Diamond Problem in Java

Last Updated on December 28, 2023 by Ankit Kochar

The "Diamond Problem" is a term used in object-oriented programming, particularly in languages that support multiple inheritance, such as Java. It refers to a situation where a class inherits from two classes that have a common ancestor. If the derived class calls a method or uses a field that is defined in the common ancestor, and both parent classes provide different implementations for that method or field, a conflict arises. Resolving such conflicts becomes a challenge, and it is named the "Diamond Problem" due to the shape of the class hierarchy diagram.
In Java, multiple inheritance is not directly supported to avoid such complications. Instead, Java uses interfaces to achieve a form of multiple inheritance. While interfaces can provide a way to inherit from multiple sources, the Diamond Problem can still manifest in certain scenarios, often leading to confusion and the need for careful design considerations.

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
In conclusion, the Diamond Problem underscores the complexities associated with multiple inheritance in object-oriented programming. Java addresses these challenges by promoting the use of interfaces and avoiding direct support for multiple inheritance. By understanding the principles of interfaces and class hierarchies, developers can mitigate the risks associated with the Diamond Problem and create more maintainable and flexible code.
While the Diamond Problem may not be entirely eliminated, Java’s design choices aim to provide a balance between the advantages of inheritance and the need for code simplicity and clarity. As developers, being aware of these challenges and adopting best practices in design can contribute to more robust and scalable software solutions.

FAQs Related to Diamond Problem in Java

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

Q1: Can the Diamond Problem occur in languages other than Java?
A1: Yes, the Diamond Problem is not specific to Java and can occur in other object-oriented programming languages that support multiple inheritance, such as C++. However, each language may have its own mechanisms and solutions to address this issue.

Q2: How does Java address the Diamond Problem?
A2: Java avoids direct support for multiple inheritance to mitigate the Diamond Problem. Instead, it encourages the use of interfaces to achieve a form of multiple inheritance. Interfaces provide a way to inherit from multiple sources without the conflicts associated with direct multiple class inheritance.

Q3: Can the Diamond Problem still occur in Java with interfaces?
A3: While Java’s use of interfaces helps alleviate the Diamond Problem, situations can still arise where conflicts occur. Careful design, proper use of interfaces, and adherence to best practices can minimize the impact of potential issues related to the Diamond Problem in Java.

Q4: How can developers mitigate the Diamond Problem in their Java code?
A4: Developers can mitigate the Diamond Problem by using interfaces judiciously, favoring composition over inheritance, and adhering to design principles such as the Dependency Inversion Principle. Clear documentation