Synchronized Thread in Java

Question

I have 3 class like this:

Source.java

public class Source extends Thread{
    private int x= 0;
    public void increment(int id){
          x++;
          System.out.println(id+" "+x);
    }
}

Task.java

public class Task extends Thread{
      private Source source;
      private int id;
      public Task(Source source, int id){
            this.source=source;
            this.id=id;
      }
      public void run(){
             for (int i=0;i<100;i++){ 
                 try{Thread.sleep(1000);}catch(InterruptedException e){}
                 source.inc(id);
             }
      }
}

Main.java

public class Main{
      public static void main(String[] args) throws IOException{
             Source source = new Source();
             Task t1=new Task(source,1);
             Task t2=new Task(source,2);
             t1.start();
             t2.start();
      }
}

I want when the x of the class Source will be equal to 4 only one task continues to increment x until x is equal to 8, we return to normal. The result will look like this:

1 1
2 2
1 3
2 4
1 5
1 6
1 7
1 8
1 9
1 10
2 11
1 12
2 13
...

How do I modify the code to achieve the desired result?

Answer 1

Basically you have two threads that modify the same variable x. There is no garantee about the order of execution. You should synchronize.

With your current implementation you may face a problem (The race condition problem): Race condition example

Something like this is an scenario that most likely is going to happen to you:

....
1 3
2 4
2 5
1 6
1 7
2 7
1 8
2 9
1 10
2 10
...

As you can see the thread 2 (source 2) tries to increment the variable x when the variable has been already incremented but the value it has to increment is the old one.

x = 0

1. Thread 1 reads the variable x (0)
2. Thread 2 reads the variable x (0)
3. Thread 1 increments variable x + 1 (0 + 1) = 1
4. Thread 2 increments variable x + 1 (0 + 1) = 1

In order to solve this you need to synchronize your variable, an AtomicInteger would be enough. + I don't think you need the extends Thread on your Source class, you can get rid of it