"implements Runnable" vs "extends Thread" in Java

Question

From what time I've spent with threads in Java, I've found these two ways to write threads:

With implements Runnable:

public class MyRunnable implements Runnable {
    public void run() {
        //Code
    }
}
//Started with a "new Thread(new MyRunnable()).start()" call

Or, with extends Thread:

public class MyThread extends Thread {
    public MyThread() {
        super("MyThread");
    }
    public void run() {
        //Code
    }
}
//Started with a "new MyThread().start()" call

Is there any significant difference in these two blocks of code?

Answer 1

If I am not wrong, it's more or less similar to

What is the difference between an interface and abstract class?

extends establishes "Is A" relation & interface provides "Has a" capability.

Prefer implements Runnable :

1. If you don't have to extend Thread class and modify Thread API default implementation
2. If you are executing a fire and forget command
3. If You are already extending another class

Prefer "extends Thread" :

1. If you have to override any of these Thread methods as listed in oracle documentation page

Generally you don't need to override Thread behaviour. So implements Runnable is preferred for most of the times.

If you need some customization for this Thread API, you can extend Thread and modify implementation

Thread(ThreadGroup group, Runnable target, String name, long stackSize)

Allocates a new Thread object so that it has target as its run object, has the specified name as its name, and belongs to the thread group referred to by group, and has the specified stack size.

On a different note, using advanced ExecutorService or ThreadPoolExecutorService API provides more flexibility and control.

Have a look at this SE Question:

ExecutorService vs Casual Thread Spawner

Answer 2

tl;dr: implements Runnable is better. However, the caveat is important.

In general, I would recommend using something like Runnable rather than Thread because it allows you to keep your work only loosely coupled with your choice of concurrency. For example, if you use a Runnable and decide later on that this doesn't in fact require its own Thread, you can just call threadA.run().

Caveat: Around here, I strongly discourage the use of raw Threads. I much prefer the use of Callables and FutureTasks (From the javadoc: "A cancellable asynchronous computation"). The integration of timeouts, proper cancelling and the thread pooling of the modern concurrency support are all much more useful to me than piles of raw Threads.

Follow-up: There is a FutureTask constructor that allows you to use Runnables (if that's what you are most comfortable with) and still get the benefit of the modern concurrency tools. To quote the javadoc:

If you don't need a particular result, consider using constructions of the form:

Future<?> f = new FutureTask<Object>(runnable, null)

So, if we replace their runnable with your threadA, we get the following:

new FutureTask<Object>(threadA, null)

Another option that allows you to stay closer to Runnables is a ThreadPoolExecutor. You can use the execute method to pass in a Runnable to execute "the given task sometime in the future".

If you'd like to try using a thread pool, the code fragment above would become something like the following (using the Executors.newCachedThreadPool() factory method):

ExecutorService es = Executors.newCachedThreadPool();
es.execute(new ThreadA());

Answer 3

If you want to separate your concerns like, if you want to create Tasks and it does not matter to you who is the worker working on your task then use Runnable and define your work as a task. Just like when you use executor service, you define tasks in form of Runnable/callable and give them to executor service which will then hold the responsibility of workers.

If you want to create a worker by yourself and assign that worker your task, you want more control over the worker thread then use Thread class.

Answer 4

Yes: implements Runnable is the preferred way to do it, IMO. You're not really specialising the thread's behaviour. You're just giving it something to run. That means composition is the philosophically "purer" way to go.

In practical terms, it means you can implement Runnable and extend from another class as well... and you can also implement Runnable via a lambda expression as of Java 8.

Answer 5

1. Extending the thread interface, is like you are making your class to behave as a thread only. Your new class will be like a enhanced thread.

jshell> public class Test extends Thread{
   ...> public Test(String name){
   ...> super(name);
   ...> }
   ...> public void run(){
   ...> System.out.println(Thread.currentThread().getName());
   ...> }
   ...> }
|  created class Test

jshell> Test t1=new Test("MyThread");
t1 ==> Thread[MyThread,5,main]

It creates a thread, not the Test object. So it's going to act like a single thread. You can not share the instance of Test class between threads.

2. Implementing the runnable interface.

jshell> public class Test1 implements Runnable{
   ...> public void run(){
   ...> System.out.println(Thread.currentThread().getName());
   ...> }
   ...> public String getName(){
   ...> return "testing";}
   ...> }
|  created class Test1

jshell> Test1 t1=new Test1();
t1 ==> Test1@396a51ab  --> this creates Test1 object.

This object can be shared across threads by,

jshell> Thread t1=new Thread(t1,"Hai");
t ==> Thread[Hai,5,main]

jshell> Thread t=new Thread(t1,"Hai");
t ==> Thread[Hai,5,main]

I think there's already been a lot that's been discussed on this topic, thought this might be helpful around the basics.

Answer 6

If you want to implements or extends any other class then Runnable interface is most preferable, otherwise, if you do not want any other class to extend or implement then Thread class is preferable.

The most common difference is

When you extends Thread class, after that you can’t extend any other class which you required. (As you know, Java does not allow inheriting more than one class).

When you implements Runnable, you can save space for your class to extend any other class in the future or now.

• Java doesn't support multiple inheritances, which means you can only extend one class in Java so once you extended Thread class you lost your chance and cannot extend or inherit another class in Java.

• In Object-oriented programming, extending a class generally means, adding new functionality, and modifying or improving behaviors. If we are not making any modification on Thread then use Runnable interface instead.

• Runnable interface represents a Task which can be executed by either plain Thread or Executors or any other means. so logical separation of Task as Runnable than Thread is a good design decision.

• Separating task as Runnable means we can reuse the task and also has the liberty to execute it from different means. since you can not restart a Thread once it completes. again Runnable vs Thread for task, Runnable is winner.

• Java designer recognizes this and that's why Executors accept Runnable as Task and they have worker thread which executes those task.

• Inheriting all Thread methods are additional overhead just for representing a Task which can be done easily with Runnable.

Courtesy from javarevisited.blogspot.com

These were some of the notable differences between Thread and Runnable in Java. If you know any other differences on Thread vs Runnable than please share it via comments. I personally use Runnable over Thread for this scenario and recommends to use Runnable or Callable interface based on your requirement.

However, the significant difference is.

When you extends Thread class, each of your thread creates a unique object and associate with it. When you implements Runnable, it shares the same object to multiple threads.

Answer 7

The simplest explanation would be by implementing Runnable we can assign the same object to multiple threads and each Thread shares the same object states and behavior.

For example, suppose there are two threads, thread1 puts an integer in an array and thread2 takes integers from the array when the array is filled up. Notice that in order for thread2 to work it needs to know the state of array, whether thread1 has filled it up or not.

Implementing Runnable lets you to have this flexibility to share the object whereas extends Thread makes you to create new objects for each threads therefore any update that is done by thread1 is lost to thread2.

Answer 8

The main difference between Thread and Runnable is: - Thread is like: Worker (execute Runnable) - Runnable is like: Job (to be executed by Thread)

Answer 9

I would say actual task is decoupled from the thread. We can pass around the task to Thread, Executor framework etc in case of Runnable, whereas with extending Thread task is coupled with thread object itself. The task isolation cannot be done in case of extending Thread. It's like we burn the task to Thread object just something like IC chip (and more specifically will not get any handle to task).

Answer 10

You can use them jointly

Example :

public class A implements Runnable{

    @Override
    public void run() {


        while(true){
             System.out.println("Class A is running");
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }}

    }

}



public class Test {

    public static void main(String[] args) {

        Thread myThread =new Thread(new A());// 1
        myThread.start();
        System.out.println(" executed after thread A");//will never be reached

    }

} 

Answer 11

Thread class defines several methods that can be overriden by the the extending class. But to create a thread we must override the run() method. Same applies to Runnable as well.

However Runnable is a preferred method of creating a thread. The primary reasons are:

1. Since Runnable is an interface, you can extend other classes. But if you extend Thread then that option is gone.

2. If you are not modifying or enhancing a whole lot of Thread functionalities and extending the Thread class is not a preferred way.

Answer 12

Difference between Extending Thread and Implementing Runnable are:

Answer 13

By extending the thread class , the derived class can not extend any other base class because java only allow single inheritance. on the contrary, By implementing the runnable interface the class still extend other base class.

The most significant difference between implementing Runnable and extending Thread is given below :

By extending Thread, derived class itself is a thread object, whereas Implementing Runnable interface it shares the same object to multiple Threads.

Answer 14

In the rare case you only run it once, you should extend Thread because of DRY. If you call it multiple times, you should implement Runnable because the same thread should not be restarted.

Answer 15

One thing that I'm surprised hasn't been mentioned yet is that implementing Runnable makes your class more flexible.

If you extend thread then the action you're doing is always going to be in a thread. However, if you implement Runnable it doesn't have to be. You can run it in a thread, or pass it to some kind of executor service, or just pass it around as a task within a single threaded application (maybe to be run at a later time, but within the same thread). The options are a lot more open if you just use Runnable than if you bind yourself to Thread.

Answer 16

Actually, It is not wise to compare Runnable and Thread with each other.

This two have a dependency and relationship in multi-threading just like Wheel and Engine relationship of motor vehicle.

I would say, there is only one way for multi-threading with two steps. Let me make my point.

Runnable:
When implementing interface Runnable it means you are creating something which is run able in a different thread. Now creating something which can run inside a thread (runnable inside a thread), doesn't mean to creating a Thread.
So the class MyRunnable is nothing but a ordinary class with a void run method. And it's objects will be some ordinary objects with only a method run which will execute normally when called. (unless we pass the object in a thread).

Thread:
class Thread, I would say A very special class with the capability of starting a new Thread which actually enables multi-threading through its start() method.

Why not wise to compare?
Because we need both of them for multi-threading.

For Multi-threading we need two things:

• Something that can run inside a Thread (Runnable).
• Something That can start a new Thread (Thread).

So technically and theoretically both of them is necessary to start a thread, one will run and one will make it run (Like Wheel and Engine of motor vehicle).

That's why you can not start a thread with MyRunnable you need to pass it to a instance of Thread.

But it is possible to create and run a thread only using class Thread because Class Thread implements Runnable so we all know Thread also is a Runnable inside.

Finally Thread and Runnable are complement to each other for multithreading not competitor or replacement.

Answer 17

You should implement Runnable, but if you are running on Java 5 or higher, you should not start it with new Thread but use an ExecutorService instead. For details see: How to implement simple threading in Java.

Answer 18

The best way for most worker threads is to have the threading completely encapsuled in the worker class so that nothing can interfere from the outside and cause unwanted and invalid thread/class states.

I've just posted an example, so I'll also share this with you:

/**
 * This worker can only run once
 * @author JayC667
 */
public class ProperThreading {

    private final Thread        mThread         = new Thread(() -> runWorkingLoop());   // if you want worker to be able to run multiple times, move initialisation into startThread()
    private volatile boolean    mThreadStarted  = false;
    private volatile boolean    mStopRequested  = false;

    private final long          mLoopSleepTime;

    public ProperThreading(final long pLoopSleepTime /* pass more arguments here, store in members */ ) {
        mLoopSleepTime = pLoopSleepTime;
    }

    public synchronized void startThread() {
        if (mThreadStarted) throw new IllegalStateException("Worker Thread may only be started once and is already running!");
        mThreadStarted = true;
        mThread.start();
    }

    private void runWorkingLoop() {
        while (!mStopRequested /* && other checks */ ) {
            try {
                // do the magic work here
                Thread.sleep(mLoopSleepTime);

            } catch (final InterruptedException e) {
                break;
            } catch (final Exception e) {
                // do at least some basic handling here, you should NEVER ignore exception unless you know exactly what you're doing, and then it should be commented!
            }
        }
    }

    public synchronized void stopThread() {
        if (!mThreadStarted) throw new IllegalStateException("Worker Thread is not even running yet!");
        mStopRequested = true;
        mThread.interrupt();
    }

}

Answer 19

With the release of Java 8, there is now a third option.

Runnable is a functional interface, which means that instances of it can be created with lambda expressions or method references.

Your example can be replaced with:

new Thread(() -> { /* Code here */ }).start()

or if you want to use an ExecutorService and a method reference:

executor.execute(runner::run)

These are not only much shorter than your examples, but also come with many of the advantages stated in other answers of using Runnable over Thread, such as single responsibility and using composition because you're not specializing the thread's behaviour. This way also avoids creating an extra class if all you need is a Runnable as you do in your examples.

Answer 20

Moral of the story:

Inherit only if you want to override some behavior.

Or rather it should be read as:

Inherit less, interface more.

Answer 21

Thread holds behaviour which is not intended to be accessed;

• it's synchronized lock is used for join etc.
• it has methods you can access by accident.

however if you sub-class Thread have to consider more Thread is implemented.

public class ThreadMain {
    public int getId() {
        return 12345678;
    }

    public String getName() {
        return "Hello World";
    }

    public String getState() {
        return "testing";
    }

    public void example() {
        new Thread() {
            @Override
            public void run() {
                System.out.println("id: "+getId()+", name: "+getName()+", state: "+getState());
            }
        }.start();
    }

    public static void main(String[] args) {
        new ThreadMain().example();
    }
}

If you run this you might expect

id: 12345678, name: Hello World, state: testing

however, you are not calling the methods you think you are because you are using the method in Thread not ThreadMain and instead you see something like

id: 11, name: Thread-0, state: RUNNABLE

Answer 22

One difference between implementing Runnable and extending Thread is that by extending Thread, each of your threads has a unique object associated with it, whereas implementing Runnable, many threads can share the same object instance.

A class that implements Runnable is not a thread and just a class. For a Runnable to be executed by a Thread, you need to create an instance of Thread and pass the Runnable instance in as the target.

In most cases, the Runnable interface should be used if you are only planning to override the run() method and no other Thread methods. This is important because classes should not be subclassed unless the programmer intends on modifying or enhancing the fundamental behavior of the class.

When there is a need to extend a superclass, implementing the Runnable interface is more appropriate than using the Thread class. Because we can extend another class while implementing Runnable interface to make a thread. But if we just extend the Thread class we can't inherit from any other class.

Answer 23

Adding my two cents here - Always whenever possible use implements Runnable . Below are two caveats on why you should not use extends Threads

1. Ideally you should never extend the Thread class; the Thread class should be made final. At least its methods like thread.getId(). See this discussion for a bug related to extending Threads.

2. Those who like to solve puzzles can see another side effect of extending Thread. The below code will print unreachable code when nobody is notifying them.

Please see http://pastebin.com/BjKNNs2G.

public class WaitPuzzle {

    public static void main(String[] args) throws InterruptedException {
        DoNothing doNothing = new DoNothing();
        new WaitForever(doNothing).start();
        new WaitForever(doNothing).start();
        new WaitForever(doNothing).start();
        Thread.sleep(100);
        doNothing.start();
        while(true) {
            Thread.sleep(10);
        }
    }


    static class WaitForever extends  Thread {

        private DoNothing doNothing;

        public WaitForever(DoNothing doNothing) {
            this.doNothing =  doNothing;
        }

        @Override
        public void run() {
            synchronized (doNothing) {
                try {
                    doNothing.wait(); // will wait forever here as nobody notifies here
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Unreachable Code");
            }
        }
    }

    static class DoNothing extends Thread {

        @Override
        public void run() {
            System.out.println("Do Nothing ");
        }
    } 
}

Answer 24

Since this is a very popular topic and the good answers are spread all over and dealt with in great depth, I felt it is justifiable to compile the good answers from the others into a more concise form, so newcomers have an easy overview upfront:

1. You usually extend a class to add or modify functionality. So, if you don't want to overwrite any Thread behavior, then use Runnable.

2. In the same light, if you don't need to inherit thread methods, you can do without that overhead by using Runnable.

3. Single inheritance: If you extend Thread you cannot extend from any other class, so if that is what you need to do, you have to use Runnable.

4. It is good design to separate domain logic from technical means, in that sense it is better to have a Runnable task isolating your task from your runner.

5. You can execute the same Runnable object multiple times, a Thread object, however, can only be started once. (Maybe the reason, why Executors do accept Runnables, but not Threads.)

6. If you develop your task as Runnable, you have all flexibility how to use it now and in the future. You can have it run concurrently via Executors but also via Thread. And you still could also use/call it non-concurrently within the same thread just as any other ordinary type/object.

7. This makes it also easier to separate task-logic and concurrency aspects in your unit tests.

8. If you are interested in this question, you might be also interested in the difference between Callable and Runnable.

Answer 25

Simple way to say is: If you implement interface that means you are implementing all methods of it and if you extending the class you are inheriting method of your choice... In this case,there is only a one method named Run() so better to implement Runnable interface..

Answer 26

Difference between Thread and runnable .If we are creating Thread using Thread class then Number of thread equal to number of object we created . If we are creating thread by implementing the runnable interface then we can use single object for creating multiple thread.So single object is shared by multiple Thread.So it will take less memory

So depending upon the requirement if our data is not senstive. So It can be shared between multiple Thread we can used Runnable interface.

Answer 27

This is discussed in Oracle's Defining and Starting a Thread tutorial:

Which of these idioms should you use? The first idiom, which employs a Runnable object, is more general, because the Runnable object can subclass a class other than Thread. The second idiom is easier to use in simple applications, but is limited by the fact that your task class must be a descendant of Thread. This lesson focuses on the first approach, which separates the Runnable task from the Thread object that executes the task. Not only is this approach more flexible, but it is applicable to the high-level thread management APIs covered later.

In other words, implementing Runnable will work in scenarios where your class extends a class other than Thread. Java does not support multiple inheritance. Also, extending Thread will not be possible when using some of the high-level thread management APIs. The only scenario where extending Thread is preferable is in a small application that won't be subject to updates in future. It is almost always better to implement Runnable as it is more flexible as your project grows. A design change won't have a major impact as you can implement many interfaces in java, but only extend one class.

Answer 28

Can we re-visit the basic reason we wanted our class to behave as a Thread? There is no reason at all, we just wanted to execute a task, most likely in an asynchronous mode, which precisely means that the execution of the task must branch from our main thread and the main thread if finishes early, may or may not wait for the branched path(task).

If this is the whole purpose, then where do I see the need of a specialized Thread. This can be accomplished by picking up a RAW Thread from the System's Thread Pool and assigning it our task (may be an instance of our class) and that is it.

So let us obey the OOPs concept and write a class of the type we need. There are many ways to do things, doing it in the right way matters.

We need a task, so write a task definition which can be run on a Thread. So use Runnable.

Always remember implements is specially used to impart a behaviour and extends is used to impart a feature/property.

We do not want the thread's property, instead we want our class to behave as a task which can be run.

Answer 29

if you use runnable you can save the space to extend to any of your other class.

Answer 30

One reason you'd want to implement an interface rather than extend a base class is that you are already extending some other class. You can only extend one class, but you can implement any number of interfaces.

If you extend Thread, you're basically preventing your logic to be executed by any other thread than 'this'. If you only want some thread to execute your logic, it's better to just implement Runnable.