How Does the Java Compiler Resolve Method Overriding Conflicts When Implementing Multiple Interfaces with Identical Signatures?

Implementing Interfaces with Overridden Methods: How the Compiler Resolves Identity

When a class implements multiple interfaces, the issue arises of how the compiler identifies which method to override when both interfaces declare methods with identical signatures. To delve into this, let's analyze the provided code example:

interface A {
  int f();
}

interface B {
  int f();
}

class Test implements A, B {   
  public static void main(String... args) throws Exception{   
  }

  @Override
  public int f() {  // from which interface A or B
    return 0;
  }
}   

The Compilation Process

In this scenario, the compiler first examines the interfaces A and B. Since they both define a method f with the same signature, they effectively represent a single method in the implementation. The compiler doesn't need to differentiate between them, as they are considered @Override-equivalent (JLS 8.4.2).

When the compiler reaches the implementation in class Test, it encounters the @Override annotation on the f method, which indicates that the method overrides an inherited method from a superclass or interface. Since both A and B declare f, the compiler checks for conflicts in the method signature. In this case, there is no conflict, as the signatures are identical.

Therefore, the f method defined in Test overrides the single inherited method f from the combination of interfaces A and B. This means that the compiler treats the implementation as a replacement for both inherited methods.

Compatibility and Incompatibility

In general, implementing multiple interfaces with @Override-equivalent methods doesn't pose issues during compilation. However, incompatibilities can arise if the inherited methods have different return types or if they conflict with methods in a superclass.

Consider the following example:

interface Gift  { void present(); }
interface Guest { boolean present(); }

interface Presentable extends Gift, Guest { } // DOES NOT COMPILE!!!

In this case, the compilation fails because the inherited methods present() have incompatible return types. The compiler cannot resolve this conflict and generates an error indicating that the methods are incompatible.

Conclusion

When implementing multiple interfaces with @Override-equivalent methods, the compiler identifies the methods based on their combined signature, which effectively merges the methods from both interfaces. This process allows for clean and unambiguous method overriding in such scenarios.

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