Running the run method directly on a Thread

[Massimo Battestini]

Why this code prints out hello?
I was expecting the 'run' method of the Thread class to run (which won't print anything) and not the run method of TestThread4; in fact I'm running the 'run' method on a new Thread instance....

Any idea?

public class TestThread4 implements Runnable {
    public static void main(String[] args) {
        new Thread(new TestThread4()).run();
    }
    public void run() {
        System.out.println("hello");
    }
}    

[Henry Wong]

I was expecting the 'run' method of the Thread class to run (which won't print anything) and not the run method of TestThread4; in fact I'm running the 'run' method on a new Thread instance....

The run() method of the Thread class, actually does stuff. It is called by the new thread that is created by the start() method. And if you don't override it, the run() method looks for a runnable object that has been passed to the Thread object via the constructor and call its run() method.

In this case, your main thread called it directly, but it still does the same thing -- look for the runnable passed via the constructor and call its run() method.

Henry

[Ireneusz Kordal]

Check the description of run() method in the api: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html

void run()
If this thread was constructed using a separate Runnable run object, then that Runnable object's run method is called; otherwise, this method does nothing and returns.

Thread class implements Runnable, so new Thread4() creates the Runnable object.
You pass this Runnable (TestThread4 object) to the new Thread instance (second Thread), so if you call the run() of this second instance,
it calls run() of the Runnable passed to it's constructor - TestThread4.run().

[Ankit Garg]

The code to what Henry said would look like this

package java.lang;
public class Thread implements Runnable {

   Runnable other;
   public Thread(Runnable other) {
      this.other = other;
   }
   public void run() {
      if(other != null) {
         other.run();
      }
   }

   public void start() {
      //invoke a new thread and call the run method
   }
}

So if you use your custom Thread sub-class, then results might not be as expected

public class MyThread extends Thread {
   public void run() {
      System.out.println("a");
   }
}

public class TestThread4 implements Runnable {
    public static void main(String[] args) {
        new MyThread(new TestThread4()).run();
    }
    public void run() {
        System.out.println("hello");
    }
}    

Here output would be a and not hello...

[Hukm chand]

I have one doubt.
In line 9 you invoking constructor of MyThread which don't have single argument constructor.
how this would print a.
i got 2 error while executing this above code.
please help me and clear my doubt.
you made one mistake I learned that one source file there will be one public class and it would be class containing main method.

[Bob Wheeler]

I think Ankit forgot the constructor?

  
class MyThread extends Thread { // default access modifier
	    public MyThread(Runnable other) {  // constructor with call to super
	    	super(other);
	    } 
	   public void run() {
	      System.out.println("a");
	   }
}

public class TestThread implements Runnable {
    public static void main(String[] args) {
        new MyThread(new TestThread()).run();
    }
    public void run() {
        System.out.println("hello");
    }
}    

  

output: a

[David Marco]

I learned that one source file there will be one public class and it would be class containing main method

The last is wrong. A file can contain only one public class, and the name of the file must match the public class name if any. But another class can contain the main method. In fact, ALL the classes in a file can contain a main method and you can execute anyone of them after compile the .java file.

Greetings.

[Hukm chand]

Thank you. David for u'r help.

[Ankit Garg]

Bob Wheeler wrote:I think Ankit forgot the constructor?

You got me

[Bob Wheeler]

Ankit Garg wrote:

Bob Wheeler wrote:I think Ankit forgot the constructor?

You got me

Yeah. I feel like I can get 102% in the SCJP exam. LOL

cheers
Bob