Question

class Counter{

  private int count = 0;

  public void inc(){

   count++;  

  }

  public int get(){

    return count;

  }

}

class Worker extends Thread{

Counter count;

  Worker(Counter count){

    this.count = count;

  }

  public void run(){

    for (int i = 0; i < 1000;i++){

      synchronized(this){

        count.inc();

      }}

  }

}

public class Test

{  

  public static void main(String args[]) throws InterruptedException

  {

    Counter c = new Counter();

    Worker w1 = new Worker(c);

    Worker w2 = new Worker(c);

    w1.start();

    w2.start();

    w1.join();

    w2.join();

    System.out.println(c.get());

     

  }

}

The above code should print 2000 because inc() method is synchronized. However, it is printing a different number ranging from 1000 to 2000 everytime it runs. Can you explain why? How would you fix it?

Answer 1

This is beacause the inc() method is not synchronized in Counter class. When both threads try to execute count++ simultaneously, there may be an anomaly. Hence we are getting a different answer each time. If we synchronize this method or only the statement inside it, it will print 2000 everytime.

The following code will always print 2000.

class Counter{

private int count = 0;

public void inc(){
synchronized(this){
  
count++;
  
}

}

public int get(){

return count;

}

}

class Worker extends Thread{

Counter count;

Worker(Counter count){

this.count = count;

}

public void run(){

for (int i = 0; i < 1000;i++){

synchronized(this){

count.inc();

}}

}

}

public class Test

{

public static void main(String args[]) throws InterruptedException

{

Counter c = new Counter();

Worker w1 = new Worker(c);

Worker w2 = new Worker(c);

w1.start();

w2.start();

w1.join();

w2.join();

System.out.println(c.get());

}

}