Java. Threads. I do the task according to the book and the conclusion is different each time, although it should be the same. Why ?

Question

Java

Friends, tell a newbie.
I do the task according to the book and the output is different each time, although it should be the same 98098.98099. Why is this and what to read about it?

Answer 1

Many thanks to everyone for the replies. The problem turns out to be that incrementing (counter += add;) is not an atomic operation. I did not know that. From here and each time different and incorrect values ​​​​at the output. It goes something like this:
T1: Thread 1 reads i from memory, it is equal to 1.
T2: Thread 1 increments to 2 in its register.
T3: A context switch occurs, thread 1 is suspended, the contents of its registers are flushed to temporary storage, and the contents of the registers for thread 2 are written to the processor, and this thread starts to work.
T4: Thread 2 reads i from memory, it is equal to 1.
T5: Thread 2 increments to 2 in its register.
T6: Thread writes to memory 2.
T7: Context switch again, first thread starts running.
T8: Thread 1 is writing to memory 2.

Answer 2

You have two threads using the same object. There is no synchronization between threads in actions.
Therefore, the answers will be different.
What does it mean to read? We have a habr, type "java streams" in the search, select articles according to your level and read =) In general, Schildt is good.

Answer 3

I once read this book: www.amazon.com/Java-Concurrency-Practice-Brian-Goe...
The basics haven't changed, but concurrency has been greatly improved in Java 7.
And first, it’s probably better to read elementary things from Eckel or Schildt (but I personally don’t like Schildt’s style)