How to Run Multiple Threads in Java at the Same Time

Written by Darlene Antonelli, MA

Last Updated: March 1, 2021

This wikiHow will teach you how to run multiple threads in Java. You'll want to run multiple threads to create a program that processes multiple actions at once; the more CPU your computer has, the more processes it can run concurrently.

Steps

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1
Enter the following code:
```
public void run( )
```
- This code provides a beginning point for your multiple threads to run.
2
Enter the following code:
```
Thread(Runnable threadObj, String threadName);
```
- 'threadObj' is the class that starts the runnable thread and 'threadName' is the name of the thread.
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void start();

Use this code after you've fleshed out a thread object and this code will start it.

Your finished code could look like this

class RunnableDemo implements Runnable {
   private Thread t;
   private String threadName;
   
   RunnableDemo( String name) {
      threadName = name;
      System.out.println("Creating " +  threadName );
   }
   
   public void run() {
      System.out.println("Running " +  threadName );
      try {
         for(int i = 4; i > 0; i--) {
            System.out.println("Thread: " + threadName + ", " + i);
            // Let the thread sleep for a while.
            Thread.sleep(50);
         }
      } catch (InterruptedException e) {
         System.out.println("Thread " +  threadName + " interrupted.");
      }
      System.out.println("Thread " +  threadName + " exiting.");
   }
   
   public void start () {
      System.out.println("Starting " +  threadName );
      if (t == null) {
         t = new Thread (this, threadName);
         t.start ();
      }
   }
}

public class TestThread {

   public static void main(String args[]) {
      RunnableDemo R1 = new RunnableDemo( "Thread-1");
      R1.start();
      
      RunnableDemo R2 = new RunnableDemo( "Thread-2");
      R2.start();
   }   
}

4
Execute your code. If you used the coding from the example, the output should read
```
Creating Thread-1
Starting Thread-1
Creating Thread-2
Starting Thread-2
Running Thread-1
Thread: Thread-1, 4
Running Thread-2
Thread: Thread-2, 4
Thread: Thread-1, 3
Thread: Thread-2, 3
Thread: Thread-1, 2
Thread: Thread-2, 2
Thread: Thread-1, 1
Thread: Thread-2, 1
Thread Thread-1 exiting.
Thread Thread-2 exiting.
```