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多线程面试问题:顺序输出多个ABC序列

发布时间:2010年9月10日 / 分类:DOTNET / 8,405 次浏览 / 评论

多线程面试问题:顺序输出多个ABC序列,不使用Sleep。本来多线程是用来更高效的利用CPU的时间,所以执行的顺序是不太好控制的。这里不对这个问题的意义进行评述,说一下解决方案。参考了java的解决方案,主要方法就是指定下一个输出的字符,然后再输出的方法里边判断是不是当前字符就可以了。不知道有没有更好的方法。

先来看看代码吧:

public class ThreadDemo
    {
        public void ThreadStart()
        {
            MyLock myLock = new MyLock();
            Thread th1 = new Thread(new ThreadStart((new PrintCalss("A", "B", myLock)).Print));
            Thread th2 = new Thread(new ThreadStart((new PrintCalss("B", "C", myLock)).Print));
            Thread th3 = new Thread(new ThreadStart((new PrintCalss("C", "A", myLock)).Print));

            th1.Start();
            th2.Start();
            th3.Start();

            myLock.CurrentStr = "A";
        }
    }

    public class MyLock
    {
        public string CurrentStr { get; set; }
    }
    public class PrintCalss
    {
        MyLock myLock;
        public string CurrStr { get; set; }
        public string NextStr { get; set; }

        public PrintCalss(string currStr, string nextStr, MyLock curLock)
        {
            CurrStr = currStr;
            NextStr = nextStr;
            myLock = curLock;
        }

        public void Print()
        {
            int count = 10;
            while (count > 0)
            {
                if (myLock.CurrentStr != null && myLock.CurrentStr == CurrStr)
                {
                    //从程序执行的的顺序分析,这个lock似乎意义不大了
                    lock (myLock)
                    {
                        Console.Write(myLock.CurrentStr);
                        count--;
                        myLock.CurrentStr = NextStr;
                    }
                }
            }
        }
    }

根据上边的代码,又做了些简化:

public class ThreadDemo
    {
        string lockStr = string.Empty;
        string currStr = string.Empty;

        public void ThreadStart()
        {
            Thread th1 = new Thread(new ThreadStart(PrintA));
            Thread th2 = new Thread(new ThreadStart(PrintB));
            Thread th3 = new Thread(new ThreadStart(PrintC));

            th1.Start();
            th2.Start();
            th3.Start();

            lockStr = "A";
        }

        public void PrintA()
        {
            int count = 10;
            while (count > 0)
            {
                if (Print("A"))
                    count--;
            }
        }

        public void PrintB()
        {
            int count = 10;
            while (count > 0)
            {
                if (Print("B"))
                    count--; ;
            }
        }

        public void PrintC()
        {
            int count = 10;
            while (count > 0)
            {
                if (Print("C"))
                    count--;
            }
        }

        public bool Print(string printStr)
        {
            if (lockStr == printStr)
            {
                lock (this)
                {
                    Console.Write(lockStr);
                    lockStr = GetNextStr();
                    return true;
                }
            }

            return false;
        }

        public string GetNextStr()
        {
            if (lockStr == "A")
            {
                return "B";
            }
            else if (lockStr == "B")
            {
                return "C";
            }
            else if (lockStr == "C")
            {
                return "A";
            }

            return string.Empty;
        }
    }
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