题目：有A,B,C三个线程, A线程输出A, B线程输出B, C线程输出C，要求, 同时启动三个线程, 按顺序输出ABC, 循环10次。

解题思路：要按顺序输出ABC, 循环10次，就要控制三个线程同步工作，也就是说要让三个线程轮流输出，直到10个ABC全部输出则结束线程。这里用一个Lock对象来控制三个线程的同步。用一个int型变量state标识由那个线程输出。

1 package com.thread; 2 3 import java.util.concurrent.locks.Lock; 4 import java.util.concurrent.locks.ReentrantLock; 5 6 public class ABC { 7     private static int state = 0; 8 9     public static void main(String[] args) {10         final Lock l = new ReentrantLock();1112         Thread A = new Thread(new Runnable(){13             @Override14             public void run() {15                 while (state<=30) {16                     l.lock();17                     if(state%3==0){18                         System.out.println(“A”);19                         state ++;20                     }21                     l.unlock();22                 }23             }24         });25         Thread B = new Thread(new Runnable(){26             @Override27             public void run() {28                 while (state<=30) {29                     l.lock();30                     if(state%3==1){31                         System.out.println(“B”);32                         state ++;33                     }34                     l.unlock();35                 }36             }37         });38         Thread C = new Thread(new Runnable(){39             @Override40             public void run() {41                 while (state<=30) {42                     l.lock();43                     if(state%3==2){44                         System.out.println(“C”);45                         state ++;46                     }47                     l.unlock();48                 }49             }50         });51         A.start();52         B.start();53         C.start();54     }5556 }

运行结果：