为什么在多线程中run（）方法不能是静态的？

Question

Am new to multi threading programming, when I declare the run() method as static its giving the compiler error as

"This static method cannot hide the instance method from Thread".

I didn't understand what that means, so please help me.

public class hello extends Thread {

    public static synchronized  void run() {
        for(int i=0;i<1000;i++)
            System.out.println(i);
    }

    public static void main(String[] args) {
        hello t1 = new hello();
        hello t2 = new hello();

        t1.start();
        t2.start();

    }
}

Answer 1

It is not necessarily relevent to multi-threaded programming, it is true of Java in general. If you have a class:

public class MySuperclass {
  public void myMethod() {
    //do stuff
  }
}

You cannot then over-ride it to make it static:

public class MySubclass extends MySuperclass {
  public static void myMethod() {
    //do other stuff
  }
}

This is not allowed. That's what the error message means.

Answer 2

public void run();

is a Method declared in class Thread . As it's not static in Thread you cannot "change" it into a static method in your subclass. And given your example, you don't even need to do that.

If you need to have the code executed inside the Thread public and static and synchronized , I'd advise to refactor that part out:

@Override
public void run() {
  staticRun();
}

public static synchronized void staticRun() {
  for(int i=0;i<1000;i++)
        System.out.println(i);
}

Answer 3

In the main() method you create two instances of Thread - t1 and t2 and then you call start() on them and that is correct you cannot run/start the class but an instance. That is why run() method is not supposed to be static. It needs an object (a Thread object) to be executed. Just remove static from your declaration and it shall be almost fine. Other thing is that you dont need to make your run method synchronized (it is counter productive - you use thread to execute in paralel and ynchronize only on specific parts never whole run method).

Answer 4

The instance method run() is already available in hello class due to inheritance . You are trying to create another method (static) with same name which is run().

If the method is not static, it will automatically override the implementation and hide the method instance of parent class..

So, is the Error.. Simple!

Answer 5

Because java is Object Oriented language and anything is java is object. And task which you are trying to perform in multiple threads is object too and you should create this object. When object is created you should call start() method which is defined on Thread class. So to implement your own logic, you should override this behavior by implementing run() method in Hello subclass.

• Run method defines logic which should be performed in other thread (note, that run method doesn't create new thread)
• Start method tells java to create new native thread and perform your run method in this new thread

Try:

public class Hello extends Thread{

    public void run()
    {
       for(int i=0;i<1000;i++)
       System.out.println(i);
    }
}

public class Main
{
    public static void main(String[] args)
    {
          Hello hello t1 = new hello();
          Hello hello t2 = new hello();
          t1.start();
          t2.start();
    }
}

Answer 6

The thread Class has a run method, therefore if you define it again in the hello class then you are overriding it, (direct inheritance since Hello extends Thread ) now, it makes no sense to turn a Object method into a Class method, (run method belongs to an object and not to the class) that is the reason why your compiler is complaining with the message:

"...you can not This static method cannot hide the instance method from Thread".

in other words, you are violating inheritance rules with it.