Question

If I want to lock the whole array I can use the synchronized keyword like this:

int arr[];

synchronized void inc(int a, int b){
    arr[a]=arr[a]+b;
}

But can I lock just the item arr[a] so that other threads can still read/write other items of the array at the same time?

Answer 1

Not out-of-the-box, but you can create an Object array that is the same size as your int array and populate the array with distinct Objects. So when you want to lock on a particular element in the int array, you lock on that Object at the corresponsible index:

final Object[] locks = new Object[arr.length]:
for(int i = 0; i < arr.length; i++) {
 locks[i] = new Object();
}

When locking: do

synchronized(locks[a]) {
  // do something here
}