Thread class provide constructors and methods to create and perform operations on a thread.Thread class extends Object class and implements Runnable interface.

Commonly used Constructors of Thread class:

• Thread()
• Thread(String name)
• Thread(Runnable r)
• Thread(Runnable r,String name)

Commonly used methods of Thread class:

1. public void run(): is used to perform action for a thread.
2. public void start(): starts the execution of the thread.JVM calls the run() method on the thread.
3. public void sleep(long miliseconds): Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds.
4. public void join(): waits for a thread to die.
5. public void join(long miliseconds): waits for a thread to die for the specified miliseconds.
6. public int getPriority(): returns the priority of the thread.
7. public int setPriority(int priority): changes the priority of the thread.
8. public String getName(): returns the name of the thread.
9. public void setName(String name): changes the name of the thread.
10. public Thread currentThread(): returns the reference of currently executing thread.
11. public int getId(): returns the id of the thread.
12. public Thread.StategetState(): returns the state of the thread.
13. public booleanisAlive(): tests if the thread is alive.
14. public void yield(): causes the currently executing thread object to temporarily pause and allow other threads to execute.
15. public void suspend(): is used to suspend the thread(depricated).
16. public void resume(): is used to resume the suspended thread(depricated).
17. public void stop(): is used to stop the thread(depricated).
18. public booleanisDaemon(): tests if the thread is a daemon thread.
19. public void setDaemon(boolean b): marks the thread as daemon or user thread.
20. public void interrupt(): interrupts the thread.
21. public booleanisInterrupted(): tests if the thread has been interrupted.
22. public static booleaninterrupted(): tests if the current thread has been interrupted.

Runnable interface:

The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread. Runnable interface have only one method named run().

1. public void run(): is used to perform action for a thread.
Starting a thread:
start() method of Thread class is used to start a newly created thread. It performs following tasks:
• A new thread starts(with new callstack).
• The thread moves from New state to the Runnable state.
• When the thread gets a chance to execute, its target run() method will run.

1) Java Thread Example by extending Thread class

1. class Multi extends Thread{ 
2. public void run(){ 
3. System.out.println("thread is running..."); 
4. } 
5. public static void main(String args[]){ 
6. Multi t1=new Multi(); 
7. t1.start(); 
8. } 
9. }

Output:thread is running…

2) Java Thread Example by implementing Runnable interface

1. class Multi3 implements Runnable{ 
2. public void run(){ 
3. System.out.println("thread is running..."); 
4. } 
5. 
6. public static void main(String args[]){ 
7. Multi3 m1=new Multi3(); 
8. Thread t1 =new Thread(m1); 
9. t1.start(); 
10. } 
11. }

Output:thread is running…

If you are not extending the Thread class,your class object would not be treated as a thread object.So you need to explicitely create Thread class object.We are passing the object of your class that implements Runnable so that your class run() method may execute.

Thread Scheduler in Java

Thread scheduler in java is the part of the JVM that decides which thread should run.
There is no guarantee that which runnable thread will be chosen to run by the thread scheduler.
Only one thread at a time can run in a single process.
The thread scheduler mainly uses preemptive or time slicing scheduling to schedule the threads.

Difference between preemptive scheduling and time slicing

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Sleep method in java

The sleep() method of Thread class is used to sleep a thread for the specified amount of time.
Syntax of sleep() method in java
The Thread class provides two methods for sleeping a thread:
• public static void sleep(long miliseconds)throws InterruptedException
• public static void sleep(long miliseconds, int nanos)throws InterruptedException

Example of sleep method in java

1. class TestSleepMethod1 extends Thread{ 
2. public void run(){ 
3. for(int i=1;i<5;i++){ 
4. try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e);} 
5. System.out.println(i); 
6. } 
7. } 
8. public static void main(String args[]){ 
9. TestSleepMethod1 t1=new TestSleepMethod1(); 
10. TestSleepMethod1 t2=new TestSleepMethod1(); 
11. 
12. t1.start(); 
13. t2.start(); 
14. } 
15. }

Output:
1
1
2
2
3
3
4
4

As you know well that at a time only one thread is executed. If you sleep a thread for the specified time,the thread shedular picks up another thread and so on.

Can we start a thread twice

No. After starting a thread, it can never be started again. If you does so, an IllegalThreadStateException is thrown. In such case, thread will run once but for second time, it will throw exception.
Let’s understand it by the example given below:

1. public class TestThreadTwice1 extends Thread{ 
2. public void run(){ 
3. System.out.println("running..."); 
4. } 
5. public static void main(String args[]){ 
6. TestThreadTwice1 t1=new TestThreadTwice1(); 
7. t1.start(); 
8. t1.start(); 
9. } 
10. }

running
Exception in thread “main” java.lang.IllegalThreadStateException

What if we call run() method directly instead start() method?

• Each thread starts in a separate call stack.
• Invoking the run() method from main thread, the run() method goes onto the current call stack rather than at the beginning of a new call stack.

1. class TestCallRun1 extends Thread{ 
2. public void run(){ 
3. System.out.println("running..."); 
4. } 
5. public static void main(String args[]){ 
6. TestCallRun1 t1=new TestCallRun1(); 
7. t1.run();//fine, but does not start a separate call stack 
8. } 
9. }

Output:running…

java 45

Problem if you direct call run() method

1. class TestCallRun2 extends Thread{ 
2. public void run(){ 
3. for(int i=1;i<5;i++){ 
4. try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e);} 
5. System.out.println(i); 
6. } 
7. } 
8. public static void main(String args[]){ 
9. TestCallRun2 t1=new TestCallRun2(); 
10. TestCallRun2 t2=new TestCallRun2(); 
11. 
12. t1.run(); 
13. t2.run(); 
14. } 
15. }

Output:1
2
3
4
5
1
2
3
4
5

As you can see in the above program that there is no context-switching because here t1 and t2 will be treated as normal object not thread object.

The join() method

The join() method waits for a thread to die. In other words, it causes the currently running threads to stop executing until the thread it joins with completes its task.
Syntax:
public void join()throws InterruptedException
public void join(long milliseconds)throws InterruptedException
Example of join() method

1. class TestJoinMethod1 extends Thread{ 
2. public void run(){ 
3. for(int i=1;i<=5;i++){ 
4. try{ 
5. Thread.sleep(500); 
6. }catch(Exception e){System.out.println(e);} 
7. System.out.println(i); 
8. } 
9. } 
10. public static void main(String args[]){ 
11. TestJoinMethod1 t1=new TestJoinMethod1(); 
12. TestJoinMethod1 t2=new TestJoinMethod1(); 
13. TestJoinMethod1 t3=new TestJoinMethod1(); 
14. t1.start(); 
15. try{ 
16. t1.join(); 
17. }catch(Exception e){System.out.println(e);} 
18. 
19. t2.start(); 
20. t3.start(); 
21. } 
22. }

Output:1
2
3
4
5
1
1
2
2
3
3
4
4
5
5

As you can see in the above example,when t1 completes its task then t2 and t3 starts executing.

Example of join(long miliseconds) method

1. class TestJoinMethod2 extends Thread{ 
2. public void run(){ 
3. for(int i=1;i<=5;i++){ 
4. try{ 
5. Thread.sleep(500); 
6. }catch(Exception e){System.out.println(e);} 
7. System.out.println(i); 
8. } 
9. } 
10. public static void main(String args[]){ 
11. TestJoinMethod2 t1=new TestJoinMethod2(); 
12. TestJoinMethod2 t2=new TestJoinMethod2(); 
13. TestJoinMethod2 t3=new TestJoinMethod2(); 
14. t1.start(); 
15. try{ 
16. t1.join(1500); 
17. }catch(Exception e){System.out.println(e);} 
18. 
19. t2.start(); 
20. t3.start(); 
21. } 
22. }

Output:1
2
3
1
4
1
2
5
2
3
3
4
4
5
5

In the above example,when t1 is completes its task for 1500 miliseconds(3 times) then t2 and t3 starts executing.

getName(),setName(String) and getId() method:

public String getName()
public void setName(String name)
public long getId()

1. class TestJoinMethod3 extends Thread{ 
2. public void run(){ 
3. System.out.println("running..."); 
4. } 
5. public static void main(String args[]){ 
6. TestJoinMethod3 t1=new TestJoinMethod3(); 
7. TestJoinMethod3 t2=new TestJoinMethod3(); 
8. System.out.println("Name of t1:"+t1.getName()); 
9. System.out.println("Name of t2:"+t2.getName()); 
10. System.out.println("id of t1:"+t1.getId()); 
11. 
12. t1.start(); 
13. t2.start(); 
14. 
15. t1.setName("Sonoo Jaiswal"); 
16. System.out.println("After changing name of t1:"+t1.getName()); 
17. } 
18. }

Output:Name of t1:Thread-0
Name of t2:Thread-1
id of t1:8
running…
After changling name of t1:Sonoo Jaiswal
running…

The currentThread() method:

The currentThread() method returns a reference to the currently executing thread object.
Syntax:
public static Thread currentThread()
Example of currentThread() method

1. class TestJoinMethod4 extends Thread{ 
2. public void run(){ 
3. System.out.println(Thread.currentThread().getName()); 
4. } 
5. } 
6. public static void main(String args[]){ 
7. TestJoinMethod4 t1=new TestJoinMethod4(); 
8. TestJoinMethod4 t2=new TestJoinMethod4(); 
9. 
10. t1.start(); 
11. t2.start(); 
12. } 
13. }

Output:Thread-0
Thread-1

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