Interface in Java

An interface in Java is a blueprint of a class. It has static constants and abstract methods.

The interface in Java is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not method body. It is used to achieve abstraction and multiple inheritance in Java.

In other words, you can say that interfaces can have abstract methods and variables. It cannot have a method body.

Java Interface also represents the IS-A relationship.
It cannot be instantiated just like the abstract class.
Since Java 8, we can have default and static methods in an interface.
Since Java 9, we can have private methods in an interface.

Why use Java interface?

There are mainly three reasons to use interface. They are given below.

It is used to achieve abstraction.
By interface, we can support the functionality of multiple inheritance.
It can be used to achieve loose coupling.

How to declare an interface?

An interface is declared by using the interface keyword. It provides total abstraction; means all the methods in an interface are declared with the empty body, and all the fields are public, static and final by default. A class that implements an interface must implement all the methods declared in the interface.

Syntax:

interface <interface_name>{
    
    // declare constant fields
    // declare methods that abstract 
    // by default.
}
</interface_name>

Example:

interface Animal {
    void eat();
    void sleep();
}

In this example, the Animal interface declares two method signatures: eat() and sleep(). Any class implementing the Animal interface must provide concrete implementations for these methods.

Java 8 Interface Improvement

Since Java 8, interface can have default and static methods which is discussed later.

Internal addition by the compiler

The Java compiler adds public and abstract keywords before the interface method. Moreover, it adds public, static and final keywords before data members.

In other words, Interface fields are public, static and final by default, and the methods are public and abstract.

Relationship Between Classes and Interfaces

As shown in the figure given below, a class extends another class, an interface extends another interface, but a class implements an interface.

The relationship between class and interface

Java Interface Example

In this example, the Printable interface has only one method, and its implementation is provided in the A6 class.

File Name: InterfaceExample.java

interface printable{  
void print();  
}  
class InterfaceExample implements printable{  
public void print(){System.out.println("Hello");}  
public static void main(String args[]){  
InterfaceExample obj = new InterfaceExample();  
obj.print();  
 }  
}  

Output:

Hello

Java Interface Example: Drawable

In this example, the Drawable interface has only one method. Its implementation is provided by Rectangle and Circle classes. In a real scenario, an interface is defined by someone else, but its implementation is provided by different implementation providers. Moreover, it is used by someone else. The implementation part is hidden by the user who uses the interface.

File: TestInterface1.java

//Interface declaration: by first user
interface Drawable{
void draw();
}
//Implementation: by second user
class Rectangle implements Drawable{
public void draw(){System.out.println("drawing rectangle");}
}
class Circle implements Drawable{
public void draw(){System.out.println("drawing circle");}
}
//Using interface: by third user
class TestInterface1{
public static void main(String args[]){
Drawable d=new Circle();//In real scenario, object is provided by method e.g. getDrawable()
d.draw();
}}

Test it Now

Output:

drawing circle

Java Interface Example: Bank

Let's see another example of java interface which provides the implementation of Bank interface.

File: TestInterface2.java

interface Bank{
float rateOfInterest();
}
class SBI implements Bank{
public float rateOfInterest(){return 9.15f;}
}
class PNB implements Bank{
public float rateOfInterest(){return 9.7f;}
}
class TestInterface2{
public static void main(String[] args){
Bank b=new SBI();
System.out.println("ROI: "+b.rateOfInterest());
}}

Test it Now

Output:

ROI: 9.15

Multiple Inheritance in Java by Interface

If a class implements multiple interfaces, or an interface extends multiple interfaces, it is known as multiple inheritance.

File Name: InterfaceDemo.java

interface Printable{
void print();
}
interface Showable{
void show();
}
class A7 implements Printable,Showable{
public void print(){System.out.println("Hello");}
public void show(){System.out.println("Welcome");}

public static void main(String args[]){
A7 obj = new A7();
obj.print();
obj.show();
 }
}

Test it Now

Output:

Hello
Welcome

Q) Multiple inheritance is not supported through class in java, but it is possible by an interface, why?

As we have explained in the inheritance chapter, multiple inheritance is not supported in the case of class because of ambiguity. However, it is supported in case of an interface because there is no ambiguity. It is because its implementation is provided by the implementation class. For example:

interface Printable{
void print();
}
interface Showable{
void print();
}

class TestInterface3 implements Printable, Showable{
public void print(){System.out.println("Hello");}
public static void main(String args[]){
TestInterface3 obj = new TestInterface3();
obj.print();
 }
}

Test it Now

Output:

Hello

As we can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class TestTnterface1, so there is no ambiguity in the program.

Interface Inheritance

A class implements an interface, but one interface extends another interface.

File Name: TestInterface4.java

interface Printable{
void print();
}
interface Showable extends Printable{
void show();
}
class TestInterface4 implements Showable{
public void print(){System.out.println("Hello");}
public void show(){System.out.println("Welcome");}

public static void main(String args[]){
TestInterface4 obj = new TestInterface4();
obj.print();
obj.show();
 }
}

Test it Now

Output:

Hello
Welcome

Interfaces and Polymorphism

One of the significant benefits of interfaces is their ability to support polymorphism. Since Java supports interface reference variables, we can use an interface type to refer to any object of a class that implements the interface. It enables flexibility and extensibility in your codebase.

Animal myAnimal = new Dog();
myAnimal.eat();
myAnimal.sleep();

In this example, myAnimal is an interface reference variable of type Animal, referring to a Dog object. It demonstrates polymorphic behavior, where the method calls are resolved at runtime based on the actual object type.

Java 8 Default Method in Interface

Since Java 8, we can have method body in interface. But we need to make it default method. Let's see an example:

File: TestInterfaceDefault.java

interface Drawable{
void draw();
default void msg(){System.out.println("default method");}
}
class Rectangle implements Drawable{
public void draw(){System.out.println("drawing rectangle");}
}
class TestInterfaceDefault{
public static void main(String args[]){
Drawable d=new Rectangle();
d.draw();
d.msg();
}}

Test it Now

Output:

drawing rectangle
default method

Java 8 Static Method in Interface

Since Java 8, we can have static method in interface. Let's see an example:

File: TestInterfaceStatic.java

interface Drawable{
void draw();
static int cube(int x){return x*x*x;}
}
class Rectangle implements Drawable{
public void draw(){System.out.println("drawing rectangle");}
}

class TestInterfaceStatic{
public static void main(String args[]){
Drawable d=new Rectangle();
d.draw();
System.out.println(Drawable.cube(3));
}}

Test it Now

Output:

drawing rectangle
27

Q) What is marker or tagged interface?

An interface which has no member is known as a marker or tagged interface, for example, Serializable, Cloneable, Remote, etc. They are used to provide some essential information to the JVM so that JVM may perform some useful operation.

//How Serializable interface is written?
public interface Serializable{
}

Nested Interface in Java

Note: An interface can have another interface which is known as a nested interface. We will learn it in detail in the nested classes chapter. For example:

interface printable{
 void print();
 interface MessagePrintable{
   void msg();
 }
}

More about Nested Interface

Best Practices

Follow the Single Responsibility Principle (SRP): Design interfaces with a clear and single purpose. Each interface should represent a cohesive set of behaviors.
Use Interfaces for Abstraction: Interfaces should focus on what needs to be done without concerning themselves with how it's done. This promotes abstraction and decouples the implementation from the interface.
Prefer Composition over Inheritance: Interfaces are a key component of achieving composition over inheritance, enabling flexible and modular designs.
Keep Interfaces Lean: Avoid bloating interfaces with unnecessary methods. Keep them concise and focused to make them easier to understand and implement.

Conclusion

Interfaces play a crucial role in Java programming, enabling the creation of modular, extensible, and maintainable codebases. By defining contracts for classes to adhere to, interfaces promote abstraction, polymorphism, and separation of concerns. Understanding how to effectively use interfaces empowers Java developers to write more flexible and scalable applications.

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