Interfaces usage

Question

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.

Thanks

Updated:

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.

• khan: I think You need to clarify your question? What Do you want to say?
• user207421: The question doesn't make sense. The interface methods are only declarations. You can't 'use' them in any way.

Answer 1

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.

• Daniel: 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.
• aioobe: Naah, you would typically want to program in a way such that errors are caught at compile-time, not at run-time.

Answer 2

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();
 }
}

Answer 3

No, that's not the point of an Interface.

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

• Habib: I believe that doesn't address the question.
• MadProgrammer: @Habib Hay, I'm half a sleep, so I'd appreciate if you could help and explain what I missed :P
• Habib: 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
• MadProgrammer: @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

Answer 4

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.

Answer 5

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.

• Daniel: Thanks for the examples...helpful.