Why does a java.lang.Thread not call the run() method of its explicit java.lang.Runnable when started?

Question

The Java docs state that if we supplied a Runnable target when creating a new thread, .start() of that thread would run the run() method of the supplied runnable.

If that's the case, shouldn't this test code prints "a" (instead of printing "b") ?

public class test {
    public static void main(String[] args) {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                System.out.println("a");
            }
        };
        Thread t = new Thread(r) {
            @Override
            public void run() {
                System.out.println("b");
            }
        };
        t.start();
    }
}

Answer 1

Because you are overriding Thread.run() method.

Here is the implementation of Thread.run():

@Override
public void run() {
    if (target != null) {
        target.run();
    }
}

try:

}) {
    @Override
    public void run() {
        super.run(); // ADD THIS LINE
        System.out.println("b");
    }
}.start();

You will get ab.

Answer 2

The default implementation is to call the Runnable. However you are overriding the default implementation and NOT calling the runnable. The simplest way to fix this is

    new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("a");
        }
    }) {
        @Override
        public void run() {
            super.run(); // call the default implementation.
            System.out.println("b");
        }
    }.start();