多线程操作共享变量顺序输出abc 记一次al面试题

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class D {
    private static volatile int index = 0;
    public static Lock lock =  new ReentrantLock();
    public static void main(String[] args) {
        new Thread(() -> {
            while (true){
                lock.lock();
                try {
                    if (index % 3 == 0) {
                        index++;
                        System.out.print("a");
                    }
                }finally {
                    lock.unlock();
                }
            }
        }).start();

        new Thread(() -> {
            while (true){
                lock.lock();
                try {
                    if (index % 3 == 1) {
                        index++;
                        System.out.print("b");
                    }
                }finally {
                    lock.unlock();
                }
            }
        }).start();

        new Thread(() -> {
            while (true){
                lock.lock();
                try {
                    if (index % 3 == 2) {
                        index++;
                        System.out.print("c");
                    }
                }finally {
                    lock.unlock();
                }
            }
        }).start();

    }

}
面试的时候大脑空白什么也没写出来，惨~~~