Points To Remember

• TimerTask is a class that can run a Thread repeatedly after a specific period.
• You can either implement your own class also by implementing Runnable Interface.
• Then you may need to put your thread to sleep and then resume the thread after a desired interval.

How to schedule a Thread to run after fixed interval

• cancel - to cancel the timer task when needed.
• scheduledExecutionTime - gives the time when the task was last executed successfully.
• run - an abstract method that needs to be implemented.

import java.util.TimerTask;
import java.util.Date;

class MyTask extends TimerTask {

 static int count = 0;
 
 @Override
 public void run(){
  if(count < 5){
     System.out.println(" MyTask called  ### count = "+ (++count));
  }
  else {
     cancel(); 
     System.out.println("#########    TASK CANCELLED    #######");
  }
 }

}

import java.util.TimerTask;
import java.util.Timer;

class TestTimerTask {

 public static void main(String args[]){

   TimerTask task = new MyTask();
   Timer timer = new Timer();
   timer.scheduleAtFixedRate(task,0,1000);

 }

}

• We can implement Runnable interface or extend Thread class to create threads.
• We have to override the run() method to define the functionality of thread.
• We should call start() method to start execution of a thread as a new entity, if we call run() method on a thread then it will not start a new thread.
• We cannot guarantee which thread will execute first even if we prioritize the threads.
• A thread whose run() method is executed completely reaches the dead state and cannot be invoked again.

class Test extends Thread{

  String name;

  public Test(String name){
    this.name = name;
  }

  public static void main(String args[]){  
    System.out.println("main started");

    Thread t1 = new Thread(new Test("Thread t1"));
    Thread t2 = new Thread(new Test("Thread t2"));
    Thread t3 = new Thread(new Test("Thread t3"));
    t1.start();
    t2.start();
    t3.start();

    System.out.println("main ended");

  }

  @Override
  public void run(){
    System.out.println(name);
  }

}

main started
Thread t1
Thread t3
Thread t2
main ended

main started
main ended
Thread t2
Thread t1
Thread t3

main started
main ended
Thread t1
Thread t3
Thread t2

• We can make threads in java by implementing Runnable Interface. Any class that implements this interface must override its run() method.
• The call to start() method internally calls the run() method of the thread.
• We can not guarantee which thread would be executed first. Thread Priority only promises the cpu time and not that it will be run first. 

class Test implements Runnable{
 String msg;

public void run(){
System.out.println("Running Thread" + msg);
}

public Test(String msg){
this.msg = msg;
}

public static void main(String args[]){

System.out.println("Stating main");
Thread t1 = new Thread(new Test("t1"));
Thread t2 = new Thread(new Test("t2"));
t1.start();
t2.start();

System.out.println("Ending main");
}

}

Stating main
Ending main
Running Threadt2
Running Threadt1

Stating main
Ending main
Running Threadt2
Running Threadt1

class Sample implements Runnable{

  String msg;

  public void run(){
     System.out.println("Running Thread" + msg);
  }

  public Sample(String msg){
     this.msg = msg;
  }
}


class Test{

  public static void main(String args[]){

    System.out.println("Stating main");
    Thread t1 = new Thread(new Sample("t1"));
    Thread t2 = new Thread(new Sample("t2"));
    t1.start();
    t2.start();
    System.out.println("Ending main");
  }

}

Stating main
Ending main
Running Threadt1
Running Threadt2

Stating main
Running Threadt1
Ending main
Running Threadt2