Threads in details

Neo10101

See the following code: Unsafe Counter

public class Counter implements Counting
{
private int value = 0;

public int incrementAndGet()
{
    return this.value++;
}

public int decrementAndGet()
{
    return this.value--;
}

}

CounterClient

public class CounterClient implements Runnable
{
//shared data
private Counting counter;

public CounterClient(Counting counter)
{
    this.counter = counter;
}

public void run()
{
    while(true)
    {
        System.out.println("Incrementing from " + Thread.currentThread().getName() );                
        System.out.println("Counter value in " + Thread.currentThread().getName() + " = " + counter.incrementAndGet());                
        
        try
        {
            Thread.sleep(2000L);
        } catch (InterruptedException ex)
        {
            Logger.getLogger(CounterClient.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

}

TestCounter

public class TestCounter
{
public static void main(String args[])
{
Counter counter = new Counter();
SafeCounter safeCounter = new SafeCounter();

    Executor exec = Executors.newCachedThreadPool();
    
    exec.execute(new CounterClient(counter));
    exec.execute(new CounterClient(counter));        
    /\*
    exec.execute(new CounterClient(safeCounter));
    exec.execute(new CounterClient(safeCounter));
     \*/
     
}

}

I did a test run for this code and one of the outputs was this:

Incrementing from pool-1-thread-1
Incrementing from pool-1-thread-2
Counter value in pool-1-thread-1 = 0
Counter value in pool-1-thread-2 = 1
Incrementing from pool-1-thread-2
Incrementing from pool-1-thread-1
Counter value in pool-1-thread-2 = 2
Counter value in pool-1-thread-1 = 3
Incrementing from pool-1-thread-2
Incrementing from pool-1-thread-1
Counter value in pool-1-thread-1 = 4
Counter value in pool-1-thread-2 = 5
Incrementing from pool-1-thread-2
Incrementing from pool-1-thread-1
Counter value in pool-1-thread-1 = 6
Counter value in pool-1-thread-2 = 7
Incrementing from pool-1-thread-1
Counter value in pool-1-thread-1 = 8
Incrementing from pool-1-thread-2
Counter value in pool-1-thread-2 = 9
Incrementing from pool-1-thread

Nagy Vilmos

The code you have will return the value then increment, or decrement. To perform the change before hand try:

return ++this.value;

It is not really thread safe, but it is such a trivial example that you won't see a problem.

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