Question

I am not fully understanding what the synchronization block is doing nor why it is necessary.
Can someone explain in a “synchronizing for dummies” kind of way?

In a book I am reading, the author tells me “The synchronization is necessary, since the members we manipulate within the
synchronized block could be manipulated in the onPause() method on the UI thread.”
He creates an Object named stateChanged and instantiates it as a new object.
Then, in the synchronization block he uses the stateChanged object as the argument.
This whole thing is throwing me off and I do not like to move on until I have a pretty good understanding of what is going on.

Answer 1

The classic example is: Imagine you have two threads of operation, and both of them reference the same method:

public void addToGlobalVar(int y) {
int x = globalVar;  //what if a thread stops right after this line?
x += y;
globalVar = y;
}

where globalVar is some other predefined number that this method can interact with and set. Lets say globalVar is 50.

Threads get computing time on a somewhat arbitrary basis, so you never fully know the precise nanosecond one stops and the other gets CPU time.

In this example, if you launched an AsyncTask in addition to the UI thread, and both at some point use addToGlobalVar(10), what can happen is that one thread might be interrupted at line 2 of that code block. If the other thread goes through while that one is sleeping, it will successfully set globalVar to 60. But when the other one wakes up, it still thinks x = 50, and its going to then set it to 60. So in essence you just made 50+10+10 = 60. Hopefully you can see how this becomes a problem.

You can fix this simple example by making the calculation atomic (skip declaring x, 1 line, all calcs done) or if the logic wasn’t able to be condensed to 1 line, you make a block of code atomic by using synchronized.