Google面试题（java）—有四个线程1、2、3、4。线程1的功能就是输出1，线程2的功能就是输出2，以此类推.........现在有四个文件ABCD

Google面试题—有四个线程1、2、3、4。线程1的功能就是输出1，线程2的功能就是输出2，以此类推.........现在有四个文件ABCD

程序运行起来，4个文件的写入结果如下：    A：12341234...    B：23412341...    C：34123412...    D：41234123...

同学给我出了道面试题，自己首先想到的是用信号灯，感觉自己应该理解错了。不可能这么简单。

java1.5以前的手法写了一个，感觉不对，后来发觉应该是斜向输出。这道题大概主要是考察对多线程的流程控制。

import java.util.ArrayList;import java.util.HashMap;import java.util.List;public class CallThread {private static class MyRunable implements Runnable {int index = 0;int max = 4 * 2;HashMap<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();{for (int i = 0; i < 4; i++) {map.put(i, new ArrayList<Integer>());}}boolean isRunning = true;public synchronized void run() {while (true) {int name = Integer.valueOf(Thread.currentThread().getName());while (index % 4 != name) {try {this.wait();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}if (!isRunning) {return;}}try {Thread.sleep(1000);} catch (InterruptedException e) {}for (int i = 0, arrIndex = index; i < 4; i++ ) {try {if (arrIndex-- < 0) {break;}List<Integer> list = map.get(i);if (list.size() < max) {list.add(name + 1);}} catch (Exception e) {}}System.err.println("A:" + map.get(0));System.err.println("B:" + map.get(1));System.err.println("C:" + map.get(2));System.err.println("D:" + map.get(3));System.err.println("-----------------------");if (map.get(map.size()-1).size() == max) {isRunning = false;this.notifyAll();return;} else {index++;}this.notifyAll();}}}public static void main(String[] args) throws InterruptedException {Runnable runable = new MyRunable();for (int i = 0; i < 4; i++) {Thread t = new Thread(runable);t.setName(String.valueOf(i));t.start();}}}

程序打印结果A:[1]B:[]C:[]D:[]-----------------------A:[1, 2]B:[2]C:[]D:[]-----------------------A:[1, 2, 3]B:[2, 3]C:[3]D:[]-----------------------A:[1, 2, 3, 4]B:[2, 3, 4]C:[3, 4]D:[4]-----------------------A:[1, 2, 3, 4, 1]B:[2, 3, 4, 1]C:[3, 4, 1]D:[4, 1]-----------------------A:[1, 2, 3, 4, 1, 2]B:[2, 3, 4, 1, 2]C:[3, 4, 1, 2]D:[4, 1, 2]-----------------------A:[1, 2, 3, 4, 1, 2, 3]B:[2, 3, 4, 1, 2, 3]C:[3, 4, 1, 2, 3]D:[4, 1, 2, 3]-----------------------A:[1, 2, 3, 4, 1, 2, 3, 4]B:[2, 3, 4, 1, 2, 3, 4]C:[3, 4, 1, 2, 3, 4]D:[4, 1, 2, 3, 4]-----------------------A:[1, 2, 3, 4, 1, 2, 3, 4]B:[2, 3, 4, 1, 2, 3, 4, 1]C:[3, 4, 1, 2, 3, 4, 1]D:[4, 1, 2, 3, 4, 1]-----------------------A:[1, 2, 3, 4, 1, 2, 3, 4]B:[2, 3, 4, 1, 2, 3, 4, 1]C:[3, 4, 1, 2, 3, 4, 1, 2]D:[4, 1, 2, 3, 4, 1, 2]-----------------------A:[1, 2, 3, 4, 1, 2, 3, 4]B:[2, 3, 4, 1, 2, 3, 4, 1]C:[3, 4, 1, 2, 3, 4, 1, 2]D:[4, 1, 2, 3, 4, 1, 2, 3]-----------------------

jdk1.5写法

import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public class CallThread2 {static int index = 0;static int max = 4 * 2;static HashMap<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();static {for (int i = 0; i < 4; i++) {map.put(i, new ArrayList<Integer>());}}static boolean isRunning = true;public static class MyRunnable implements Runnable {int name;Lock lock;Condition condition;private MyRunnable(int name, Lock lock, Condition condition) {super();this.name = name;this.lock = lock;this.condition = condition;}public boolean isMyName(int index) {while (index != name) {myWait();if(!isRunning)return false;return true;}return false;}public void myWait() {lock.lock();try {condition.await();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}lock.unlock();}public void myNotifyAll() {lock.lock();condition.signalAll();lock.unlock();}@Overridepublic void run() {// TODO Auto-generated method stubwhile (isRunning) {while (isMyName(index % 4)) {}if(!isRunning)return;for (int i = 0, arrIndex = index; i < 4; i++) {try {if (arrIndex-- < 0) {break;}List<Integer> list = map.get(i);if (list.size() < max) {list.add(name + 1);}} catch (Exception e) {}}System.err.println("A:" + map.get(0));System.err.println("B:" + map.get(1));System.err.println("C:" + map.get(2));System.err.println("D:" + map.get(3));System.err.println("-----------------------");if (map.get(map.size() - 1).size() == max) {isRunning = false;myNotifyAll();return;} else {index++;}myNotifyAll();}}}public static void main(String[] args) {// TODO Auto-generated method stubfinal Lock lock = new ReentrantLock();final Condition condition = lock.newCondition();ExecutorService threadPool = Executors.newCachedThreadPool();for (int i = 0; i < 4; i++) {threadPool.execute(new MyRunnable(i, lock, condition));} threadPool.shutdown();}}

感觉自己应该理解错了。不可能这么简单。