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Can you have a class which implements an interface, and choose whether to use the methods in the interface during instantiation of this class? Therefore having object A which uses the interface and object B which does not use it.

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Assuming you have a Professor class and this class implements an interface called Employer, which has employ(rAssist x) abstract method.

Now I want to instantiated 2 objects from the Professor class implementing this interface Object A - Professor can employ a research assistant and Object B - Professor cannot employ research assistants.

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  • I think You need to clarify your question? What Do you want to say? Commented Jul 30, 2012 at 5:22
  • The question doesn't make sense. The interface methods are only declarations. You can't 'use' them in any way. Commented Jul 30, 2012 at 22:26

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Can you have a class which implements an interface, and choose whether to use the methods in the interface during instantiation of this class?

No, if class C implements the interface, then all instances of C will provide the methods declared in the interface.

What you can do is something like

class MyClass implements MyInterface { @Override void interfaceMethod() { System.out.println("Interface method"); } }

and then do

MyClass x = new MyClass(); MyClass y = new MyClass() { @Override void interfaceMethod() { throw new UnsupportedOperationException(); } };

In effect, x supports the use of interfaceMethod while y does not. Note however that...

  • The usage of y.interfaceMethod is not prevented at compile-time, i.e. it will not be enforced by the type system.

  • With this solution, you are in fact creating an (anonymous) subclass of MyClass and assigning an instance of it to y.

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2 Comments

Ok I understand. Now from a coder's point of view is this a neat solution? I am asking cause I need to understand this entirely.
Naah, you would typically want to program in a way such that errors are caught at compile-time, not at run-time.
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Do you mean you want class A and Class B to implement a common Interface but you dont want to implement all methods in Class B?

An Interface in simple terms means it is sort of a contract and all the classes which implement it should follow that contract.So if you want Class B to implement the interface , Class B should also follow the same contract. But if you dont want to implement any methos you can always do this.

class ISampleInterface { void sampleMethod(); void optionalMethod(); } Class A implements ISampleInterface { void sampleMethod() { //Your Implementation } void optionalMethod() { //Your Implementation } } class B implements ISampleInterface { void sampleMethod() { //Your Implementation } void optionalMethod() { throw new UnsupportedMethodException(); } }
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No, that's not the point of an Interface.

An Interface is contract that guarantees that implementations WILL implement it's signature

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I believe that doesn't address the question.
@Habib Hay, I'm half a sleep, so I'd appreciate if you could help and explain what I missed :P
see the answer of aioobe, What you are saying is perfectly right with respect to interfaces, but OP is asking about objects of a same class using methods from interface based on some condition
@Habib Ah, okay, obviously need more sleep, thanks for taking the time to point it out, much appreciated :) - Yes, I thought aioode's answer was much better as well
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The idea of interface is to establish a obligation for the class that implements the interface. If your's is a requirement, you can use the java.lang.reflect.Method reflection class to change the visibility of the method at runtime. However, this is not a clean way.

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1. Interfaces were introduced in Java because Multiple Inheritance was not allowed in Java.

2. But as far as Design Pattern are concerned, following are the uses..

- To implement certain Roles.

Consider Dog a Super class, but then Pet dog and Wild dog can be interfaces, which can be implemented by the Sub Classes of Dog class.

- Used when Behaviors keeps changing.

Consider you have a Class Drawing, and paint method() in it, now paint can be stroking, shading, etc... You must Encapsulate such behaviors in an Interface or an Abstract class.

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Thanks for the examples...helpful.

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