题目：编写程序，开启 3 个程序，这 3 个线程的 ID 分别是 A、B、C，每个线程将自己的 ID 在屏幕上打印 10 遍，要求输出结果必须按 ABC 的顺序显示，如 ABCABC…以此递推

代码：

package train20181010;

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

/* 编写一个程序，开启 3 个线程，
 * 这三个线程的 ID 分别为 A、B、C，
 * 每个线程将自己的 ID 在屏幕上打印 10 遍，
 * 要求输出的结果必须按顺序显示。
 * 如：ABCABCABC…… 依次递归 
 */
public class TestABCAlternate {
	public static void main(String[] args) {
AlternateDemo ad = new AlternateDemo();
    new Thread(new Runnable() {
    	@Override
    	public void run() {
    		for (int i = 1; i <= 10; i++) {
    		ad.loopA(i);
    		}
    		}
    }, "A").start();
    new Thread(new Runnable() {
    	@Override
    	public void run() {
    		for (int i = 1; i <= 10; i++) {
    				ad.loopB(i);
    				}
    		}
    	}, "B").start();    
    new Thread(new Runnable() {
    	@Override
    	public void run() {
    	for (int i = 1; i <= 10; i++) {
    		ad.loopC(i);
    		System.out.println("-----------------------------------");
    		}
    	}
    	}, "C").start();
    }
}
class AlternateDemo{
    private int number = 1; //当前正在执行线程的标记        
    	private Lock lock = new ReentrantLock();
    	private Condition condition1 = lock.newCondition();
    	private Condition condition2 = lock.newCondition();
    	private Condition condition3 = lock.newCondition();
     /**     
      * @param totalLoop : 循环第几轮     
      */
    	public void loopA(int totalLoop){
    		lock.lock();
    		try {
    			//1. 判断            
    			if(number != 1){
    				condition1.await();
    				}
    			//2. 打印
    			for (int i = 1; i <= 1; i++) {
    				System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
    			}
    			//3、唤醒
    			number = 2;
    			condition2.signal();
    			} catch (Exception e) {
    				e.printStackTrace();
    				} finally {
    					lock.unlock();
    			}
    		}
      public void loopB(int totalLoop){
    			lock.lock();
    			try {
    				//1. 判断            
    				if(number != 2){
    					condition2.await();
    					}
    				//2. 打印            
    				for (int i = 1; i <= 1; i++) {
    					System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
    				}
    				//3. 唤醒           
    				number = 3;
    				condition3.signal();
    				} catch (Exception e) {
    					e.printStackTrace();
    					} finally {
    						lock.unlock();
    				}
    			}
      public void loopC(int totalLoop){
    	  lock.lock();try {
    		  //1. 判断
    		  if(number != 3){
    						condition3.await();
    					}   					
    					//2. 打印            
    					for (int i = 1; i <= 1; i++) {
    				       System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
    					}
    					//3. 唤醒
    					number = 1;
    					condition1.signal();
    					} catch (Exception e) {
    						e.printStackTrace();
    						} finally {
    						lock.unlock();
    	}
    }
}

运行结果：