一道经典的面试题目：两个线程，分别打印AB，其中线程A打印A，线程B打印B，各打印10次，使之出现ABABABABA.. 的效果

package com.shangshe.path; public class ThreadAB { /** * @param args */ public static void main(String[] args) { final Print business = new Print(); new Thread(new Runnable() { public void run() { for(int i=0;i<10;i++) { business.print_A(); } } }).start(); new Thread(new Runnable() { public void run() { for(int i=0;i<10;i++) { business.print_B(); } } }).start(); } } class Print { private boolean flag = true; public synchronized void print_A () { while(!flag) { try { this.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } System.out.print("A"); flag = false; this.notify(); } public synchronized void print_B () { while(flag) { try { this.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } System.out.print("B"); flag = true; this.notify(); } }

由上面的例子我们可以设计出3个线程乃至于n个线程的程序，下面给出的例子是3个线程，分别打印A，B，C 10次，使之出现ABCABC.. 的效果

public class ThreadABC { /** * @param args */ public static void main(String[] args) { final Print business = new Print(); new Thread(new Runnable() { public void run() { for(int i=0;i<100;i++) { business.print_A(); } } }).start(); new Thread(new Runnable() { public void run() { for(int i=0;i<100;i++) { business.print_B(); } } }).start(); new Thread(new Runnable() { public void run() { for(int i=0;i<100;i++) { business.print_C(); } } }).start(); } } class Print { private boolean should_a = true; private boolean should_b = false; private boolean should_c = false; public synchronized void print_A () { while(should_b || should_c) { try { this.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } System.out.print("A"); should_a = false; should_b = true; should_c = false; this.notifyAll(); } public synchronized void print_B () { while(should_a || should_c) { try { this.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } System.out.print("B"); should_a = false; should_b = false; should_c = true; this.notifyAll(); } public synchronized void print_C () { while(should_a || should_b) { try { this.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } System.out.print("C"); should_a = true; should_b = false; should_c = false; this.notifyAll(); } }

再一次证明了软件工程的重要性了；在多线程程序中，应该说在程序中，我们应该把那些业务逻辑代码放到同一个类中，使之高内聚，低耦合