要求：
（1）自定義容器，提供新增元素（add）和獲取元素數量（size）的方法。
（2）啟動兩個線程。線程1向容器中新增10個數據。線程2監聽容器元素數量，當容器元素數量為5時，線程2輸出信息并終止。

方式一：

public class Test1 {
    public static void main(String[] args) {
        Test1_Container t = new Test1_Container();
        new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < 10; i++) {
                    System.out.println("add Object to Container " + i);
                    t.add(new Object());
                    try {
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();

        new Thread(new Runnable() {
            public void run() {
                while (true) {
                    if (t.size() == 5) {
                        System.out.println("Container's size = 5");
                        break;
                    }
                }
            }
        }).start();
    }
}

class Test1_Container {
    volatile List<Object> container = new ArrayList<>();

    public void add(Object o) {
        this.container.add(o);
    }

    public int size() {
        return container.size();
    }
}

方式二：

public class Test2 {
    public static void main(String[] args) {
        final Test2_Container t = new Test2_Container();
        final Object lock = new Object();

        new Thread(new Runnable() {
            public void run() {
                synchronized (lock) {
                    if (t.size() != 5) {
                        try {
                            lock.wait(); // 線程進入等待隊列。
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println("size = 5");
                    lock.notifyAll(); // 喚醒其他等待線程
                }
            }
        }).start();

        new Thread(new Runnable() {
            public void run() {
                synchronized (lock) {
                    for (int i = 0; i < 10; i++) {
                        System.out.println("add Object to Container " + i);
                        t.add(new Object());
                        if (t.size() == 5) {
                            lock.notifyAll();
                            try {
                                lock.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        try {
                            TimeUnit.SECONDS.sleep(1);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }).start();
    }
}

class Test2_Container {
    List<Object> container = new ArrayList<>();

    public void add(Object o) {
        this.container.add(o);
    }

    public int size() {
        return container.size();
    }
}

方式三：

public class Test3 {
    public static void main(String[] args) {
        Test3_Container t = new Test3_Container();
        CountDownLatch latch = new CountDownLatch(1);// 門閂

        new Thread(new Runnable() {
            public void run() {
                if (t.size() != 5) {
                    try {
                        latch.await();// 等待門閂開啟，不是進入等待隊列
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("size = 5");
            }
        }).start();

        new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < 10; i++) {
                    System.out.println("add Object to Container " + i);
                    t.add(new Object());
                    if (t.size() == 5) {
                        latch.countDown();// 門閂-1
                    }
                    try {
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }
}

class Test3_Container {
    List<Object> container = new ArrayList<>();

    public void add(Object o) {
        this.container.add(o);
    }

    public int size() {
        return container.size();
    }
}