Question

Why is it that i must extend Thread in an inner class instead of implementing Runnable
The following code works:-

class Outer{

 Inner innerObj;

 Outer(){
  innerObj = new Inner();
 }

 public void begin(){

  innerObj.start();
 }

 class Inner extends Thread{

  Inner(){
   System.out.println("Thread initialized!");
  }

  public void run(){

    System.out.println("Thread running!");

  }  
 }
}

class Driver{

 public static void main(String[] args){

  Outer o1 = new Outer();
  o1.begin();

 }

}

However using a Runnable causes a compile error:-

class Outer{

 Inner innerObj;

 Outer(){
  innerObj = new Inner();
 }

 public void begin(){

  innerObj.start();
 }

 class Inner implements Runnable{

  Inner(){
   System.out.println("Thread initialized!");
  }

  public void run(){

    System.out.println("Thread running!");

  }  
 }
}

class Driver{

 public static void main(String[] args){

  Outer o1 = new Outer();
  o1.begin();

 }

}

Answer 1

That’s because implementing Runnable doesn’t give a class the start method. You will need to create a Thread to run the Inner.

Inner i = new Inner();
Thread t = new Thread( i );