Difference Between Class and Interface in Java

In Java, the differences between class and interface are syntactically similar but different in various aspects. Both the class and interface have methods, variables, and constants. In this section, we will discuss the differences between class and interface.

What is a Class?

A class is a blueprint for creating objects. It contains variables and methods that define the properties and behaviour of objects.

Example of a Class

Here is an example of a class in Java.

Here, Animal is a class with a variable (name) and a method (makeSound).

Advantages of Using Classes

There are several advantages of classes in Java. Some of them are as follows:

1. Encapsulation: A class groups related data and methods together, preventing direct access and ensuring better data security.
2. Reusability: With inheritance, a class can be extended to add new features without modifying the original code.
3. Flexibility: Polymorphism allows objects to take different forms, making code more adaptable and easier to manage.

What is an Interface?

An interface is like a contract that specifies a set of methods that a class must implement. It does not contain any actual code for methods (except in special cases).

Example of an Interface

Here is an example of an interface in Java.

Here, Animal is an interface that only declares a method but does not define how it works.

Advantages of Using Interfaces

There are several advantages of Interfaces in Java. Some of them are as follows:

1. Loose Coupling: Interfaces reduce dependencies, making it easier to modify or replace components without affecting the entire system.
2. Multiple Inheritance: A class can implement multiple interfaces, overcoming Java’s single inheritance limitation.
3. Testability: Interfaces simplify unit testing by allowing mock implementations, making testing more efficient and isolated.

When to Use a Class or Interface?

You can use the class or interfaces in the following cases:

• Use a class when we want to define real-world objects with properties and behaviours.
• Use an interface when we want to enforce rules that multiple classes must follow.

Java Class Vs. Interface

The following table shows the key differences between class and interface in Java based on the different aspects:

Understanding Class vs. Interface Using Example

Consider the following example to understand the difference between a class and an interface. In this example, Animal is an interface that defines a common behavior, while Dog and Cat are classes that implement the interface and provide their own implementations.

Output:

Dog barks
Cat meows   

Explanation

In this example, the Animal interface sets a rule that any class implementing it must define the makeSound() method. The Dog and Cat classes follow the rule by providing their versions: one prints "Dog barks," and the other prints "Cat meows." In the Main class, objects of Dog and Cat are created and assigned to the Animal type. When makeSound() is called, Java dynamically picks the correct method for each object.

Conclusion

Both classes and interfaces are essential in Java, but they serve different purposes. Classes define real-world objects, while interfaces establish a behaviour contract that multiple classes can implement. Understanding their differences helps in designing robust and scalable applications.