三个线程分别打印A，B，C，要求这三个线程一起运行，打印n次，输出形如“ABCABCABC....”的字符串。

1. 解法一：使用Lock

public class PrintABCUsingLock {
    private int times;
    private int state;
    private Lock lock = new ReentrantLock();

    public PrintABCUsingLock(int times) {
        this.times = times;
    }

    public static void main(String[] args) {
        PrintABCUsingLock printABC = new PrintABCUsingLock(10);
        new Thread(printABC::printA).start();
        new Thread(printABC::printB).start();
        new Thread(printABC::printC).start();
    }

    public void printA() {
        print("A", 0);
    }

    public void printB() {
        print("B", 1);
    }

    public void printC() {
        print("C", 2);
    }

    private void print(String name, int targetState) {
        for (int i = 0; i < times;) {
            lock.lock();
            if (state % 3 == targetState) {
                state++;
                i++;
                System.out.print(name);
            }
            lock.unlock();
        }
    }
}

2. 解法二：使用wait/notify

public class PrintABCUsingWaitNotify {
    private int times;
    private volatile int state;
    private static final Object LOCK = new Object();

    public PrintABCUsingWaitNotify(int times) {
        this.times = times;
    }

    public static void main(String[] args) {
        PrintABCUsingWaitNotify printABC = new PrintABCUsingWaitNotify(10);
        new Thread(printABC::printA).start();
        new Thread(printABC::printB).start();
        new Thread(printABC::printC).start();
    }

    public void printA() {
        try {
            print("A", 0);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void printB() {
        try {
            print("B", 1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void printC() {
        try {
            print("C", 2);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void print(String name, int targetState)
            throws InterruptedException {
        for (int i = 0; i < times; i++) {
            synchronized (LOCK) {
                while (state % 3 != targetState) {
                    LOCK.wait();
                }
                state++;
                System.out.print(name);
                LOCK.notifyAll();
            }
        }
    }
}

解法三：使用Lock/Condition

public class PrintABCUsingLockCondition {
    private int times;
    private int state;
    private Lock lock = new ReentrantLock();
    private Condition conditionA = lock.newCondition();
    private Condition conditionB = lock.newCondition();
    private Condition conditionC = lock.newCondition();

    public PrintABCUsingLockCondition(int times) {
        this.times = times;
    }

    public static void main(String[] args) {
        PrintABCUsingLockCondition printABC = new PrintABCUsingLockCondition(10);
        new Thread(printABC::printA).start();
        new Thread(printABC::printB).start();
        new Thread(printABC::printC).start();
    }

    public void printA() {
        print("A", 0, conditionA, conditionB);
    }

    public void printB() {
        print("B", 1, conditionB, conditionC);
    }

    public void printC() {
        print("C", 2, conditionC, conditionA);
    }

    private void print(String name, int targetState, Condition current,
            Condition next) {
        for (int i = 0; i < times;) {
            lock.lock();
            try {
                while (state % 3 != targetState) {
                    current.await();
                }
                state++;
                i++;
                System.out.print(name);
                next.signal();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }
}

解法四：使用Semaphore

public class PrintABCUsingSemaphore {
    private int times;
    private Semaphore semaphoreA = new Semaphore(1);
    private Semaphore semaphoreB = new Semaphore(0);
    private Semaphore semaphoreC = new Semaphore(0);

    public PrintABCUsingSemaphore(int times) {
        this.times = times;
    }

    public static void main(String[] args) {
        PrintABCUsingSemaphore printABC = new PrintABCUsingSemaphore(10);
        new Thread(printABC::printA).start();
        new Thread(printABC::printB).start();
        new Thread(printABC::printC).start();
    }

    public void printA() {
        try {
            print("A", semaphoreA, semaphoreB);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void printB() {
        try {
            print("B", semaphoreB, semaphoreC);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void printC() {
        try {
            print("C", semaphoreC, semaphoreA);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void print(String name, Semaphore current, Semaphore next)
            throws InterruptedException {
        for (int i = 0; i < times; i++) {
            current.acquire();
            System.out.print(name);
            next.release();
        }
    }
}