Java Joining Thread

In the last chapter we have seen, how to use sleep() method in the threading.

Joining the Thread :

join() method Waits for the thread to die.

public final void join()
                throws InterruptedException

Example #1 : Illustration of Java join method

package com.c4learn.thread;
public class ThreadJoin extends Thread {
public void run() {
 for (int i = 1; i < 5; i++) {
   try {
       Thread.sleep(500);
   } catch (InterruptedException e) {
       System.out.println(e);
   }
   String threadName = Thread.currentThread().getName();
   System.out.println(i + " # " + threadName );
 }
}
public static void main(String[] args) 
                   throws InterruptedException {
  ThreadJoin t0 = new ThreadJoin();
  ThreadJoin t1 = new ThreadJoin();
  ThreadJoin t2 = new ThreadJoin();
  t0.start();
  t0.join();
  t1.start(); 
  t2.start(); 
  }
}

Output :

1 # Thread-0
2 # Thread-0
3 # Thread-0
4 # Thread-0
1 # Thread-1
1 # Thread-2
2 # Thread-1
2 # Thread-2
3 # Thread-1
3 # Thread-2
4 # Thread-1
4 # Thread-2

Explanation : join()

Seeing above program may create confusion about the exact working of the join method.

Following table will explain the different meaning of the join method.

Now consider the above program, we have started thread 0 and after starting thread 0 we have called join method for thread 0.

t0.start();
t0.join();
t1.start();
t2.start();

Thus t1 & t2 thread will wait until the thread 0 finishes its execution. Thus in the output we can see first four rows as -

1 # Thread-0
2 # Thread-0
3 # Thread-0
4 # Thread-0

After finishing the t0 thread, t1 starts its execution.

Example #2 : Passing Time Parameter

package com.c4learn.thread;
public class ThreadJoin extends Thread {
public void run() {
 for (int i = 1; i < 5; i++) {
   try {
       Thread.sleep(500);
   } catch (InterruptedException e) {
       System.out.println(e);
   }
   String threadName = Thread.currentThread().getName();
   System.out.println(i + " # " + threadName );
 }
}
public static void main(String[] args) 
                   throws InterruptedException {
  ThreadJoin t0 = new ThreadJoin();
  ThreadJoin t1 = new ThreadJoin();
  ThreadJoin t2 = new ThreadJoin();
  t0.start();
  t0.join(1000);
  t1.start(); 
  t2.start(); 
  }
}

Output :

1 # Thread-0
2 # Thread-0
1 # Thread-1
1 # Thread-2
3 # Thread-0
2 # Thread-1
2 # Thread-2
4 # Thread-0
3 # Thread-1
3 # Thread-2
4 # Thread-1
4 # Thread-2

Explanation :

In this case we have passed parameter to join method.

t0.join(1000);

Thread t0 completes its task for 1000 milliseconds (2 times in this case) then t1 and t2 starts executing.