Thread Vs Runnable
This question already has an answer here:
Here's what the javadoc states
There are two ways to create a new thread of execution. One is to declare a class to be a subclass of Thread . This subclass should override the run method of class Thread. An instance of the subclass can then be allocated and started. For example, a thread that computes primes larger than a stated value could be written as follows:
The other way to create a thread is to declare a class that implements the Runnable interface. That class then implements the run method. An instance of the class can then be allocated, passed as an argument when creating Thread, and started. The same example in this other style looks like the following:
So the two ways
public class MyThread extends Thread {
// overriden from Runnable, which Thread implements
public void run() {
...
}
}
...
MyThread thread = new MyThread();
thread.start();
Or
public class MyRunnable implements Runnable{
public void run() {
...
}
}
...
Thread thread = new Thread(new MyRunnable());
thread.start();
Your counter field is an instance field.
In your first case, each of the objects created here
ExtendsThread tc1 = new ExtendsThread();
tc1.start();
Thread.sleep(1000); // Waiting for 1 second before starting next thread
ExtendsThread tc2 = new ExtendsThread();
tc2.start();
Thread.sleep(1000); // Waiting for 1 second before starting next thread
ExtendsThread tc3 = new ExtendsThread();
tc3.start();
will have their own copy (that's how instance variables work). So when you start each thread, each one increments its own copy of the field.
In your second case, you are using your Thread sub class as a Runnable argument to the Thread constructor.
ExtendsThread extendsThread = new ExtendsThread();
Thread thread11 = new Thread(extendsThread);
thread11.start();
Thread.sleep(1000);
Thread thread12 = new Thread(extendsThread);
thread12.start();
Thread.sleep(1000);
Thread thread13 = new Thread(extendsThread);
thread13.start();
Thread.sleep(1000);
It is the same ExtendsThread object that you pass, so its counter field gets incremented by all threads. It's pretty much equivalent to your previous usage of ImplementsRunnable .
To add from the comments:
First thing to understand is that the Thread class implements Runnable , so you can use a Thread instance anywhere you can use Runnable . For example,
new Thread(new Thread()); // won't do anything, but just to demonstrate
When you create a Thread with
new Thread(someRunnable);
and start it, the thread calls the given Runnable instance's run() method . If that Runnable instance happens to also be an instance of Thread , so be it. That doesn't change anything.
When you create a custom thread like
new ExtendsThread();
and start it, it calls run() on itself.
The principle difference in implementing Runnable is that you do not 'consume' your single inheritance. Consider these Class declarations:
public class HelloRunnable implements Runnable extends AbstractHello
public class HelloRunnable extends Thread
You can do more with Runnable when it comes to inheritance.
@BalwantChauhan: One common usage of Runnable interface is that as we know that multiple inheritance is not possible in case of Java. Now suppose you have a scenario where you want extends a class and also you want to implement thread. So for those scenario if we go ahead for Thread then it is not possible to achieve it. For example : Suppose (in case of Java Swing), if you want to create a frame and also in that frame class you want to implement thread, then it is not possible to extends JFrame and Thread class so in that case we extends JFrame and implement Runnable.
public class HelloFrame extends JFrame implements Runnable{
...
public void run(){
// thread code
}
...
}
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