Diamond Problem in Java? How to fix it?

In Java, the Diamond Problem is an issue that arises due to multiple inheritance. When a class inherits the same method from two different superclasses, it becomes unclear which method should be used. However, Java does not directly support multiple inheritance for classes (a class can only inherit from a single class). The Diamond Problem typically occurs with interfaces when default methods are involved.

What is the Diamond Problem?

Scenario: How the Diamond Problem Arises

When a class inherits a default method with the same name from two different interfaces, ambiguity arises. Let’s illustrate this with a diagram:

       Interface A
       /       \
      /         \
Interface B   Interface C
      \         /
       \       /
        Class D

• Explanation:
  • Interface A has a default method, let’s say methodX().
  • Interface B and Interface C implement Interface A and may either override methodX() or define their own default methodX() implementations.
  • Class D implements both Interface B and Interface C.
  • Now, when methodX() is called in Class D, it’s unclear which methodX() should be used. This is the Diamond Problem.

Solution to the Diamond Problem in Java

Java provides several mechanisms to solve the Diamond Problem. Let's explain them with diagrams:

Solution 1: Overriding the Default Method

By explicitly overriding methodX() in Class D, you resolve the ambiguity.

       Interface A
       /       \
      /         \
Interface B   Interface C
      \         /
       \       /
        Class D
         |
    Override methodX()

• Explanation:
  • Class D defines methodX() within itself and specifies the desired behavior.
  • This way, instead of using the default methods from Interface B or Interface C, Class D's own implementation is used.

Solution 2: Calling by Specifying the Super Interface

In Java, a class can explicitly specify which interface's default method it wants to use.

       Interface A
       /       \
      /         \
Interface B   Interface C
      \         /
       \       /
        Class D
         |
    methodX() -> Interface B.methodX()

• Explanation:
  • While defining methodX() in Class D, for example, methodX() from Interface B can be called.
  • Alternatively, methodX() from Interface C can be used.

Solution 3: Establishing a Hierarchy Among Interfaces

Sometimes, the Diamond Problem can be avoided by creating a hierarchy among interfaces.

Interface A
   |
Interface B
   |
Interface C
   |
 Class D

• Explanation:
  • If Interface C inherits from Interface B, and Interface B inherits from Interface A, a chained structure is formed.
  • This way, Class D only implements Interface C, eliminating ambiguity.

Thanks for reading.

Burak Boduroglu