Google multi-threaded interview questions: 4 threads Write data to 4 files, each thread can write only one value

by Long Luo

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Recently in the study Java多线程 , encountered a following pen question, the topic is as follows:

编写一个程序，程序会启动4个线程，向4个文件A，B，C，D里写入数据，每个线程只能写一个值。     线程A：只写A    线程B：只写B     线程C：只写C     线程D：只写D 4个文件A，B，C，D。 程序运行起来，4个文件的写入结果如下：     A：ABCDABCD...     B：BCDABCDA...     C：CDABCDAB...     

Online search, as if still a Google pen questions , the problem involves the knowledge points are:,, 多线程 并发 锁 , 线程间通信 .

Personal Analysis Process:

1. 创建4个线程；2. 每个线程在输出时都需要加锁；3. 操作文件的代码要加锁；4. 一个线程完成之后，通知下一个要执行的线程；

According to the above analysis, the following code can be written:

Package Com.imlongluo.practise;import Java.io.bufferedwriter;import Java.io.file;import java.io.FileOutputStream; Import Java.io.outputstreamwriter;import Java.util.concurrent.locks.condition;import Java.util.concurrent.locks.lock;import Java.util.concurrent.locks.reentrantlock;public class Practise {public        static void Main (string[] args) {Final task task = new Task ();            /** Create 4 threads and start at the same time *//** * thread A */New Thread (new Runnable () {@Override                public void Run () {for (int i = 0; i <; i++) {Task.outputa ();        }}}, "Thread A"). Start ();                /** * Thread B */New Thread (new Runnable () {@Override public void run () {                for (int i = 0; i < i++) {TASK.OUTPUTB ();        }}}, "Thread B"). Start ();    /** * Thread C * *    New Thread (New Runnable () {@Override public void run () {for (int i = 0; i < 10;                i++) {TASK.OUTPUTC ();        }}}, "Thread C"). Start ();                /** * Thread D */New Thread (new Runnable () {@Override public void run () {                for (int i = 0; i < i++) {TASK.OUTPUTD ();    }}}, "Thread D"). Start ();  }/** Task class */public static class Task {/** creates a lock lock object */private lock lock = new        Reentrantlock ();        Private BufferedWriter BW1 = null;        Private BufferedWriter bw2 = null;        Private BufferedWriter bw3 = null;        Private BufferedWriter bw4 = null;        private int ctl = 0;        /** * Condition: Thread condition */private Condition cond1 = Lock.newcondition ();  Private Condition Cond2 = Lock.newcondition ();      Private Condition Cond3 = Lock.newcondition ();        Private Condition Cond4 = Lock.newcondition ();        Private boolean[] Outputthisround = {False, True, True, true}; Public Task () {try {bw1 = new BufferedWriter (new OutputStreamWriter (New FileOutputStream (New F                Ile ("/users/luolong/code/android/workspace/multithreads/a.txt"))); BW2 = new BufferedWriter (new OutputStreamWriter (New FileOutputStream new File ("/users/luolong/code                /android/workspace/multithreads/b.txt "))); BW3 = new BufferedWriter (new OutputStreamWriter (New FileOutputStream new File ("/users/luolong/code                /android/workspace/multithreads/c.txt "))); BW4 = new BufferedWriter (new OutputStreamWriter (New FileOutputStream new File ("/users/luolong/code            /android/workspace/multithreads/d.txt "))); } catch (Exception e) {E.printStackTrace ();            }} public void Outputa () {lock.lock ();                    try {while (Outputthisround[0]) {System.out.println ("Outputa begin to Await!"); Cond1.await ();                    Block A thread} if (ctl% 4 = = 0) {bw1.write ("a");                Bw1.flush ();                    } else if (ctl% 4 = = 1) {bw4.write ("A");                Bw4.flush ();                    } else if (ctl% 4 = = 2) {bw3.write ("A");                Bw3.flush ();                    } else if (ctl% 4 = = 3) {bw2.write ("A");                Bw2.flush ();                } Outputthisround[0] = true;                OUTPUTTHISROUND[1] = false;                SYSTEM.OUT.PRINTLN ("Outputa signal outputb!"); Cond2.signal ();            Wake b Thread} catch (Exception e) {e.printstacktrace (); } finally {lock.unlock ();            }} public void Outputb () {lock.lock ();                    try {while (outputthisround[1]) {System.out.println ("OUTPUTB begin to Await!");                Cond2.await ();                    } if (ctl% 4 = = 0) {bw2.write ("B");                Bw2.flush ();                    } else if (ctl% 4 = = 1) {bw1.write ("B");                Bw1.flush ();                    } else if (ctl% 4 = = 2) {bw4.write ("B");                Bw4.flush ();                    } else if (ctl% 4 = = 3) {bw3.write ("B");                Bw3.flush ();                } outputthisround[1] = true;                OUTPUTTHISROUND[2] = false;                SYSTEM.OUT.PRINTLN ("OUTPUTB signal outputc!");            Cond3.signal ();            } catch (Exception e) {e.printstacktrace (); } finAlly {Lock.unlock ();            }} public void Outputc () {lock.lock ();                    try {while (outputthisround[2]) {System.out.println ("OUTPUTC begin to Await!");                Cond3.await ();                    } if (ctl% 4 = = 0) {bw3.write ("C");                Bw3.flush ();                    } else if (ctl% 4 = = 1) {bw2.write ("C");                Bw2.flush ();                    } else if (ctl% 4 = = 2) {bw1.write ("C");                Bw1.flush ();                    } else if (ctl% 4 = = 3) {bw4.write ("C");                Bw4.flush ();                } outputthisround[2] = true;                OUTPUTTHISROUND[3] = false;                SYSTEM.OUT.PRINTLN ("OUTPUTC signal outputd!");            Cond4.signal ();            } catch (Exception e) {e.printstacktrace (); }finally {Lock.unlock ();            }} public void Outputd () {lock.lock ();                    try {while (Outputthisround[3]) {System.out.println ("OUTPUTD begin to Await!");                Cond4.await ();                    } if (ctl% 4 = = 0) {bw4.write ("D");                Bw4.flush ();                    } else if (ctl% 4 = = 1) {bw3.write ("D");                Bw3.flush ();                    } else if (ctl% 4 = = 2) {bw2.write ("D");                Bw2.flush ();                    } else if (ctl% 4 = = 3) {bw1.write ("D");                Bw1.flush ();                } Outputthisround[3] = true;                Outputthisround[0] = false;                ctl++;                SYSTEM.OUT.PRINTLN ("OUTPUTD signal outputa!");            Cond1.signal (); } catch (Exception e) {e.printstAcktrace ();            } finally {Lock.unlock (); }        }    }}

Above.

If you have other better ways, welcome to discuss:-).

Created by Long Luo in 2015-04-09 22:14:02 @Shenzhen, China. completed by Long Luo in 2015-04-09 23:46:29 @Shenzhen, China.

Google multi-threaded interview questions: 4 threads Write data to 4 files, each thread can write only one value