JAVA :access method defined in class , but not implemented by its interface

Question

Interface:

public interface InterfaceA {
    public void PartOfIinterfaceA();
}

Class:

public class Class_To_Test_Interface_encapsulation implements InterfaceA {

public void MethodM() {

}

@Override
public void PartOfIinterfaceA() {
    // TODO Auto-generated method stub

}

}

Main class:

public class MainClass {


public static void main(String args[]) {

    InterfaceA Ia = new Class_To_Test_Interface_encapsulation();
     Ia.MethodM();
}
}

Its giving me following error : The method MethodM() is undefined for in type InterfaceA() I very well know, why its giving me error and its logical. Also, I refrred , //Use methods declared in implementation that are not defined in interface

But, my question is, is there any other way we can , where a piece of code referring to an instance of B (with type InterfaceA) can in fact access m ?

In general: if you need to use a method from a class C, then you should have the reference stored in a variable of type C. — Louis Wasserman
– Louis Wasserman, Commented Oct 26, 2017 at 23:58

Mike · Accepted Answer · 2017-10-26 23:33:46Z

2

It doesn't make sense to access m on an object (InterfaceA) that doesn't have m (the method is not defined in the interface). If you know that a subset of InterfaceA will have m defined, you need to determine membership in that subset before accessing m:

if (Ia instanceof Class_To_Test_interface_encapsulation) {
  ((Class_To_Test_interface_encapsulation)Ia).MethodM();
}

answered Oct 26, 2017 at 23:33

Mike

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1 Comment

Mihir Over a year ago

Hi Mike, thanks for the solution. I have found an alternate solution as well as posted below.

Mihir · Accepted Answer · 2017-10-27 03:04:25Z

-1

InterfaceA Ia = new Class_To_Test_Interface_encapsulation().MethodM();

This will skip compile time error, and resolve at runtime correctly and call the method MethodM()

answered Oct 27, 2017 at 3:04

Mihir

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7 Comments

omajid Over a year ago

Um. new Class_To_Test_Interface_encapsulation().MethodM() evaluates to void. You cant assign it to an InterfaceA.

Mihir Over a year ago

new Class_To_Test_Interface_encapsulation() is a child type of Interface. So when i create a new objcet of that class it would definitely gets assigned to interface which gets resolved at run-time and than the method from the class Class_To_Test_Interface_encapsulation().MethodM() gets called. From where does void came from ? can you have a look at my original code once again

omajid Over a year ago

The compiler sees InterfaceA Ia = Class_To_Test_Interface_encapsulation().MethodM(); as InterfaceA Ia = (new Class_To_Test_Interface_encapsulation().MethodM()). It will call MethodM and assign the result of that to Ia, which is not compatible with type void. Your solution doesn't even compile: ideone.com/NE8dV4

Mihir Over a year ago

I have tried it ....it's compiling and running in eclipse... I am still not getting your point with void?

Mihir Over a year ago

Hey, apologies to miss your point. Now I got it. It worked only when I do this public InterfaceA MethodM() { return new Class_To_Test_Interface_encapsulation(); } and than call the above method in Main using InterfaceA Ia = new Class_To_Test_Interface_encapsulation().MethodM(); Not sure whether this is the correct solution to my problem

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