Resolving Conflicts During Multiple Inheritance in Java

Java does not support multiple inheritance with classes to avoid the diamond problem, which causes ambiguity when multiple parents provide methods with the same signature. However, with the introduction of default methods in Java 8, multiple inheritance is supported using interfaces. While this enhances flexibility, conflicts can arise when multiple interfaces provide default methods with the same signature.

To address such conflicts, Java has well-defined rules of resolution to determine which method should be invoked. These rules ensure that ambiguity is avoided and developers can resolve conflicts systematically.

Rules for Resolving Multiple Inheritance Conflicts

1. Classes take higher precedence than interfaces: If a class in the inheritance hierarchy provides a concrete implementation of a method, it takes precedence over default methods in interfaces, regardless of the interface hierarchy.
2. Derived interfaces take higher precedence than parent interfaces: If multiple interfaces contain default methods with the same signature and one interface extends the other, the sub-interface (derived interface) method is preferred as it represents a more specific behavior.
3. Explicit implementation in the class: If the above rules cannot resolve the conflict (for example, when multiple unrelated interfaces provide methods with the same signature), the implementing class must explicitly override the conflicting method and provide its own implementation.

Rule 1: Classes Take Higher Precedence Than Interfaces

If a class provides a concrete implementation of a method, it overrides any default methods with the same signature from interfaces.

Algorithm

Step 1: Create a Parent class with a display() method.

Step 2: Create an interface InterfaceA with a default display() method.

Step 3: Define a Child class that extends Parent and implements InterfaceA.

Step 4: Instantiate the Child class in the Main class.

Step 5: Call the display() method, which uses the Parent class method.

Implementation

Output:

 
Class Parent: display()   

Explanation: When the display() method is called on the Child class object, the Parent class's display() method is executed because, according to Rule 1, a class method takes precedence over a default interface method. Therefore, the display() method from Parent is used.

Rule 2: Derived Interfaces Take Higher Precedence Than Parent Interfaces

A default method in a derived (child) interface overrides the default method with the same signature in its parent interface.

Algorithm

Step 1: Define BaseInterface with a default display() method.

Step 2: Define DerivedInterface that extends BaseInterface and overrides display().

Step 3: Create ImplementingClass that implements both BaseInterface and DerivedInterface.

Step 4: Instantiate ImplementingClass in the Main class.

Step 5: Call the display() method, which uses DerivedInterface's method.

Implementation

Output:

 
Derived Interface: display()   

Explanation: When the display() method is called on the ImplementingClass object, the DerivedInterface's method is executed because, according to Rule 2, a derived interface's method overrides its parent interface's method. Hence, the display() method from DerivedInterface is used.

Rule 3: Explicit Implementation in The Class

When two unrelated interfaces provide default methods with the same signature, the implementing class must explicitly override and resolve the conflict.

Algorithm

Step 1: Define InterfaceA with a default greet() method.

Step 2: Define InterfaceB with a default greet() method.

Step 3: Create ImplementingClass that implements both InterfaceA and InterfaceB, and explicitly overrides the greet() method.

Step 4: Instantiate ImplementingClass in the Main class.

Step 5: Call the greet() method, which uses the class's explicitly overridden method.

Implementation

Output:

 
Explicit implementation: greet()   

Explanation

When the greet() method is called on the ImplementingClass object, the explicitly overridden method in the ImplementingClass is executed. According to Rule 3, when no precedence can be established between interfaces, the implementing class must provide its own implementation to resolve the conflict. Therefore, the method System.out.println("Explicit implementation: greet()") is executed.