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java - Threads with shared integer object not working as expected

问题描述

I have a problem where i have to print the numbers in such format. 

First  1
First  2
Second  3
Second  4
First  5
First  6
Second  7
Second  8
First  9
and so on...

I have implemented my runnable interface as below.

class ThreadDemo implements Runnable {

 public volatile Integer num;

 public Object lock;

 public ThreadDemo(Integer num, Object lock) {
  this.num = num;
  this.lock = lock;
 }

 @Override
 public void run() {

  try {
   while (true) {
    int count = 0;
    synchronized(lock) {
     Thread.sleep(100);
     while (count < 2) {
      System.out.println(Thread.currentThread().getName() + "  " + num++);
      count++;

     }
     lock.notify();
     lock.wait();
    }
   }
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
}

My main class is as follows 

public class CoWorkingThreads {
 private static volatile Integer num = new Integer(1);
 public static void main(String...args) {
  Object lock = new Object();
  Thread thread1 = new Thread(new ThreadDemo(num, lock), "First");
  thread1.start();
  Thread thread2 = new Thread(new ThreadDemo(num, lock), "Second");
  thread2.start();

 }
}

when i run the program i am getting the output as follows 

First  1
First  2
Second  1
Second  2
First  3
First  4
Second  3
Second  4

Instead of previously expected results. But when I Change the integer to atomic integer type i start getting the expected result. can anyone explain what is i can do to make it run with integer instead of using atomic integer

标签： javamultithreadingjava-threadsatomicinteger

解决方案

Java Integer 不能通过引用传递。在您的代码中，每个线程都会创建变量的副本。但是 atomicInteger 可以通过引用传递。

此外，为了获得正确的结果，您可以将 num 变量更改为静态变量。

public static Integer num = 1;

public Object lock;
public ThreadDemo(Integer num, Object lock) {
    //this.num = num;
    this.lock =lock;
}

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