Java.util.Timer Class in Java

Timer class provides a method call that is used by a thread to schedule a task, such as running a block of code after some regular instant of time. Each task may be scheduled to run once or for a repeated number of executions. Each timer object is associated with a background thread that is responsible for the execution of all the tasks of a timer object. 
Note: 

• Timer class is thread-safe.
• Timer class uses binary heap data structure in order to store its task.

Constructors: 

• Timer(): Creates a new timer
• Timer(boolean isDaemon): Creates a new timer whose associated thread may be specified to run as a daemon
• Timer(String name): Creates a new timer whose associated thread has the specified name
• Timer(String name, boolean isDaemon): Creates a new timer whose associated thread has the specified name, and may be specified to run as a daemon

Declaration:  

public class Timer
        extends Object

Methods inherited from class java.lang.Object

• clone
• equals
• finalize
• getClass
• hashCode
• notify
• notifyAll
• toString
• wait

Methods: 

• cancel(): java.util.Timer.cancel() Terminates this timer, discarding any currently scheduled tasks. Does not interfere with a currently executing task (if it exists). Once a timer has been terminated, its execution thread terminates gracefully, and no more tasks may be scheduled on it 
  Syntax: 

public void cancel()

• purge(): java.util.Timer.purge() Removes all cancelled tasks from this timer's task queue 
  Syntax: 

public int purge()
Returns:
the number of tasks removed from the queue

• schedule(TimerTask task, Date time): java.util.Timer.schedule(TimerTask task, Date time) Schedules the specified task for execution at the specified time 
  Syntax: 

public void schedule(TimerTask task, Date time)
Parameters:
task - task to be scheduled.
time - time at which task is to be executed.
Throws:
IllegalArgumentException - if time.getTime() is negative.
IllegalStateException - if the task was already scheduled or cancelled, 
the timer was cancelled, or timer thread terminated.
NullPointerException - if task or time is null

• schedule(TimerTask task, Date firstTime, long period): java.util.Timer.schedule(TimerTask task, Date firstTime, long period) Schedules the specified task for repeated fixed-delay execution, beginning at the specified time 
  Syntax: 

public void schedule(TimerTask task, Date firstTime, long period)
Parameters:
task - task to be scheduled.
firstTime - First time at which task is to be executed.
period - time in milliseconds between successive task executions.
Throws:
IllegalArgumentException - if firstTime.getTime() < 0,
 or period <= 0
IllegalStateException - if task was already scheduled 
or cancelled, timer was cancelled, 
or timer thread terminated.
NullPointerException - if task or firstTime is null

// Java program to demonstrate
//schedule method calls of Timer class

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

class Helper extends TimerTask
{
    public static int i = 0;
    public void run()
    {
        System.out.println("Timer ran " + ++i);
    }
}

public class Test
{
    public static void main(String[] args)
    {
        
        Timer timer = new Timer();
        TimerTask task = new Helper();
        
        timer.schedule(task, 2000, 5000);
        
    }
}
 

Output: 

Timer ran 1
Timer ran 2
Timer ran 3
Timer ran 4
Timer ran 5
.
.
.

• schedule(TimerTask task, long delay): java.util.Timer.schedule(TimerTask task, long delay) Schedules the specified task for execution after the specified delay 
  Syntax: 

public void schedule(TimerTask task, long delay)
Parameters:
task - task to be scheduled.
delay - delay in milliseconds before task is to be executed.
Throws:
IllegalArgumentException - if delay is negative,
or delay + System.currentTimeMillis() is negative.
IllegalStateException - if a task was already scheduled 
or cancelled, the timer was cancelled, 
or timer thread terminated.
NullPointerException - if task is null

• schedule(TimerTask task, long delay, long period): java.util.Timer.schedule(TimerTask task, long delay, long period) Schedules the specified task for repeated fixed-delay execution, beginning after the specified delaySyntax: 
   

public void schedule(TimerTask task, long delay, long period)
Parameters:
task - task to be scheduled.
delay - delay in milliseconds before task is to be executed.
period - time in milliseconds between successive task executions.
Throws:
IllegalArgumentException - if delay < 0, 
or delay + System.currentTimeMillis() < 0, or 
period <= 0
IllegalStateException - if task was already scheduled 
or cancelled, timer was cancelled, 
or timer thread terminated.
NullPointerException - if task is null

• scheduleAtFixedRate(TimerTask task, Date firstTime, long period): java.util.Timer.scheduleAtFixedRate(TimerTask task, Date firstTime, long period) Schedules the specified task for repeated fixed-rate execution, beginning at the specified timeSyntax: 
   

public void scheduleAtFixedRate(TimerTask task, Date firstTime, long period)
Parameters:
task - task to be scheduled.
firstTime - First time at which task is to be executed.
period - time in milliseconds between successive task executions.
Throws:
IllegalArgumentException - if firstTime.getTime() <
0 or period <= 0
IllegalStateException - if task was already scheduled
 or cancelled, timer was cancelled, 
or timer thread terminated.
NullPointerException - if task or firstTime is null

• scheduleAtFixedRate(TimerTask task, long delay, long period): java.util.Timer.scheduleAtFixedRate(TimerTask task, long delay, long period) Schedules the specified task for repeated fixed-rate execution, beginning after the specified delaySyntax: 
   

public void scheduleAtFixedRate(TimerTask task, long delay, long period)
Parameters:
task - task to be scheduled.
delay - delay in milliseconds before task is to be executed.
period - time in milliseconds between successive task executions.
Throws:
IllegalArgumentException - if delay < 0, 
or delay + System.currentTimeMillis() < 0, or 
period <= 0
IllegalStateException - if task was already 
scheduled or cancelled, timer was cancelled, 
or timer thread terminated.
NullPointerException - if task is null

// Java program to demonstrate
// scheduleAtFixedRate method of Timer class

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


class Helper extends TimerTask
{
    public static int i = 0;
    public void run()
    {
        System.out.println("Timer ran " + ++i);
        if(i == 4)
        {
            synchronized(Test.obj)
            {
                Test.obj.notify();
            }
        }
    }
    
}


public class Test
{
    protected static Test obj;
    public static void main(String[] args) throws InterruptedException
    {
        obj = new Test();
        
        //creating a new instance of timer class
        Timer timer = new Timer();
        TimerTask task = new Helper();

        //instance of date object for fixed-rate execution
        Date date = new Date();

        timer.scheduleAtFixedRate(task, date, 5000);
        
        System.out.println("Timer running");
        synchronized(obj)
        {
            //make the main thread wait
            obj.wait();
            
            //once timer has scheduled the task 4 times, 
            //main thread resumes
            //and terminates the timer
            timer.cancel();
            
            //purge is used to remove all cancelled 
            //tasks from the timer'stack queue
            System.out.println(timer.purge());
        }
    }
}

Output: 

Timer running
Timer ran 1
Timer ran 2
Timer ran 3
Timer ran 4
0

Reference:  

• Oracle