Interface

• Only Method declaration
• No method body-only method prototype
• In Interface we can declare the variables,variables are by default static in nature
• Variables value will not be changed
• No static method in interface
• No main method in interface
• We can  not create the object of interface
• Interface is abstract in nature 

• Interface is the blue print of class:

           interface Test Int

          {

              //varibles

             //methods

           }

• Interface contains only static and final variables(by default)
• In interface by default methods are public
• In interface methods are by default abstract.

           Abstract:A method have only definition but not implementation

            interface Testint

             {

                   void m1(); //abstract method

             }

             class TestClass implements TestInt

             {

                   void m1()

                {

                      //implement the body

                 }

             }

• We cannot instantiate interface

             TestInt testobj=new Testint();//not valid

             TestInt testobj=new TestClass();//valid

• Multiple Inheritance can be implemented using interface 

Example:

Interface:
public interface USBank {
    int min_bal=100;
    public void credit();
    public void debit();
    public void transferMoney();

 }
public interface BrazilBank {
    public void mutualFund();
}

Implementing Interface:
public class HSBCBank implements USBank,BrazilBank{
//we are achieving multiple inheritance is a relationship
//If a class is implementing interface ,then it is mandatory to define/override all the methods of interface
                                                              //overriding from USBank
    public void credit() {
        System.out.println("HSBCBank----credit");
    }
    public void debit() {
        System.out.println("HSBCBank----debit");
    }
    public void transferMoney() {
        System.out.println("HSBCBank-----transferMoney");
    }
   
                                      //seperate method of HSBCBank class
    public void educationLoan() {
        System.out.println("HSBCBank----eductionLoan");
    }
    public void carLoan() {
        System.out.println("HSBCBank----carLoan");
    }
   
                  //brazil bank method:Override from brazil bank Interface
    public void mutualFund() {
        System.out.println("HSBCBank---mutual fund");
    }
}

Main Method: 
public static void main(String[] args) {
                                                                              //static polymorphism
        HSBCBank hsbc=new HSBCBank();
        hsbc.credit();
        hsbc.debit();
        hsbc.transferMoney();
        hsbc.educationLoan();
        hsbc.carLoan();
      
                              //dynamic polymorphism
                        //child class object can be referred with interface reference var
        USBank usbank=new HSBCBank();
        usbank.credit();
        usbank.debit();
        usbank.transferMoney();
      
    }