Signup/Sign In

Java Thread Class

Thread class is the main class on which Java's Multithreading system is based. Thread class, along with its companion interface Runnable will be used to create and run threads for utilizing Multithreading feature of Java.

It provides constructors and methods to support multithreading. It extends object class and implements Runnable interface.

Signature of Thread class

public class Thread extends Object implements Runnable

Thread Class Priority Constants

Field Description
MAX_PRIORITY It represents the maximum priority that a thread can have.
MIN_PRIORITY It represents the minimum priority that a thread can have.
NORM_PRIORITY It represents the default priority that a thread can have.

Constructors of Thread class

  1. Thread()
  2. Thread(String str)
  3. Thread(Runnable r)
  4. Thread(Runnable r, String str)
  5. Thread(ThreadGroup group, Runnable target)
  6. Thread(ThreadGroup group, Runnable target, String name)
  7. Thread(ThreadGroup group, Runnable target, String name, long stackSize)
  8. Thread(ThreadGroup group, String name)

Thread Class Methods

Thread class also defines many methods for managing threads. Some of them are,

setName()to give thread a name
getName()return thread's name
getPriority()return thread's priority
isAlive()checks if thread is still running or not
join()Wait for a thread to end
run()Entry point for a thread
sleep()suspend thread for a specified time
start()start a thread by calling run() method
activeCount()Returns an estimate of the number of active threads in the current thread's thread group and its subgroups.
checkAccess()Determines if the currently running thread has permission to modify this thread.
currentThread()Returns a reference to the currently executing thread object.
dumpStack()Prints a stack trace of the current thread to the standard error stream.
getId()Returns the identifier of this Thread.
getState()Returns the state of this thread.
getThreadGroup()Returns the thread group to which this thread belongs.
interrupt()Interrupts this thread.
interrupted()Tests whether the current thread has been interrupted.
isAlive()Tests if this thread is alive.
isDaemon()Tests if this thread is a daemon thread.
isInterrupted()Tests whether this thread has been interrupted.
setDaemon(boolean on)Marks this thread as either a daemon thread or a user thread.
setPriority(int newPriority)Changes the priority of this thread.
yield()A hint to the scheduler that the current thread is willing to yield its current use of a processor.

Some Important points to Remember

  1. When we extend Thread class, we cannot override setName() and getName() functions, because they are declared final in Thread class.
  2. While using sleep(), always handle the exception it throws.
    static void sleep(long milliseconds) throws InterruptedException

Runnable Interface

It also used to create thread and should be used if you are only planning to override the run() method and no other Thread methods.


public interface Runnable

Runnable Interface Method

It provides only single method that must be implemented by the class.

run()It runs the implemented thread.

Shutdown hook

In Java, Shutdown hook is used to clean-up all the resource, it means closing all the files, sending alerts etc. We can also save the state when the JVM shuts down. Shutdown hook mostly used when any code is to be executed before any JVM shuts down. Following are some of the reasons when the JVM shut down:

  • Pressing ctrl+c on the command prompt
  • When the System.exit(int) method is invoked.
  • When user logoff or shutdown etc

addShutdownHook(Thread hook)

The addShutdownHook(Thread hook) method is used to register the thread with the virtual machine. This method is of Runtime class.


class Demo6 extends Thread
    public void run()
	System.out.println("Shutdown hook task is Now completed...");  

public class ShutdownDemo1
	public static void main(String[] args)throws Exception 

		Runtime obj=Runtime.getRuntime();  
		obj.addShutdownHook(new Demo6());  
		System.out.println("Now main method is sleeping... For Exit press ctrl+c");  
		catch (Exception e) {}  


OutOfMemory Exception

In Java, as we know that all objects are stored in the heap. The objects are created using the new keyword. The OutOfMemoryError occurs as follow:


This error occurs when Java Virtual Machine is not able to allocate the object because it is out of memory and no memory can be available by the garbage collector.

The meaning of OutOfMemoryError is that something wrong is in the program. Many times the problem can be out of control when the third party library caches strings.

Basic program in which OutOfMemoryError can occur


import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class OutOfMemoryDemo1 {
	public static void main(String[] args) {
		Listobj = new ArrayList<>();
		Random obj1= new Random();
		while (true)


Program in which OutOfMemoryError can occur because of low memory


public class OutOfMemoryErrorDemo2
	public static void main(String[] args) 
		Integer[] a = new Integer[100000*10000*1000];


Program in which OutOfMemoryError can occur, when Garbage Collector exceed the limit


import java.util.*; 

public class OutOfMemoryErrorDemo2{ 
public static void main(String args[]) throws Exception 
        Map a = new HashMap(); 
        a = System.getProperties(); 
        Random b = new Random(); 
        while (true) { 
a.put(b.nextInt(), "randomValue"); 
        }   }