Java Thread Example – Extending Thread Class and Implementing Runnable Interface

Processes and Threads are two basic units of execution. Java concurrency programming is more concerned with threads.

Process

A process is a self contained execution environment and it can be seen as a program or application. However a program itself contains multiple processes inside it. Java runtime environment runs as a single process which contains different classes and programs as processes.

Thread

Thread can be called lightweight process. Thread requires less resources to create and exists in the process, thread shares the process resources.

Java Multithreading

Every java application has at least one thread – main thread. Although there are so many other threads running in background like memory management, system management, signal processing etc. But from application point of view – main is the first thread and we can create multiple threads from it.

Multithreading refers to two or more threads executing concurrently in a single program. A computer single core processor can execute only one thread at a time and time slicing is the OS feature to share processor time between different processes and threads.

Benefits of Threads

  1. Threads are lightweight compared to processes, it takes less time and resource to create a thread.
  2. Threads share their parent process data and code
  3. Context switching between threads is usually less expensive than between processes.
  4. Thread intercommunication is relatively easy than process communication.

Java provides two ways to create a thread programmatically.

  1. Implementing the java.lang.Runnable interface.
  2. Extending the java.lang.Thread class.

Java Thread Example by implementing Runnable interface

To make a class runnable, we can implement java.lang.Runnable interface and provide implementation in public void run() method. To use this class as Thread, we need to create a Thread object by passing object of this runnable class and then call start() method to execute the run() method in a separate thread.

Here is a java class example implementing Runnable interface.

package com.journaldev.threads;

public class HeavyWorkRunnable implements Runnable {

    @Override
    public void run() {
        System.out.println("Doing heavy processing - START "+Thread.currentThread().getName());
        try {
            Thread.sleep(1000);
            //Get database connection, delete unused data from DB
            doDBProcessing();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Doing heavy processing - END "+Thread.currentThread().getName());
    }

    private void doDBProcessing() throws InterruptedException {
        Thread.sleep(5000);
    }

}

Java Thread Example by extending Thread class

We can extend java.lang.Thread class to create our own thread class and override run() method. Then we can create it’s object and call start() method to execute our custom thread class run method.

Here is a simple example showing how to extend Thread class.

package com.journaldev.threads;

public class MyThread extends Thread {

    public MyThread(String name) {
        super(name);
    }

    @Override
    public void run() {
        System.out.println("MyThread - START "+Thread.currentThread().getName());
        try {
            Thread.sleep(1000);
            //Get database connection, delete unused data from DB
            doDBProcessing();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("MyThread - END "+Thread.currentThread().getName());
    }

    private void doDBProcessing() throws InterruptedException {
        Thread.sleep(5000);
    }
    
}

Here is a test program showing how to create a thread and execute it.

package com.journaldev.threads;

public class ThreadRunExample {

    public static void main(String[] args){
        Thread t1 = new Thread(new HeavyWorkRunnable(), "t1");
        Thread t2 = new Thread(new HeavyWorkRunnable(), "t2");
        System.out.println("Starting Runnable threads");
        t1.start();
        t2.start();
        System.out.println("Runnable Threads has been started");
        Thread t3 = new MyThread("t3");
        Thread t4 = new MyThread("t4");
        System.out.println("Starting MyThreads");
        t3.start();
        t4.start();
        System.out.println("MyThreads has been started");
        
    }
}

Output of the above java program is:

Starting Runnable threads
Runnable Threads has been started
Doing heavy processing - START t1
Doing heavy processing - START t2
Starting MyThreads
MyThread - START Thread-0
MyThreads has been started
MyThread - START Thread-1
Doing heavy processing - END t2
MyThread - END Thread-1
MyThread - END Thread-0
Doing heavy processing - END t1

Once we start any thread, it’s execution depends on the OS implementation of time slicing and we can’t control their execution. However we can set threads priority but even then it doesn’t guarantee that higher priority thread will be executed first.

Run the above program multiple times and you will see that there is no pattern of threads start and end.

Runnable vs Thread

If your class provides more functionality rather than just running as Thread, you should implement Runnable interface to provide a way to run it as Thread. If your class only goal is to run as Thread, you can extend Thread class.

Implementing Runnable is preferred because java supports implementing multiple interfaces. If you extend Thread class, you can’t extend any other classes.

Tip: As you have noticed that thread doesn’t return any value but what if we want our thread to do some processing and then return the result to our client program, check our Java Callable Future.

Update: From Java 8 onwards, Runnable is a functional interface and we can use lambda expressions to provide it’s implementation rather than using anonymous class. For more details, check out Java 8 Lambda Expressions Tutorial.

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