How can multiple inheritance be implemented in Java?

Directly — not at all. Java does not support multiple inheritance of classes to avoid the diamond problem.

Multiple inheritance of behavior (implementing multiple interfaces) is possible.

// Example of implementing multiple interfaces
public interface InterfaceA {
    void methodA();
}

public interface InterfaceB {
    void methodB();
}

public class MyClass implements InterfaceA, InterfaceB {
    @Override
    public void methodA() {
        // Implementation of methodA
    }

    @Override
    public void methodB() {
        // Implementation of methodB
    }
}

For a certain resemblance of multiple inheritance of functionality, you can use:

• Interfaces with default and static methods (Java 8+): Allow providing default implementations of methods in interfaces.

  public interface InterfaceWithDefaults {
      default void defaultMethod() {
          System.out.println("Default implementation");
      }
      static void staticMethod() {
          System.out.println("Static method");
      }
  }
  
  public class AnotherClass implements InterfaceWithDefaults {
      // Uses defaultMethod from the interface
      // Or can override it
  }
  

• Composition: A class contains instances of other classes and delegates method calls to them. This is the preferred approach for "has-a" relationship instead of "is-a".

  public class FeatureA {
      public void doFeatureA() {
          System.out.println("Doing feature A");
      }
  }
  
  public class FeatureB {
      public void doFeatureB() {
          System.out.println("Doing feature B");
      }
  }
  
  public class CombinedClass {
      private FeatureA featureA = new FeatureA();
      private FeatureB featureB = new FeatureB();
  
      public void performCombinedAction() {
          featureA.doFeatureA();
          featureB.doFeatureB();
      }
  }
  

Comparison of approaches:

Approach	Multiple inheritance of state	Multiple inheritance of behavior	Diamond problem solution
Class inheritance	No	Yes (single base class)	Solved
Interface implementation	No	Yes	Not applicable
Interfaces with default/static	No	Yes	Solved (explicit call)
Composition	Yes	No (delegation)	Not applicable