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Why interface methods do not have a body

To achieve multiple inheritance, we must use interfaces, but why do interface methods not have bodies and why should they be overridden in a derived class?

I really need a clear answer, not related to too much computer jargon, I don't seem to understand this, I mentioned various links

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8 answers

Because Java, unlike languages ​​like C ++ or Eiffel, has only multiple type inheritance (e.g. interfaces, as well as one class), and not multiple state and behavior inheritance. The latter ones add tremendous complexity (especially state).

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interface A {
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interface B {
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interface C extends A, B {
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}

public class C_Implementer implements C{
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public interface FaceOne {
    public void method() {
        System.out.println("FaceOne Version");
    }
}

public interface FaceTwo {
    public void method() {
        System.out.println("FaceTwo Version");
    }
}

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public class Inheriter implements FaceOne, FaceTwo {

}

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  • Use FaceOne.super.method()to make the inherited default implementation result "FaceOne Version".

  • Use FaceTwo.super.method()to make the inherited default implementation result "FaceTwo Version".

  • Use a combination of the above:

One implementation may be:

@Override
public void method() {
    FaceOne.super.method();
    FaceTwo.super.method();
    System.out.println("Inheriter Version");
}

Withdrawal:

FaceOne Version

FaceTwo Version

Inheriter version

Edit: according to this question , this seems to be exactly how the default implementations are structured in Java 8.

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