How to create a thread-safe singleton in Java using double-checked locking?

Question

import java.util.ArrayList;
import java.util.List;

public class ThreadB extends Thread {
    private static List<String> list1   = new ArrayList<String>();
    private static List<String> list2   = new ArrayList<String>();
    private static final Object mutex   = new Object();

    public void run() {
        try {
            sleep(2000l);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        synchronized (mutex) {
            list1.add("Hello");
            System.out.println("Added for 1   " + list1.size());
            list2.add("Added for 2");
            System.out.println("Added for 2   " + list2.size());
            mutex.notify();
        }
    }

    public static void main(String[] args) {
        ThreadB a1 = new ThreadB();
        ThreadB a2 = new ThreadB();
        a1.start();
        a2.start();
        new ThreadB().m1();    
    }

    private void m1() {         
        synchronized (list1) {
            System.out.println(list1.size());
            System.out.println(list2.size());
            try {
                list1.add("hello");
                System.out.println("m1  :::   " + list1.size());
                sleep(5000l);
                System.out.println("Oh mY God");
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        }
    }
}

Output is

0
0
m1  :::   1
Added for 1   2
Added for 2   1
Added for 1   3
Added for 2   2
Oh mY God

After locking list1, how other threads are going to access that list?