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See the following code:
Unsafe Counter

Java

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

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

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

CounterClient

Java

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

Java

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-2
Counter value in pool-1-thread-2 = 10
Incrementing from pool-1-thread-1
Counter value in pool-1-thread-1 = 11
Incrementing from pool-1-thread-1
Incrementing from pool-1-thread-2
Counter value in pool-1-thread-2 = 13
Counter value in pool-1-thread-1 = 12

I am trying to understand what exactly is going on in the thread activity here that is not thread safe. As you can see, some values are out of order: e.g. 13,12

This is what I can deduct from the output:
Thread-1 increments (value++)
Thread-2 increments (value++)

Thread-1's value should be 1 now, but I see this:
Counter value in pool-1-thread-1 = 0
Is this because thread-1 is still reading and didn't write the result of the register back to memory?

Why is it 0? Didn't it just increment?
Thread-2's value is 1. That's logical.

This is all a bit confusing to me and I would like to know exactly what's going on in that output.

Thanks

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