经典笔试题：监控容器元素的数量（采用LockSupport实现）

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

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

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

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

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.LockSupport;

public class Test6 {

    public static void main(String[] args) {
        Container container = new Container();
        Thread t1 = null;

        Thread t2 = new Thread(() -> {
            System.out.println("t2启动");
            if (container.size() != 5) {
                LockSupport.park();
            }
            System.out.println("size=5");
            System.out.println("t2 结束");
        }, "t2");

        t2.start();

        t1 = new Thread(() -> {
            System.out.println("t1启动");
            for (int i = 0; i < 10; i++) {
                container.add(new Object());
                System.out.println("add " + i);

                if (container.size() == 5) {
                    LockSupport.unpark(t2);
                }

                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e1) {
                    e1.printStackTrace();
                }
            }

            System.out.println("t1 结束");
        }, "t1");

        t1.start();

    }
}