gistfile1.java

public class PrintAB {

    // The code below would print overlapped A and B sequences like:
    // ...
    // A
    // A
    // B
    // B
    // ...
    //
    // Please add multi-threading protection code to make sure that
    // As and Bs are printed in the order of:
    // ...
    // A
    // B
    // A
    // B
    // ...

    private static ReentrantLock lock = new ReentrantLock();

    private static void printA() {
        lock.lock();
        System.out.println("A");
    }

    private static void printB() {
        System.out.println("B");
        lock.unlock();
    }

    // DO NOT touch the code below

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        printA();
                        printB();
                    }
                }
            }).start();
        }
    }

}