多线程java程序，可以交替打印偶数和奇数 [英] Multi threaded java program to print even and odd numbers alternatively

问题描述

我被要求在一次采访中写一个双线程的Java程序。在这个程序中，一个线程应该打印偶数，另一个线程应该交替打印奇数。

I was asked to write a two-threaded Java program in an interview. In this program one thread should print even numbers and the other thread should print odd numbers alternatively.

示例输出：

线程1：1

线程2：2

线程1：3

线程2：4
......等等

Thread2: 4 ... and so on

我编写了以下程序。一个类任务，其中包含两种分别打印偶数和奇数的方法。从main方法，我创建了两个线程来调用这两个方法。面试官让我进一步改进，但我想不出任何进步。有没有更好的方法来编写相同的程序？

I wrote the following program. One class Task which contains two methods to print even and odd numbers respectively. From main method, I created two threads to call these two methods. The interviewer asked me to improve it further, but I could not think of any improvement. Is there any better way to write the same program?

class Task
{
    boolean flag;

    public Task(boolean flag)
    {
        this.flag = flag;
    }
    public void printEven()
    {
        for( int i = 2; i <= 10; i+=2 )
        {
            synchronized (this)
            {
                try
                {
                    while( !flag )
                        wait();
                    System.out.println(i);
                    flag = false;
                    notify();
                }
                catch (InterruptedException ex)
                {
                    ex.printStackTrace();
                }
            }
        }
    }
    public void printOdd()
    {
        for( int i = 1; i < 10; i+=2 )
        {
            synchronized (this)
            {
                try
                {
                    while(flag )
                        wait();
                    System.out.println(i);
                    flag = true;
                    notify();
                }
                catch(InterruptedException ex)
                {
                    ex.printStackTrace();
                }
            }
        }
    }
}

public class App {
    public static void main(String [] args)
    {
        Task t = new Task(false);
        Thread t1 = new Thread( new Runnable() {
           public void run()
           {
               t.printOdd();
           }
        });
        Thread t2 = new Thread( new Runnable() {
            public void run()
            {
                t.printEven();
            }
        });
        t1.start();
        t2.start();
    }
}

推荐答案

好，有很多选择。我可能会使用 SynchronousQueue （我不喜欢低级等待 / 通知并尝试使用更高级别的并发原语。另外 printOdd 和 printEven 可以合并为单个方法，不需要额外的标志：

Well, there are many alternatives. I would probably use a SynchronousQueue instead (I don't like low-level wait/notify and try to use higher-level concurrency primitives instead). Also printOdd and printEven could be merged into single method and no additional flags are necessary:

public class App {
    static class OddEven implements Runnable {
        private final SynchronousQueue<Integer> queue = new SynchronousQueue<>();

        public void start() throws InterruptedException {
            Thread oddThread = new Thread(this);
            Thread evenThread = new Thread(this);
            oddThread.start();
            queue.put(1);
            evenThread.start();
        }

        @Override
        public void run() {
            try {
                while (true) {
                    int i = queue.take();
                    System.out.println(i + " (" + Thread.currentThread() + ")");
                    if (i == 10)
                        break;
                    queue.put(++i);
                    if (i == 10)
                        break;
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        new OddEven().start();
    }
}

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