一道java多线程笔试上机题

jsjzhou

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多线程 Java 面试 thread C

上个星期我到诚迈科技参加面试.面试完毕后面试官让我把笔试卷上的一道多线程题在计算机上编程实现.题目如下:

四个线程a,b,c,d. 线程a,b对变量i加一. 线程c,d对变量i减去一.四个线程顺序执行, 每个线程每次只执行一次.i的初始值为0，打印结果0 1 2 1 0 1 2 1 0 1 2...

这道题还是有一定的难度的. 因为要求顺序执行. 不能简单用同步.

经考虑,我决定用一个队列来对四个线程顺序调度.代码如下:

package org.jenfer.struts2demo.web.struts2.action;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;

/**
 * 四个线程a,b,c,d. 线程a,b对变量i加一.
 * 线程c,d对变量i减去一.四个线程顺序执行,
 * 每个线程每次只执行一次.i的初始值为0，
 * 打印结果0 1 2 1 0 1 2 1 0 1 2... 
 * 
 * @author 武汉科技大学08级研究生周剑华
 *
 */
public class MultiThreadTest {
	//variable i 
	private int i=0;
	//queue to control thread invoke
	private BlockingQueue<Integer> queue=new LinkedBlockingQueue<Integer>();
	
	public MultiThreadTest() {
		queue.offer(1);
		queue.offer(2);
		queue.offer(3);
		queue.offer(4);
	}
	
	public synchronized void inc(int con) throws InterruptedException{
		while(true){
			int c=queue.peek();
			if(c==con){
				break;
			}else{
				notifyAll();
				wait();
			}
		}
		
		queue.offer(queue.take());
		
		i++;
		System.out.println(Thread.currentThread().getName()+",i="+i);
	}
	
	public synchronized void dec(int con) throws InterruptedException{
		while(true){
			int c=queue.peek();
			if(c==con){
				break;
			}else{
				notifyAll();
				wait();
			}
		}
		
		queue.offer(queue.take());
		i--;
		System.out.println(Thread.currentThread().getName()+",i="+i);
	}
	
	private class IncThread implements Runnable{
		private int condition;
		public IncThread(int condition) {
			this.condition=condition;
		}
		public void run() {
			while(true){
				try {
					inc(condition);
				} catch (InterruptedException e) {
					System.err.println(Thread.currentThread().getName()+" exit now");
					break;
				}
			}
		}
	}
	
	private class DecThread implements Runnable{
		private int condition;
		public DecThread(int condition) {
			this.condition=condition;
		}
		public void run() {
			while(true){
				try {
					dec(condition);
				} catch (InterruptedException e) {
					System.err.println(Thread.currentThread().getName()+" exit now");
					break;
				}
			}
		}
	}
	
	public static void main(String[] args) {
		MultiThreadTest test=new MultiThreadTest();
		ExecutorService exec=Executors.newFixedThreadPool(4);
		exec.submit(test.new IncThread(1));
		exec.submit(test.new IncThread(2));
		exec.submit(test.new DecThread(3));
		exec.submit(test.new DecThread(4));
		
		exec.shutdown();
	}
}

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unicode汉字范围 | 编程打印全排列

2010-04-18 21:14
浏览 3407
评论(9)
分类:编程语言
查看更多

9 楼 leoricxu 2014-06-25

package com.thread.test;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;

/**
* 重新用队列写了下
* @author Administrator
*
*/
public class Task4Test2 {
public Task4Test2() {
print();
}

public static void main(String[] args) throws InterruptedException {
final Task4Test2 test = new Task4Test2();
test.queue1.put("start");

Thread a = new Thread(new Runnable() {
public void run() {
while (true) {
try {
test.queue1.take();
test.increment();
test.queue2.put("a");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "a");

Thread b = new Thread(new Runnable() {
public void run() {
while(true){
try {
test.queue2.take();
test.increment();
test.queue3.put("b");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "b");

Thread c = new Thread(new Runnable() {
public void run() {
while(true){
try {
test.queue3.take();
test.decrement();
test.queue4.put("c");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "c");

Thread d = new Thread(new Runnable() {
public void run() {
while(true){
try {
test.queue4.take();
test.decrement();
test.queue1.put("d");
} catch (InterruptedException e) {
e.printStackTrace();
}
}

}
}, "d");
a.start();
b.start();
c.start();
d.start();
}

public void decrement() {
index--;
print();
}

public void increment() {
index++;
print();
}

public void print() {
System.out.println(Thread.currentThread().getName() +" "+index);
}

private int index;
private BlockingQueue<String> queue1 = new LinkedBlockingQueue<String>(1);
private BlockingQueue<String> queue2 = new LinkedBlockingQueue<String>(1);
private BlockingQueue<String> queue3 = new LinkedBlockingQueue<String>(1);
private BlockingQueue<String> queue4 = new LinkedBlockingQueue<String>(1);
}

8 楼 leoricxu 2014-06-25

最近在学习多线程。
package com.thread.test;

import java.util.concurrent.Semaphore;

public class Task4Test {
private static Semaphore semp = new Semaphore(1);

public static void main(String args[]) throws InterruptedException {
final Task4Test number = new Task4Test();
while(true){
Thread a = new Thread(new Runnable() {
public void run() {
number.increment();

}
},"a");

Thread b = new Thread(new Runnable() {
public void run() {

number.increment();

}
},"b");

Thread c = new Thread(new Runnable() {
public void run() {
number.minus();

}
},"c");

Thread d = new Thread(new Runnable() {
public void run() {

number.minus();

}
},"d");
a.start();
a.join();
b.start();
b.join();
c.start();
c.join();
d.start();
d.join();

}
}

public synchronized void minus() {
try {
semp.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
if (index > 0) {
--index;
print();
semp.release();
} else {
semp.release();
}

}

public synchronized void increment() {
try {
semp.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
if (index < 2) {
++index;
print();
semp.release();
} else {
semp.release();
}

}

public Task4Test() {
System.out.println(index);
}

public void print() {
System.out.println(Thread.currentThread().getName()+" "+index);
}

private int index = 0;
}

7 楼 funnyone 2013-10-17

	private static volatile int target=0;
	private static volatile int count=0;
	private static final Object LOCk=new Object();
	
	public static void main(String[] args) throws Exception {
		for(char i='a';i<='d';i=(char)(i+1)){
			final char temp=i;
			new Thread(new Runnable() {
				public void run() {
					while(true){
						if (count%4==(int)temp-(int)'a') {
							synchronized (LOCk) {
								if (count%4==(int)temp-(int)'a') {
									++count;
									if (temp=='a'||temp=='b') {
										++target;
									}else {
										--target;
									}
								}
								System.err.println(temp+":"+target);
							}
						}
					}
				}
			}).start();
		}
	}

6 楼 yan465942872 2011-08-30

自己写的。。

public class Test {  
	public static void main(String[] args) {
		
		final ShareData shareData = new ShareData();

		new Thread(new Runnable() {
			@Override
			public void run() {
				while(true) {
					shareData.increase(0);
				}
			}
		}).start();
		
		new Thread(new Runnable() {
			@Override
			public void run() {
				while(true) {
					shareData.increase(1);
				}
			}
		}).start();
		
		new Thread(new Runnable() {
			@Override
			public void run() {
				while(true) {
					shareData.decrease(2);
				}
			}
		}).start();
		
		new Thread(new Runnable() {
			@Override
			public void run() {
				while(true) {
					shareData.decrease(3);
				}
			}
		}).start();
		
	}
} 

class ShareData {
	
	int i = 0;
	int j = 0;
	
	public synchronized void increase(int threadIndex) {
		
		while(j != threadIndex) {
			try {
				wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		System.out.println("Thread " + j + ": i = " + i);
		i++;
		j = (j + 1) % 4;
		notifyAll();
	}
	
	public synchronized void decrease(int threadIndex) {
		while(j != threadIndex) {
			try {
				wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		System.out.println("Thread " + j + ": i = " + i);
		i--;
		j = (j + 1) % 4;
		notifyAll();
	}
	
}

5 楼 jsjzhou 2011-06-05

另外我提供一个简单的方法：
/**
*
* @author 武汉科技大学周剑华
*
*/
public class ThreadTest {
private static final int THREAD_NUM = 4;
private int invoder = 0;
private int i = 0;

public synchronized void add(final int threadIndex) throws Exception {
while (threadIndex != invoder) {
wait();
}
invoder = (invoder + 1) % THREAD_NUM;
System.out.println("Thread " + threadIndex + ": i=" + i);
i++;
notifyAll();
}

public synchronized void sub(final int threadIndex) throws Exception {
while (threadIndex != invoder) {
wait();
}
invoder = (invoder + 1) % THREAD_NUM;
System.out.println("Thread " + threadIndex + ": i=" + i);
i--;
notifyAll();
}

private class Inc extends Thread {
private final int index;

public Inc(int index) {
this.index = index;
}

@Override
public void run() {
try {
while (true) {
add(index);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}

private class Dec extends Thread {
private final int index;

public Dec(int index) {
this.index = index;
}

@Override
public void run() {
try {
while (true) {
sub(index);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}

public static void main(String[] args) {
ThreadTest test = new ThreadTest();
test.new Inc(0).start();
test.new Inc(1).start();
test.new Dec(2).start();
test.new Dec(3).start();
}
}

4 楼 jsjzhou 2010-11-15

shengren0 写道

楼主的代码运行结果跟预期不一致

在你的环境运行结果是什么啊？请赐教。

3 楼 shengren0 2010-10-30

楼主的代码运行结果跟预期不一致

2 楼 ws715 2010-08-03

楼下的程序也只能叫单线程吧

1 楼 253317239 2010-07-20

/**
* @作者： cz
* 2010-7-20 下午02:03:51
* TODO:四个线程a,b,c,d. 线程a,b对变量i加一. 线程c,d对变量i减去一.
* 四个线程顺序执行, 每个线程每次只执行一次.i的初始值为0，
*   打印结果0 1 2 1 0 1 2 1 0 1 2...
*/

public class MutilThreadTest implements Runnable{
   static int i=0;
     public static void runThreadA(){
i++;
System.out.print(i);
try{
Thread.sleep(100);
}catch(Exception e){
e.printStackTrace();
}
   }
     public static void runThreadB(){
i--;
System.out.print(i);
try{
Thread.sleep(100);
}catch(Exception e){
e.printStackTrace();
}
    }

/**
* @param args
*/
public static void main(String[] args) {
System.out.print(0);
try{
while(true){
       new Thread(new MutilThreadTest()).start();
       Thread.sleep(400);
}
}catch(Exception e){
e.printStackTrace();
}
}

/* (non-Javadoc)
* @see java.lang.Runnable#run()
*/
public void run() {
try{
new MutilThreadTest().runThreadA();
new MutilThreadTest().runThreadA();
new MutilThreadTest().runThreadB();
new MutilThreadTest().runThreadB();
}catch(Exception e){
e.printStackTrace();
}

}
}

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