Java8 - Multiple inheritance and conflict resolution

Category : Java | Sub Category : Java8 Features | By Prasad Bonam Last updated: 2023-11-13 04:57:17 Viewed : 661

In Java, interfaces support multiple inheritance, meaning a class can implement multiple interfaces. However, conflicts can arise when a class implements two or more interfaces that declare methods with the same signature (name and parameter types). This is known as a "diamond problem" or "interface method conflict." Java provides rules and mechanisms to resolve such conflicts:

Interface Method Conflict:

Consider the following example:

javaCopy code
interface A {
    void method();
}

interface B extends A {
    // No method declaration here
}

interface C extends A {
    // No method declaration here
}

class D implements B, C {
    // Class D must provide an implementation for method()
    @Override
    public void method() {
        System.out.println("Implementation in class D");
    }
}

In this example, both interfaces B and C extend interface A, and class D implements both B and C. The common method method() is declared in A, and class D must provide an implementation to resolve the conflict.

Resolution Strategies:

1. Class Overrides the Method:

   • If the class directly provides an implementation for the conflicting method, that implementation is used.

   javaCopy code
   class D implements B, C {
       // Class D provides its own implementation
       @Override
       public void method() {
           System.out.println("Implementation in class D");
       }
   }
   

2. Interface Provides a Default Method:

   • If the conflicting method is declared as a default method in one of the interfaces, the default method is used unless the class provides its own implementation.

   javaCopy code
   interface B extends A {
       // Default method provided in interface B
       default void method() {
           System.out.println("Default implementation in interface B");
       }
   }
   
   interface C extends A {
       // Default method provided in interface C
       default void method() {
           System.out.println("Default implementation in interface C");
       }
   }
   
   class D implements B, C {
       // Class D does not provide its own implementation, uses default from interface B
   }
   
   }
   
   interface C extends A {
       // Default method provided in interface C
       default void method() {
           System.out.println("Default implementation in interface C");
       }
   }
   
   class D implements B, C {
       // Class D does not provide its own implementation, uses default from interface B
   }
   

3. Class Explicitly Chooses Interface Method:

   • If the class explicitly wants to use the method from a specific interface, it can do so by invoking the method on that interface.

   javaCopy code
   class D implements B, C {
       // Class D explicitly chooses the method from interface B
       @Override
       public void method() {
           B.super.method();
       }
   }
   

   Here, B.super.method() explicitly calls the method() from interface B.

Priority of Resolution:

1. Class Hierarchy:

   • If the class hierarchy provides an implementation, that takes precedence.

2. Interface Default Methods:

   • If no class in the hierarchy provides an implementation, the default methods in interfaces are considered.

3. Explicit Interface Invocation:

   • If a class explicitly invokes a method from a specific interface, that method is used.

Example:

javaCopy code
public interface A {
    default void method() {
        System.out.println("Default implementation in interface A");
    }
}

public interface B extends A {
    // No method declaration here
}

public interface C extends A {
    // No method declaration here
}

public class D implements B, C {
    // No implementation here
}

class Main {
    public static void main(String[] args) {
        D d = new D();
        d.method(); // Output: Default implementation in interface A
    }
}

In this example, interface A provides a default method method(). Interfaces B and C extend A. Class D implements both B and C. Since class D does not provide its own implementation, it inherits the default implementation from interface A.