Modern threading for not-quite-beginners

import java.util.concurrent.locks.*;
import java.util.*;
import java.text.*;

public class lock_example {
    public static Lock this_lock = new ReentrantLock();
    public static void main(String[] args) throws Exception {
    ListenerThread lt = new ListenerThread();
        SpeakerThread st = new SpeakerThread();
    lt.start();
        st.start();
    }
    public static int message = 0;
    public static void report(String label) {
        Date now = new Date();
        SimpleDateFormat sdf = new SimpleDateFormat("hh:mm:ss.S");
        String time_of_day = sdf.format(now.getTime());
    System.out.format("%s:  %s\n", time_of_day, label);
    }
}

class ListenerThread extends Thread {
    public void run() {
    Thread this_thread = Thread.currentThread();
        while (true) {
            lock_example.report("The Listener requests the lock.");
            lock_example.this_lock.lock();
            lock_example.report("The Listener acquires the lock.");
            if (lock_example.message == 0) {
                lock_example.report("The Speaker has sent nothing.");
                lock_example.report("The Listener will check again later.");
                try {
                    this_thread.sleep(800);
                } catch (InterruptedException ie) {
                    // ie
                }
            } else {
                lock_example.report("The Listener just received " + Integer.toString(lock_example.message) + " from the Speaker.");
                lock_example.message = 0;
            }
            lock_example.report("The Listener releases the lock.");
            lock_example.this_lock.unlock();
        }
    }
}

class SpeakerThread extends Thread {
    int counter = 0;
    public void run() {
    Thread this_thread = Thread.currentThread();
        while (true) {
            while (0 != lock_example.message) {
                try {
                    lock_example.report("The Speaker waits for the Listener to catch up.");
                    this_thread.sleep(30);
                } catch (InterruptedException ie) {
                }
            }
            counter++;
            lock_example.report("The Speaker requests the lock.");
            lock_example.this_lock.lock();
            lock_example.report("The Speaker acquires the lock.");
            lock_example.report("The Speaker takes a nap--a model for real work it might otherwise do.");
            try {
                this_thread.sleep((int) (2000 * Math.random()));
            } catch (InterruptedException ie) {
            }
            lock_example.message = counter;
            lock_example.report("The Speaker releases the lock.");
            lock_example.this_lock.unlock();
        }
    }
}