Creating Thread by Extending Thread class

Creating a new thread is a 3 steps process

Step 1: Create a class by extending the Thread class

     Ex:

     class MyThread extends Thread{

     }

Step 2: Override the run method

Step 3: Call the start method of the Thread

Example

class MyThread extends Thread{
 public void run(){
  for(int i=0; i < 5; i++){
   System.out.println(i);
  }
 }

 public static void main(String args[]){
  MyThread t1 = new MyThread();
  t1.start();
 }
}

 

Output

0
1
2
3
4

Explanation

MyThread t1 = new MyThread();

Statement creates a thread object, and the reference t1 point to the object. When t1.start() is called it immediately call the run method and starts execution.

A Thread is alive until it finishes its execution (A Thread can stop its execution by calling its stop(), anyway it is deprecated).

Can I start a Thread twice ?

No

What will happen if I start a thread twice

Program will compile fine. But java run time throws IllegalThreadStateException at run time.

 Example

class MyThread extends Thread{
 public void run(){
  for(int i=0; i < 4; i++){
   System.out.println(i);
  }
 }

 public static void main(String args[]){
  MyThread t1 = new MyThread();
  t1.start();
  t1.start();
 }
}

Sample Output

0
1
2
3
4
Exception in thread "main" java.lang.IllegalThreadStateException
  at java.lang.Thread.start(Unknown Source)
  at MyThread.main(MyThread.java:13)

You may not see the output like above when you ran the progrm. Since you can't expect the output of an Asynchronous program.

Thread scheduling is non deterministic and depends on many factors, including what else is currently running in the operating system. I.e, you can't guess the outcome of threads.

class MyThread extends Thread{
 public void run(){
  for(int i=0; i < 5; i++){
   System.out.println("In My Thread " + i);
  }
 }

 public static void main(String args[]){
  MyThread t1 = new MyThread();
  t1.start();
  for(int i=0; i<5; i++){
   System.out.println("In Main Thread " + i);
  }
 }
}

   

Possible outcomes are:

Output for first run

In Main Thread 0
In Main Thread 1
In My Thread 0
In My Thread 1
In Main Thread 2
In My Thread 2
In Main Thread 3
In My Thread 3
In Main Thread 4
In My Thread 4

Output for next run

In Main Thread 0
In Main Thread 1
In Main Thread 2
In My Thread 0
In My Thread 1
In Main Thread 3
In My Thread 2
In Main Thread 4
In My Thread 3
In My Thread 4

  

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