Runnable Interface in Java

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In this article, we will learn about the Runnable interface and its usage with examples.
The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread. The Runnable interface defines a single run() method, which contains a code that executed in the thread. The Runnable object is passed to the Thread constructor.

Runnable Interface Source Code in Java Library

package java.lang;

@FunctionalInterface
public interface Runnable {
     public abstract void run();
}
Note that Runnable interface is a functional interface so we can use Java 8 Lambda Expressions on a Runnable interface.

Runnable Interface Method

The Runnable interface defines a single method, run, meant to contain the code executed in the thread.
  • void run() - When an object implementing interface Runnable is used to create a thread, starting the thread causes the object's run method to be called in that separately executing thread.

Runnable Interface Examples

Let's demonstrate how to use Runnable interface with an example.
First, create a Task or WorkerThread using Runnable interface.

WorkerThread.java

public class WorkerThread implements Runnable {
    private String data;
 
    public WorkerThread(final String anyData) {
        this.data = anyData;
    }
 
    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println("[" + Thread.currentThread().getName() + "] [data=" + 
            this.data + "] Message " + i);
             try {
                  Thread.sleep(200);
             } catch (final InterruptedException e) {
                 e.printStackTrace();
             }
        }
    }
}
Second, let's see how to use WorkerThread in the main thread. in this example the main() method is the main thread which will create another using above WorkerThread class.
public class InstantiateUsingRunnable {
 
    public static void main(final String[] args) {
  
       System.out.println("Thread main started");
  
       final Thread thread1 = new Thread(new WorkerThread("Process data through Runnable interface")); 
       thread1.start();
       thread1.setName("Demo Thread");
  
       System.out.println("Thread main finished");
   }
}
Output:
Thread main started
Thread main finished
[Demo Thread] [data=Process data through Runnable interface] Message 0
[Demo Thread] [data=Process data through Runnable interface] Message 1
[Demo Thread] [data=Process data through Runnable interface] Message 2
[Demo Thread] [data=Process data through Runnable interface] Message 3
[Demo Thread] [data=Process data through Runnable interface] Message 4

Runnable Interface Example using Anonymous Class

Anonymous classes enable you to make your code more concise. They enable you to declare and instantiate a class at the same time:
public class RunnableExampleUsingAnonymousClass {

    public static void main(final String[] args) {
        System.out.println(" main thread started : " + Thread.currentThread().getName());

        System.out.println("Creating Runnable...");

        final Runnable runnable = new Runnable() {
   
            @Override
            public void run() {
                System.out.println("Inside : " + Thread.currentThread().getName()); 
            }
        }; 

        System.out.println("Creating Thread...");
        final Thread thread = new Thread(runnable);

        System.out.println("Starting Thread...");
        thread.start();
        
        System.out.println(" main thread ended : " + Thread.currentThread().getName());
    }
}
Output:
 main thread started : main
Creating Runnable...
Creating Thread...
Starting Thread...
 main thread ended : main
Inside  : Thread-0

Runnable Interface Example Using Lambda Expression

The above example can be made even shorter by using Java 8 Lambda Expressions -
public class RunnableExampleUsingLambda {

    public static void main(final String[] args) {
        System.out.println(" main thread started : " + Thread.currentThread().getName());

        System.out.println("Creating Runnable...");

        final Runnable runnable = () -> System.out.println("Inside  : " + Thread.currentThread().getName()); 

        System.out.println("Creating Thread...");
        final Thread thread = new Thread(runnable);

        System.out.println("Starting Thread...");
        thread.start();
        
        System.out.println(" main thread ended : " + Thread.currentThread().getName());
    }
}
Output:
Creating Runnable...
Creating Thread...
Starting Thread...
 main thread ended : main
Inside  : Thread-0

Reference

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