Synchronizing elements in an array

Question

I am new to multi-threading in Java and don't quite understand what's going on.

From online tutorials and lecture notes, I know that the synchronized block, which must be applied to a non-null object, ensures that only one thread can execute that block of code. Since an array is an object in Java, synchronize can be applied to it. Further, if the array stores objects, I should be able to synchronize each element of the array too.

My program has several threads updated an array of numbers, hence I created an array of Long objects:

synchronized (grid[arrayIndex]){
    grid[arrayIndex] += a.getNumber();
}

This code sits inside the run() method of the thread class which I have extended. The array, grid, is shared by all of my threads. However, this does not return the correct results while running the same program on one thread does.

Answer 1

This will not work. It is important to realize that grid[arrayIndex] += ... is actually replacing the element in the grid with a new object. This means that you are synchronizing on an object in the array and then immediately replacing the object with another in the array. This will cause other threads to lock on a different object so they won't block. You must lock on a constant object.

You can instead lock on the entire array object, if it is never replaced with another array object:

synchronized (grid) {
    // this changes the object to another Long so can't be used to lock
    grid[arrayIndex] += a.getNumber();
}

This is one of the reasons why it is a good pattern to lock on a final object. See this answer with more details:

Why is it not a good practice to synchronize on Boolean?