About Interface in Java

You must know about Java SE 8 Interface Default and Static ClickMe

Interface is a group of related methods with empty bodies along with below, And most impotent What to do, but don't know how to do? 

Advantage

Members of a Java interface are public by default

Variables declared in a Java interface is by default final

Methods of a Java interface are implicitly abstract and cannot have implementations

Java interface should be implemented using keyword “implements”

An interface can extend another Java interface only

A Java class can implement multiple interfaces

The purpose of interfaces is abstraction, or decoupling from implementation

By interface, we can support the functionality of multiple inheritance

It can be used to achieve loose coupling
Java Interface also represents IS-A relationship

Dis-Advantage

We can’t add additional method later in Interface, if so then need to implement in all implementation class

Writing on basis of below software's:

1. JDK1.6 or +
2. Eclipse
3. Windows OS
4. Basic knowledge of java

Verify the JDK installation

Open the command prompt and verify the java installation
Type the 
C:\>javac -version
javac 1.7.0_25

C:\>java -version
java version "1.7.0_25"
Java(TM) SE Runtime Environment (build 1.7.0_25-b17)
Java HotSpot(TM) Client VM (build 23.25-b01, mixed mode, sharing)

Now java installation got verified, as installed JDK1.7.0_25

Verify the Eclipse installation

Go the the installation directory of eclipse and click on the eclipse.exe

e.g C:\eclipse\eclipse-java-mars-1-win32\eclipse\eclipse.exe

Create the new java project in Eclipse

Open the eclipse
File
New
Java project
Type the project name: Interface_In_Java_Programming

Next
Finish

Java project created in eclipse with directory structure:
 Interface_In_Java_Programming
     - src
     - JRE System Library

How to create Interface in eclipse? 

Right click on the src folder under Interface_In_Java_Programming project
New
Interface

Type the package name: com.atozexamples
Type the name: Calculator
Finish

Calculator.java

package com.atozexamples;

public interface Calculator {

}

Note: Above code are also called tag interface (also called a marker interface) in java, good examples is java.io.Serializable.

public interface Serializable {

}

More about Calculator:

Keyword: interface
We can't user interface as name or package name. 

e.g:
1. com.interface.atzexample
  if we put in package name then we will get error: 
  "Invalid package name. 'interface' is not a valid Java identifier"
2. public interface interface {}
   if we put in interface name then we will get:
   "Syntax error on token "interface", Identifier expected"

Added fields as a and b, and addition method.

package com.atozexamples;

public interface Calculator {

       int a = 10;

       int b = 20;

       int addition(int a, int b);

}

Above Calculator and below Calculator interface is same.

package com.atozexamples;

public interface Calculator {

       public static final int a = 10;

       public static final int b = 20;

       public abstract int addition(int a, int b);

}

if we not include the "public static final" even though javac (compiler) understand and consider filed in interface is public static final.

And method are in interface by default public abstract.

By default method in interface "public abstract".

By default field in interface "public static final".

package com.atozexamples;

public interface Calculator {

       public static final int a = 10;

       public static final int b = 20;

       int c;

       public abstract int addition(int a, int b);

}

if we not initialized the field in interface will get "The blank final field c may not have been initialized".

Multiple inheritance in Java by interface

Add two more interface in Interface_In_Java_Programming.

Printable.java

package com.atozexamples;

public interface Printable {

       void print();

}

Showable.java

package com.atozexamples;

public interface Showable {

 void display();

}

Now add the SimpleCalculator class.

Right click on the src 

New

Class

class name: SimpleCalculator 

Finish

SimpleCalculator.java 

package com.atozexamples;

public class SimpleCalculator {

       public SimpleCalculator() {

       }

       public static void main(String[] args) {

       }

}

 SimpleCalculator class implements Calculator, Printable, Showable interface

package com.atozexamples;

public class SimpleCalculator implements Calculator, Printable, Showable {

       public SimpleCalculator() {

       }

       @Override

       public int addition(int a, int b) {

              return a + b;

       }

       @Override

       public void print() {

              addition(10, 20);

       }

       @Override

       public void display() {

              addition(10, 20);

       }

       public static void main(String[] args) {

              SimpleCalculator obj = new SimpleCalculator();

              obj.addition(10, 20);

              obj.display();

              obj.print();

       }

}

Interface inheritance

Calculator interface extends Printable, Showable 

package com.atozexamples;

public interface Calculator extends Printable, Showable {

       public static final int a = 10;

       public static final int b = 20;

       public abstract int addition(int a, int b);

}

Then SimpleCalculator will only implements Calculator interface 

package com.atozexamples;

public class SimpleCalculator implements Calculator {

       public SimpleCalculator() {

       }

       @Override

       public int addition(int a, int b) {

              return a + b;

       }

       @Override

       public void print() {

              addition(10, 20);

       }

       @Override

       public void display() {

              addition(10, 20);

       }

       public static void main(String[] args) {

              SimpleCalculator obj = new SimpleCalculator();

              obj.addition(10, 20);

              obj.display();

              obj.print();

       }

 }

We will get if not implement any method, which is there in Interface and implemented.

"The type SimpleCalculator must implement the inherited abstract method Showable.display()"

Nested Interface in Java

Calculator interface have Printable interface as nested and SimpleCalculator class implementing the both.

package com.atozexamples;

public class SimpleCalculator implements Calculator, Calculator.Printable {

       public SimpleCalculator() {

       }

       @Override

       public void print() {

              // Print

       }

       @Override

       public int addition(int a, int b) {

              return a + b;

       }

       public static void main(String[] args) {

              SimpleCalculator obj = new SimpleCalculator();

              obj.addition(10, 20);

              obj.print();

       }

}

Happy learning and implementation!!!