JavaWhat are Interfaces in Java

What are Interfaces in Java

Java Programming tutorials
In Java, an interface is a type that defines a set of abstract methods (methods without implementation) and constants (final variables).

This programming tutorial presents a discussion on interfaces in Java with relevant code examples to help developers understand the object oriented programming (OOP) concepts discussed.

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What is an Interface in Java?

An interface in Java is a set of abstract methods with no implementations. Interfaces specify what an implementing class must do, without specifying how the class should do it. Interfaces in Java allows programmers to implement multiple inheritance, which would otherwise be impossible in Java.

Interfaces are often used to define the contracts between different modules or components of a system, which can then be implemented by classes to form a working system. By using interfaces, we can also benefit from polymorphism, as a class can implement multiple interfaces, each providing its own set of methods and behavior.

Interfaces allow for easier integration of new features as they create a contract that each class should comply with, resulting in fewer errors and increased readability. They also help reduce coupling between classes by providing a standard way for different parts of code to communicate with each other.

Additionally, the use of interfaces in Java provides a reliable mechanism of communication between different parts of an application and allows developers to rely on the same contract even when other parts of the application are changed and updated.

Before delivering further, if you are newer to the principles of OOP software development, you may wish to read some of the following tutorials:

Why Use Interfaces in Java?

Interfaces are an important part of Java programming and have several benefits, including for Java developers, including encapsulation, polymorphism, flexibility, and improved testing:

  • Encapsulation: Interfaces provide a way to encapsulate behavior that is not tied to a particular class. This allows for more modular and reusable code.
  • Polymorphism: Interfaces allow for polymorphism, which means that multiple classes can implement the same interface, allowing them to be used interchangeably.
  • Flexibility: Interfaces allow for greater flexibility in the design of a program, as they allow for classes to be designed to implement multiple interfaces.
  • Testing: Interfaces make it easier to test code, as they provide a clear definition of the behavior that is expected from an object.
  • Future-proofing: Interfaces provide a way to future-proof a program, as they allow for the addition of new functionality without breaking existing code, provided that the new functionality is added through new interfaces.

Syntax of Java Interfaces

The following is the syntax for using interfaces in Java:

interface {  
    // define constant fields here
    // define methods that are abstract by default.  

How to Define an Interface in Java

To declare an interface in Java, you should use the interface keyword followed by the name of the interface. It should be noted that all methods in an interface are public and abstract by default. Therefore, there is no need to explicitly declare the methods as such.

The following code example shows how you can define an interface in Java:

public interface MyInterface {
    double getABC();
    double getXYZ();

This interface defines two abstract methods, getABC() and getXYZ(), which any class implementing the interface must implement. These methods define the basic behavior of any shape, but the specific implementation of each method will depend on the individual shape class.

Why Use Interfaces in Java?

Interfaces can be used in Java to define the behavior of a class while allowing the class to maintain flexibility. By defining methods and fields within an interface, classes that implement the interface are required to contain those methods and fields, ensuring consistency while allowing for reuse of code.

Interfaces also provide a level of flexibility in programs by allowing multiple classes to maintain the same functionality while providing different implementations. As such, interfaces are important tools for code reuse that should be taken advantage of when programming with Java.

They also provide a way to separate implementation details which helps to maintain the integrity of your code by preventing changes in implementation from affecting users of the interface.

How to Program Interfaces in Java

An interface can be used as a contract between two objects, specifying the methods that one object should implement to interact with the other. To create an interface in Java, you use the interface keyword followed by the interface name, and then define the methods and constants within braces.

Here is a code example of an interface in Java:

public interface Shape {
    double getArea();
    //other abstract methods...

In the preceding code example, we define an interface called Shape that has one abstract method: getArea(). These methods do not have any implementation and do not specify any access modifiers because they are abstract.

We also define a constant variable called TYPE and initialize it to the string “Shape”. By default, all variables in an interface are public, static, and final. This means they are constants that can be accessed directly through the interface.

For a class that extends an interface, it is imperative that it implements all members of the interface. For example, let’s say we have a class called Circle that implements the Shape interface:

public class Circle implements Shape {
    private double radius;
    public Circle(double radius) {
        this.radius = radius;
    public double getArea() {
        return Math.PI * radius * radius;

In the preceding code example, the Circle class implements the Shape interface by providing an implementation for all of its methods. It also has a private instance variable radius, and a constructor that takes a radius argument.

By implementing the Shape interface, the Circle class ensures that it will provide a specific set of methods to interact with other objects that expect a Shape. This can be useful for polymorphism and code reuse, where a single interface can be implemented by multiple classes with different implementations.

If you have an abstract class that extends an interface, the abstract class must provide concrete implementations of the members of the interface. For example, suppose we have an interface called MyInterface:

public interface MyInterface {
public void someMethod();

Here is an abstract class that implements this interface in Java:

public abstract class MyAbstractClass implements MyInterface {
public void someMethod() {
// do something here

You can learn more about constructors in our tutorial: How to Work with Constructors in Java.

Interfaces vs Abstract Classes in Java

Developers can use both interfaces and abstract classes in Java to define requirements that a class is required to implement. Interfaces consist of a collection of abstract methods that define the method signatures but not the implementation.

An abstract class may provide an implementation for some or all of its methods, while allowing subclasses to implement others. While you can use any access modifier (public, private, or protected) to declare its members, an interface can only have public access for its methods and constants.

While you can have constructors in an abstract class, you cannot have a constructor in an interface. Abstract classes extend the Object class by default, whereas interfaces do not. Interfaces are useful for defining a set of requirements without any implementation that a class extending the interface must fulfill.

Final Thoughts on Java Interfaces

Interfaces provide a powerful tool for creating modular, reusable, and flexible code that is easier to test and maintain over time. Abstract classes are useful for providing a common implementation for related classes while still allowing some flexibility in the implementation.

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