Interface in Java

Interface

• In Object Oriented Programming, an interface is a contract.
• It defines a set of methods with specific signatures and any class that implements that interface must implement those methods.
• Example: List and Map

Understanding through Code

Details.java (You can understand the folder path through package)

package com.example.java;    import java.util.List;  import com.example.java.model.Bike;    public interface Details {  	  	public void getDetails(List<Bike> bikes);  //we are not implementing the method here    	public void setColor(String color);    }

Bike.java

package com.example.java.model;    public class Bike {    	public static final String HONDA = "Honda";  	public static final String YAMAHA = "Yamaha";    	private String name = HONDA;  	private String color = "Red";  	private int gearOrNot = 1;      	public Bike(String name, String color, int gearOrNot) {  		this.name = name;   		this.color = color;  		this.gearOrNot = gearOrNot;  	}    	public String getName() { return name; }    	public String getColor() { return color; }    	public int getGearOrNot() { return gearOrNot; }    }  

Honda.java

package com.example.java.model;    public class Honda extends Bike {     	public Honda() {  		super(Bike.HONDA, "Red", 1);  	}      }

 VehicleTypes.java

package com.example.java;    //FOr demonstrating instance method  import java.util.List;  import com.example.java.model.Bike;    public class VehicleTypes implements Details {    	private String colorChoice;    	public void getDetails(List<Bike> bikes) {  //we must define the method of interface    		String colorChoiceOfCustomer = colorChoice;    		for(Bike b: bikes) {  		 	System.out.println(b.getName() +" " + b.getColor() + " " + b.getGearOrNot());  		}    		System.out.println("Color choice is: " +colorChoiceOfCustomer);    	}    	//now we need to either set setColor method as abstract or define it.  	//abstract just means you are putting the signature of method but not defining it.    	public void setColor(String color) {  		colorChoice = color;  	}    }

 Vehicle.java

package com.example.java;    import com.example.java.model.*;	  import java.util.List;  import java.util.ArrayList;    public class Vehicle {  	public static void main( String[] args ){    		List<Bike> bikes = new ArrayList<>();    		bikes.add(new Honda());  		bikes.add(new Bike(Bike.YAMAHA, "Black", 1));  		bikes.add(new Bike(Bike.YAMAHA, "Black", 1));    		//Instead of:  		//VehicleTypes vt = new VehicleTypes();  		//We make use of interface:  		Details vt = new VehicleTypes();  		vt.setColor("White");  		vt.getDetails(bikes);    	}  }

 Execution

• K:\Interface>javac com\example\java\model\Bike.java
• K:\Interface>javac com\example\java\Details.java
• K:\Interface>javac com\example\java\VehicleTypes.java
• K:\Interface>javac com\example\java\Vehicle.java
• K:\Interface>java com.example.java.Vehicle
• Honda Red 1
• Yamaha Black 1
• Yamaha Black 1
• Color choice is: White

Notes

• Instead of setting data-type as VehicleTypes class, we set as Details which implies:
  • VehicleTypes is an instance of Details interface
• We could call setColor() method since VehicleTypes class by implementing the interface promised to define the method.
• If you instantiate interface directly then you need to define all the methods yourself.
  • That is: Details vt1 = new Details();