Diamond Problem in Java

Last Updated on March 24, 2023 by Prepbytes

Java is one of the most popular languages that support object-oriented programming. In object-oriented programming, inheritance is one of the four pillars and it is widely used as it enables the creation of new classes from existing ones. Inheritance provides various advantages to the developer but along with this it also causes some problems one of the problems is the diamond problem in java. While moving further in this article we will learn all about the diamond problem in java, its causes, its solution, etc.

What is the Diamond Problem in Java?

The diamond problem in java is the problem that occurs due to multiple inheritance. It occurs when the child class inherits from more than one parent class and the parent classes have the same name function and the object of the child class calls the function then the compiler does not know as the function is present in both the class. Let’s understand the diamond problem in detail with the help of the example that we have explained above.
Consider a scenario when the parent class is used to create two child classes. The parent class has a method name sum which will be inherited by both the child classes or subclasses. Now suppose we further create grand child class that will inherit from both the child classes and now the user makes an object of the grand child class and the object calls the sum function now the compiler will not be able to identify which sum function they are calling as it is present in both the child classes and because of this ambiguity the java does not support multiple inheritance. And this problem is generally referred to as the diamond problem in java.

Inheritance in Java

The diamond problem in java occurs because of inheritance so before directly moving to understand the diamond problem in java let’s first have a brief discussion about inheritance in java.
Inheritance is basically a concept with the help of which the child class can inherit the features, attributes, and methods of the parent class. This allows the developers to create a hierarchy of classes where the subclasses are inherited from the parent classes.

There are many types of inheritance available in java like

• Single inheritance.
• Multiple inheritance
• Multi-level inheritance
• Hierarchical inheritance
• Hybrid inheritance

To learn about inheritance in detail you can refer to Inheritance in Java.

Why Multiple Inheritance is not allowed in Java?

Multiple inheritance is nothing but when a child inherits the properties of more than one parent class.
Consider a scenario when a base class is used to create two subclasses p1, and p2. So the methods of the base class will be inherited by the subclass. Let’s suppose the base class has a method name sum() now it will be inherited by both p1 and p2. Now if we further use multiple inheritance to create a child class c1 from both p1 and p2. And now the c1 will merit the sum() method from both p1 and p2. Now suppose a case when the user made an object of c1 class and wants to call the sum() method. Now the compiler will be confused as the sum() is present in both the parent class. This is the problem faced during multiple inheritance and because of this multiple inheritance is not allowed in java. Let’s understand this with an example.

Example
Here is an example of multiple inheritance with code implementation.

import java.util.*;
 
class Sample1{
     public void sum(int x,int y){
           System.out.println("Sum= "+ x+y);
     }
}
 
class Sample2{
     public void sum(int x,int y){
           System.out.println("Sum= "+ x+y);
     }
}
 
public class Dummy1 extends Sample1, Sample2{
     public static void main(String args[]){
           Dummy1 obj = new Dummy1();
           obj.sum();
     }
}

Output

Main.java:15: error: '{' expected
public class Dummy1 extends Sample1, Sample2{

Explanation of the above example
In the above example we can see the ambiguity with multiple inheritance and because of this reason multiple inheritance is not allowed in java.

Solution of Diamond Problem in Java: Interfaces

You might think that we have said that we cannot implement multiple inheritance in java so there is no solution to the diamond problem in java also as we cannot implement multiple inheritance. The answer is NO, We can implement multiple inheritance in java through default methods using the interface.
This was made possible in Java 8 when classes that implement interfaces were unaffected by methods with implementation. They are the methods that the interface’s definition of the term "default" designates. We are unable to add a method to the interface without using "default" or "static" ways. It is not required to override a default function if it exists in an interface in the classes that implement it. However, ambiguity results when a class implements two distinct interfaces but uses the same default methods for both.

The class can implement more than two interfaces as the interface provides the default implementation of methods. The class that is implementing it should indicate which method is to be used or which of the default methods should be overridden if the implemented interfaces contain default methods with the same method signature.

Example
Let’s solve the above-mentioned example.

import java.util.*;

interface Sample1 { 
      default void sum(int x,int y) { 
            System.out.println("Sum 1 is = "+ (x+y)); 
      } 
}
 
interface Sample2 { 
      default void sum(int x,int y) { 
            System.out.println("Sum 2 is = "+ (x+y)); 
      } 
}

 class Dummy implements Sample1, Sample2{
      @Override
      public void sum(int x,int y) { 
            System.out.println("Sum is = "+ (x+y)); 
      }
      public static void main(String args[]){
            Dummy obj = new Dummy();
            obj.sum(18,92);
      }
}

Output

Sum is = 110

Explanation of the above example
In the above example we have implemented multiple inheritance in java with the help of interfaces. We have overridden the sum method in a class that implements the interface to solve the diamond problem. The compiler will know that we have overridden this method when we override the sum method that is present in the implemented interfaces.

Conclusion
The diamond problem in java is a major issue that is occurred when multiple inheritance is allowed in object-oriented programming. Java does not allow multiple inheritance directly but we can implement it by using interfaces. We have seen the diamond problem in java along with the method to solve it. This will allow developers to use multiple inheritance in java without worrying about the diamond problem. And the developers can make the complex codes easy with the help of inheritance. It is advised to avoid multiple inheritance whenever possible due to the diamond problem in java but you can always use interfaces to solve it.

FAQs Related to Diamond Problem in Java

Here are some of the frequently asked questions about the diamond problem in java.

1. What is an interface in Java?
An interface in Java is a collection of abstract methods that define a contract for classes that implement the interface.

2. What is a default method in Java interface?
A default method in Java interface is a method that has a default implementation in the interface itself.

3. What is the super keyword in Java?
The super keyword in Java is used to refer to the superclass of a class or to call a method of the superclass.

4. How does virtual method dispatch work in Java?
Virtual method dispatch in Java works by dynamically dispatching a method based on the type of the object at runtime.

5. Can a class extend multiple interfaces in Java?
Yes, a class can extend multiple interfaces in Java.