How Does Java Resolve Overlapping Methods When Implementing Multiple Interfaces?

Implementing Multiple Interfaces with Overlapping Methods: Unveiling the Selection Criteria

In object-oriented programming, interfaces play a crucial role in defining contracts for classes to implement. However, scenarios may arise where multiple interfaces share the same method name and signature. In such cases, it is natural to wonder how the compiler identifies the method's implementation.

Overridden versus Inherited Methods

When a class implements multiple interfaces, their methods inherit the same signatures. However, a method can only be overridden once, so the compiler must determine which interface's method is being overridden. The selection criterion is based on the following rule:

• If two methods have identical signatures, they are considered "@Override-equivalent," meaning they are effectively the same method.
• If a class inherits two or more @Override-equivalent methods, it needs to implement only one of them.

Example

Consider the following example:

interface A {
  int f();
}

interface B {
  int f();
}

class Test implements A, B {
  @Override
  public int f() {
    return 0;
  }
}

In this case, both A and B define the method f() with the same signature. Since they are @Override-equivalent, Test only needs to implement one version of f(), regardless of which interface it is intended for. The compiler identifies the implemented method as the overriding method, and no further distinction is required.

Compatibility and Incompatibility

It is important to note that methods inherited from different interfaces must also comply with the rules of method overriding. If the methods have conflicting return types or annotations, a compilation error will occur.

Conclusion

In summary, when a class implements multiple interfaces with overlapping methods, the compiler identifies the overriding method based on @Override-equivalence. The implemented method serves as the single implementation for all inherited methods with matching signatures. This allows for inheritance of common behaviors while ensuring compatibility and avoiding conflicts in method definitions.

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