What is Dependency Injection

Dependency Injection is style of object configuration in which Object fields and collaborators are set by external entity. Dependency Injection is alternation to having object configure itself.

Dependency Injection design pattern allows us to remove hard-coded  dependency and make our application loosely coupled, extensible and mantainable.

Below class diagram explains Dependency Injection

Traveler:

public class Traveler {
 
 private Vehicle vehicle;
 
 public void setVehicle(Vehicle vehicle) {
  this.vehicle = vehicle;
 }
 
 public void travel(){
  vehicle.start();
  vehicle.run();
  vehicle.stop();
 }
}

Vehicle:

public interface Vehicle {
 void start();
 void run();
 void stop();
}

public class Bike implements Vehicle{

 @Override
 public void start() {
  System.out.println("Bike is started");
 }

 @Override
 public void run() {
  System.out.println("Bike is runnig");
 }
 
 @Override
 public void stop() {
  System.out.println("Bike is stopped");
 }
}

public class Bus implements Vehicle{


 @Override
 public void start() {
  System.out.println("Bus is started");
 }

 @Override
 public void run() {
  System.out.println("Bus is runnig");
 }
 
 @Override
 public void stop() {
  System.out.println("Bus is stopped");
 }
}

public class Car implements Vehicle{

 @Override
 public void start() {
  System.out.println("Car is started");
 }

 @Override
 public void run() {
  System.out.println("Car is runnig");
 }
 
 @Override
 public void stop() {
  System.out.println("Car is stopped");
 }

 
}