JAVA：access方法在类中定义，但未通过其接口实现

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 ?

Answer 1

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();
}

Answer 2

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()