In java language, as we all know that there are two ways to create threads. One using Runnable interface and another by extending Thread class. Let’s identify the differences between both ways i.e extends thread and implements runnable.
1. Create Thread using Runnable Interface vs Thread class
Let’s quickly check the java code of usage of both techniques.
1.1. Runnable interface
Java program to create thread by implementing Runnable interface.
public class DemoRunnable implements Runnable {
public void run() {
//Code
}
}
//start new thread with a "new Thread(new demoRunnable()).start()" call
1.2. Thread class
Java program to create thread by extending Thread class.
public class DemoThread extends Thread {
public DemoThread() {
super("DemoThread");
}
public void run() {
//Code
}
}
//start new thread with a "new demoThread().start()" call
2. Difference between Runnable vs Thread
There has been a good amount of debate on which is better way. Well, I also tried to find out and below is my learning.
- Implementing
Runnableis the preferred way to do it. Here, you’re not really specializing or modifying the thread’s behavior. You’re just giving the thread something to run. That means composition is the better way to go. - Java only supports single inheritance, so you can only extend one class.
- Instantiating an interface gives a cleaner separation between your code and the implementation of threads.
- Implementing
Runnablemakes your class more flexible. If you extendThreadthen the action you’re doing is always going to be in a thread. However, if you implementRunnableit 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. - If you are working on JDK 4 or lesser, then there is bug :
It’s fixed in Java 1.5 but Sun doesn’t intend to fix it in 1.4.
The issue is that at construction time, a
Threadis added to a list of references in an internal thread table. It won’t get removed from that list until itsstart()method has completed. As long as that reference is there, it won’t get garbage collected.
That’s all about differences between Runnable interface and Thread class in java. If you know something more, please put that in comments section and I will include in post content.
Happy Learning !!
By implementing Runnable interface, we can only override the run() method. But when we extend Thread class, we can use many methods based on our requirements. If we want to make our class as a thread, we can always go for extending the Thread class rather than implementing Runnable interface.
Hello Sowmya rajan,
As per my understanding, Thread is a curse in java. If you create multiple threads then there would be a chance of memory overhead and hence the performance of the app will go down.
why we extends a thread class and what are the advantage of it
Well Explained. Thanks to Lokesh !
Good points on topic
Thanks lokesh..
As per my knowledge, the advantage is – when there are multiple threads then, memory usage would be more in case of extends Thread. Because, each of your threads contains unique object associated with it. Where as, memory usage would be less in case of implements Runnable. Because, many threads can share the same runnable instance.
You are right Gopi. Point 5 state the same thought as you. Key thing is that it “CAN” share (as you also said); not always.
Awesome explanation !!
Small correction in point 4
It should be ” implements Runnable” Not “extend Runnable”
i want what the main difference in the thread & and Runnable interface?
5) By extending Thread, each of your threads has a unique object associated with it, whereas implementing Runnable, many threads can share the same runnable instance. – This statement seems wrong to me.
Many threads can share the same Thread object, same as Runnable instance.
For example,
public class A1 implements Runnable {….}
public class B1 extends Thread {….}
public class ABMain {
public static void main( String[] args ) {
A1 a = new A1(); //Create runnable instance
B1 b = new B1(); // Create Thread instance
new Thread( a ).start(); //Starts runnable instance
new Thread( b ).start(); //Starts Thread instance.
new Thread( a ).start(); // 2nd thread sharing same runnable instance A1
new Thread( b ).start(); // 2nd thread sharing same Thread instance.
}
}
Difference between Thread and Runnable I found is, Thread class provides apis to configure Thread instance based on your need.
If you extends Thread class, then
1. you will have flexibility to set priority to Thread instance,
2. you can create daemon thread,
3. you can check if thread is terminated, alive etc.
There are apis available in Thread class which is not avaialbe in Runnable. Those apis helps developers to configure Thread properties.
I modified your program a little bit, to correct the picture of usage of thread and runnable.
class A1 implements Runnable {private int counter = 0;
@Override
public void run() {
counter++;
System.out.println("Running A1 -> " + counter);
}
}
class B1 extends Thread {
private int counter = 0;
@Override
public void run() {
counter++;
System.out.println("Running B1 -> " + counter);
}
}
public class ABMain {
public static void main(String[] args) throws InterruptedException {
A1 a = new A1(); // Create runnable instance
B1 b = new B1(); // Create Thread instance
new Thread(a).start();
Thread.sleep(1000);
new Thread(a).start();
Thread.sleep(1000);
new Thread(a).start();
Thread.sleep(1000);
new B1().start();
Thread.sleep(1000);
new B1().start();
Thread.sleep(1000);
new B1().start();
}
}
Output:
1
2
3
1
1
1
1) A1 a = new A1(); does not make a thread. It’s just another class with no extra behavior. If you call A1.run() then it is not new thread. And A1.start() is not available to this class.
2) Only way to create start a thread in java is calling it’s start method.
3) If you look at constructor of Thread class, no constructor takes parameter of Thread class itself. And we know that Thread implements Runnable, effectively any call such as “new Thread(a).start();” is starting a thread with runnable mechanism.
4) Correct way to start thread (“using extend”) is calling it’s start method only directly.
e.g. new B1().start();
5) So effectively, in both techniques in your code, you are doing the same thing i.e. implementing runnable.
It’s correct post.
By extending Thread, each of your threads has a unique object associated with it, whereas implementing Runnable, many threads can share the same runnable instance.
The statement is alright but what is the advantage we gain here ?
The code is misleading. It doesn’t match the intent. It’s true that
Threadclass itself implementsRunnableinterface, so any instance of a class extending theThreadclass can be passed in to creating new thread.These two threads share the same instance.
You have a point. Removing the statement from above post this until I find a reason to include.
Thank you for the post. It’s very helpful.