Interface in Java
1. Interface
2. Example of Interface
3. Multiple inheritance by Interface
4. Why multiple inheritance is supported in Interface while it is not supported in case of
class.
5. Marker Interface
6. Nested Interface
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.
o It is used to achieve abstraction.
o By interface, we can support the functionality of multiple inheritance.
o 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:
1. interface <interface_name>{
2.
3. // declare constant fields
4. // declare methods that are abstract
5. // by default.
6. }
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.
The 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.
Java Interface Example
In this example, the Printable interface has only one method, and its implementation is
provided in the A6 class.
1. interface printable{
2. void print();
3. }
4. class A6 implements printable{
5. public void print(){System.out.println("Hello");}
6.
7. public static void main(String args[]){
8. A6 obj = new A6();
9. obj.print();
10. }
11. }
Test it Now
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
1. //Interface declaration: by first user
2. interface Drawable{
3. void draw();
4. }
5. //Implementation: by second user
6. class Rectangle implements Drawable{
7. public void draw(){System.out.println("drawing rectangle");}
8. }
9. class Circle implements Drawable{
10. public void draw(){System.out.println("drawing circle");}
11. }
12. //Using interface: by third user
13. class TestInterface1{
14. public static void main(String args[]){
15. Drawable d=new Circle();//In real scenario, object is provided by method e.g. getDrawa
ble()
16. d.draw();
17. }}
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
1. interface Bank{
2. float rateOfInterest();
3. }
4. class SBI implements Bank{
5. public float rateOfInterest(){return 9.15f;}
6. }
7. class PNB implements Bank{
8. public float rateOfInterest(){return 9.7f;}
9. }
10. class TestInterface2{
11. public static void main(String[] args){
12. Bank b=new SBI();
13. System.out.println("ROI: "+b.rateOfInterest());
14. }}
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.
1. interface Printable{
2. void print();
3. }
4. interface Showable{
5. void show();
6. }
7. class A7 implements Printable,Showable{
8. public void print(){System.out.println("Hello");}
9. public void show(){System.out.println("Welcome");}
10.
11. public static void main(String args[]){
12. A7 obj = new A7();
13. obj.print();
14. obj.show();
15. }
16. }
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:
1. interface Printable{
2. void print();
3. }
4. interface Showable{
5. void print();
6. }
7.
8. class TestInterface3 implements Printable, Showable{
9. public void print(){System.out.println("Hello");}
10. public static void main(String args[]){
11. TestInterface3 obj = new TestInterface3();
12. obj.print();
13. }
14. }
Test it Now
Output:
Hello
As you 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.
Interface inheritance
A class implements an interface, but one interface extends another interface.
1. interface Printable{
2. void print();
3. }
4. interface Showable extends Printable{
5. void show();
6. }
7. class TestInterface4 implements Showable{
8. public void print(){System.out.println("Hello");}
9. public void show(){System.out.println("Welcome");}
10.
11. public static void main(String args[]){
12. TestInterface4 obj = new TestInterface4();
13. obj.print();
14. obj.show();
15. }
16. }
Test it Now
Output:
Hello
Welcome
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
1. interface Drawable{
2. void draw();
3. default void msg(){System.out.println("default method");}
4. }
5. class Rectangle implements Drawable{
6. public void draw(){System.out.println("drawing rectangle");}
7. }
8. class TestInterfaceDefault{
9. public static void main(String args[]){
10. Drawable d=new Rectangle();
11. d.draw();
12. d.msg();
13. }}
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
1. interface Drawable{
2. void draw();
3. static int cube(int x){return x*x*x;}
4. }
5. class Rectangle implements Drawable{
6. public void draw(){System.out.println("drawing rectangle");}
7. }
8.
9. class TestInterfaceStatic{
10. public static void main(String args[]){
11. Drawable d=new Rectangle();
12. d.draw();
13. System.out.println(Drawable.cube(3));
14. }}
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.
1. //How Serializable interface is written?
2. public interface Serializable{
3. }
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:
1. interface printable{
2. void print();
3. interface MessagePrintable{
4. void msg();
5. }
6. }
Java Nested Interface
An interface i.e. declared within another interface or class is known as nested interface.
The nested interfaces are used to group related interfaces so that they can be easy to
maintain. The nested interface must be referred by the outer interface or class. It can't
be accessed directly.
Points to remember for nested interfaces
There are given some points that should be remembered by the java programmer.
o Nested interface must be public if it is declared inside the interface but it can
have any access modifier if declared within the class.
o Nested interfaces are declared static implicitly.
Syntax of nested interface which is declared within the interface
1. interface interface_name{
2. ...
3. interface nested_interface_name{
4. ...
5. }
6. }
Syntax of nested interface which is declared within the class
1. class class_name{
2. ...
3. interface nested_interface_name{
4. ...
5. }
6. }
Example of nested interface which is declared within the
interface
In this example, we are going to learn how to declare the nested interface and how we can
access it.
1. interface Showable{
2. void show();
3. interface Message{
4. void msg();
5. }
6. }
7. class TestNestedInterface1 implements Showable.Message{
8. public void msg(){System.out.println("Hello nested interface");}
9.
10. public static void main(String args[]){
11. Showable.Message message=new TestNestedInterface1();//upcasting here
12. message.msg();
13. }
14. }
Output:hello nested interface
As you can see in the above example, we are accessing the Message interface by its outer
interface Showable because it cannot be accessed directly. It is just like a cupboard inside the
room, we cannot access the cupborad directly because we must enter the room first.
Internal code generated by the java compiler for nested
interface Message
The java compiler internally creates public and static interface as displayed below:.
1. public static interface Showable$Message
2. {
3. public abstract void msg();
4. }
Example of nested interface which is declared within the class
Let's see how can we define an interface inside the class and how can we access it.
1. class A{
2. interface Message{
3. void msg();
4. }
5. }
6.
7. class TestNestedInterface2 implements A.Message{
8. public void msg(){System.out.println("Hello nested interface");}
9.
10. public static void main(String args[]){
11. A.Message message=new TestNestedInterface2();//upcasting here
12. message.msg();
13. }
14. }
Output:hello nested interface
Can we define a class inside the interface?
Yes, If we define a class inside the interface, java compiler creates a static nested class.
Let's see how can we define a class within the interface:
1. interface M{
2. class A{}
3. }
