving its implementation. They are the same thing, so you are right that once you define a method, you don't also need to implement it.
An interface declares that anything implementing this interface will defined those methods. This is part of the contract for interfaces. This allows you to call any method of an interface knowing than any concrete implementation will have such a method.
BTW In Java 8, it will support virtual extensions which means an interface can give a default implementation. This has to be defined in terms of other methods provided by the interface.
An Interface is a contract that all classes that implement it, should have a definition for the methods specified in the interface. An interface does not define the method body as such.
An interface defines a set of method which must be implemented. It says nothing on how they are implemented. This is where the class definition comes in, since it defines how these methods are implemented.
Thus, when you call a class which implements a particular interface, then you know, for sure, that you will find whatever set of methods the interface defines.
Interfaces are usually handy when you need to expose some endpoints to your application, without the need to expose the logic.
EDIT: As per your example, the printable interface defines what behaviour should a class which implements it expose, in this case print.
This will allow you to do something along the lines of printable p = new A(); p.print();.
Assuming you have something which yields an object which implements the printable interface, then, whoever is calling that method will not need to bother what is the actual implementation of the print method. The interface makes sure that whatever you are returning, will contain an implementation of that method.
#NarutoUzumaki
Welcome to Stack overflow!
I agree with Chris. You can replace the doSomething method with eat() method to get a better understanding. A dog may eat something different than a cat and to a giraffe.
Its up to you how you implement the eat method, and when using it create a reference of the interface Animal and point it to the instance of Dog, Cat or Giraffe which ever eat method you want to use. This makes your class design very extensible.
Hope you get a clear idea now.
Generally Interface based Programming is recommended, Because of the following reasons
1)Interface means rule , you should follow those rules while implementing those methods in Implemented class.
2) Dependency is less between classes while instancing your implemented class then call your methods from another class or some where.
3) You can publish your interface details only no need to disclose the implemented details of your methods to out side the world.
Defining an interface is the difference between:
public void doSomething(Dog d)
{
d.doSomething();
}
public void doSomething(Cat c)
{
c.doSomething();
}
public void doSomething(Giraffe g)
{
g.doSomething();
}
and
public void doSomething(Animal a)
{
a.doSomething();
}
Why?
Well, if all the classes just implement their own methods, there's no common reference between them. However, if they all implement the method from a common interface, they can be referred to by the same reference type; in this case Animal.
I have two classes A and B which both implment the interface Z. Now, class A should for some functions of Interface Z (Z.f1, Z.f2, Z.f3, ...) only work as dispatcher to an object of class B.
public class A implements Z{
private B b; //instantiated in constructor of A
#Override
public String f1(int p)
{
return b.f1(p);
}
...
Is there a generic way to do this in Java?
If you mean that method f1() is declared in interface Z the pattern you want to implement is called wrapper or decorator.
In java you can create generic implementation using dynamic proxy introduced to java 1.4.
I don't think so. But sometimes your IDE can assist in creating all the simple methods to delegate the calls. And sometimes you can find third part classes to do this. For example, Guava (http://code.google.com/p/guava-libraries/) has a ton of ForwardingXXX classes, which, by default, delegate everything to something else. For example, ForwardingMap delegates all calls to another Map. You need to override the methods that you do NOT want to delegate.
I'm porting some Python code to Java, and I am having trouble dealing with the following problem:
I have some classes which need to have abilities A, B, or C. Class 1 needs ability A, class 2 needs A, B and C, and class 3 needs B and C. Most importantly, I want to easily be able to change what class can have what ability in the future.
I solved this problem pretty easily with multiple inheritance in Python. I'm trying to figure out the best way to do it in Java, but I can't come up with as good of a solution. I know multiple inheritance is frowned-upon, so I'd appreciate being taught a better way.
Thanks!
It depends on your actual use case, but have you already considered decorators?
http://en.wikipedia.org/wiki/Decorator_pattern
Multiple-inheritance ain't frowned upon. What is frowned upon is "implementation inheritance" (also known as "code reuse"), because it leads to the unsolvable "diamond problem". And because, well, code-reuse really hasn't much to do with OO.
What you want to do can be solved using multiple inheritance (and, say, delegation if you need to do "code reuse").
interface A {
void move();
}
interface B {
void eat();
}
interface C {
void think();
}
class One implements A { ... }
class Two implements B { ... }
class Three implements B, C { ... }
Any OOA/OOD using multiple inheritance can be trivially translated to Java. The part where you say that you need to change the "ability" all the time is a bit scary: if, say, a Car can move(), why would it suddenly need to be able to think()?
You can use AspectJ's mixin syntax fairly easily to emulate multiple inheritance (and at compile time too). First, declare an interface for the functionality you want to mixin:
public interface A{
String getSomethingForA();
}
then define an annotation which you can use to signify that you want the mixin applied to a given class:
public #interface WithA {}
then add the annotation to the class you want to use:
#WithA
public class MyClass {}
then, to actually add some functionality:
#Aspect
public class MixinA {
public static class AImpl implements A{
public String getSomethingForA() {
return "it worked!";
}
}
#DeclareMixin("#WithA *")
public static A get() {
return new AImpl();
}
}
You'll need to use the aspectj jars and run the aspects as part of your compile process, but this lets you create truly modularized functionality and then forcibly merge it into your classes later. To access your class with the new functionality, do the following:
MyClass obj = new MyClass();
((A)obj).getSomethingForA();
You can apply the same annotation to another class and cast it as well:
#WithA
#WithB //let's pretend we created this with some other functionality
public class AnotherClass {}
AnotherClass anotherObj = new AnotherClass();
((A)anotherObj).getSomethingForA();
((B)anotherObj).andSetSomethingElseForB("something else");
Multiple inheritance is almost always a bad idea, as its effects can usually be achieved through other mechanisms. Based upon your description of the problem, it sounds like you want to
Use interfaces to define behavior (public interface A) in this scenario, each behavior should probably have its own interface.
If 2 behaviors are tightly coupled (say A & B), define an interface that implements those two atomic interfaces (public interface CombinedAandB extends A, B)
Define an abstract base class that implements the interface to provide default implementations for behaviors
public abstract class BaseAB implements A, B
{
#Override
public void A() { add(0,1); }
#Override
public void B() {add(1,0); }
private void add(int a, int b) //it doesn't return. no soup for you.
{ a + b; //If you know why this is wrong, high five yourself. }
}
Define a concrete class that extends the abstract base class, implements another interface, and provides its own behavior.
public class IDoABAndC extends BaseAB implements C
{
//stuff, etc
}
You can define the abilities in interfaces and implement them in your classes.
In java you don't have multiple inheritance, instead you can implement multiple interfaces.
So your class 1 will implement interface A and B. Class 2 will implement interface A, B and C. Class 3 will implement B and C.
If what you need is interface inheritance, then as mentioned before, you can always implement multiple interfaces.
If you're looking for implementation inheritance, you're somewhat out of luck. The best solution is probably to use delegation — replace the extra superclasses with fields, and implement methods that just delegate to those fields. It does require writing a lot of repetitive delegation methods, but it's rather unavoidable in Java (without resorting to AspectJ or other bytecode-munging tricks; careful, this way madness lies …).
This is a bit tangential, but you can have python code running in Java via Jython (http://www.jython.org/). This addresses the porting to Java part, not the solving multiple inheritance part (I think you need to determine which is relevant)
HI
I have a question If interface has got four methods,and I like to implement only two methods, how this could be achieved?
Can any explain is that possible or should I go for implementing all the methods.
You can't "partially" implement an interface without declaring the implementing class abstract, thereby requiring that some subclass provide the remaining implementation. The reason for this is that an interface is a contract, and implementing it declares "I provide the behavior specified by the interface". Some other code is going to use your class via the declared interface and will expect the methods to be there.
If you know the use case does not use the other two methods you can implement them by throwing OperationNotSupported. Whether this is valid or not very much depends on the interface and the user. If the interface can legitimately be partially implemented this way it would be a code smell that the interface is poorly designed and perhaps should have been two interfaces.
You may also be able "implement" the interface by doing nothing, though this is usually only proper for "listener" or "callback" implementations.
In short, it all depends.
If you control the design of the interface, simply split it in two. One interface specifies the two only some implementations implement, and one interface specifies the other two (or inherits the first two and adds more, your choice)
You can make the implementing class abstract and implement two of the 4 methods from the interface.
I think #sateesh 's answer is the one closer to solving your problem. Let me elaborate on it. First of all, it is imperative that any class implementing an interface should provide definitions for all of its methods. But in some cases the user may find no use for a majority of the methods in the interface save for one or two. Consider the following interface having 6 abstract methods:
public interface HugeInterface {
void a();
void b();
void c();
void d();
void e();
void f();
}
Suppose your code finds use for the method 'c()' only and you wish to provide implementation details for only the method 'c()'. You can create a new class HugeInterfaceAdapter in a separate file which implements all the methods of the interface HugeInterface like shown below:
public class HugeInterfaceAdapter implements HugeInterface {
public void a() {}
public void b() {}
public void c() {}
public void d() {}
public void e() {}
public void f() {}
}
Note that you need not provide any actual implementation code for any of the methods. Now comes the interesting part. Yes, your class in which the need to implement a huge interface arose in the first place.
public class MyClass {
HugeInterfaceAdapter mySmallInterface = new HugeInterfaceAdapter() {
#Override
public void c() {
//Your class-specific interface implementation code here.
}
};
}
Now you can use the reference variable mySmallInterface in all the places where a HugeInterface is expected. This may seem a little hackish but I may say that it is endorsed officially by Java and classes like MouseAdapter bears testimony to this fact.
It's not possible.
You can implement all four methods, but the two you don't need should throw an UnsupportedOperationException.
If you want a concrete class which is implementing this interface, then it is not possible to have unimplemented methods, but if you make have abstract class implementing this interface then you can leave any number of methods as you want to be unimplemented.
As other answers mention you cannot have a concrete class implementing only some of the methods of the interface it implements. If you have no control over the interface your class is extending, you can think of having Adapter classes.
The abstract Adapter class can provide dummy implementation for the methods of an interface and the client classes can
extend the Adapter class. (Of course the disadvantage is that you cannot extend more than one class)
This is common practice with GUI programming (using Swing) where the event listener class
might not be interested in implementing all methods specified by the EventListener interface. For example
take a look at the java.awt.event.MouseListener interface and and the corresponding adapter class java.awt.event.MouseAdapter.