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'stak queue 
            System.out.println(timer.purge()); 
        } 
    } 
} 

Output:

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

Reference:

Oracle

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