Populating array with SecureRandom: Why not out of bounds?

I am trying to populate an array (probCount) using a SecureRandom object (index) to determine probabilities. I have a for loop to run this about 100 million times:

probCount[index.nextInt(probCount.length)]++;

When I print the array, all my elements are populated correctly. How does this line does not throw an out of bounds exception?

I had it like: 

probCount.length - 1

This, however, never populated my last element. I am a bit confused.

Jon Skeet
people
quotationmark

From the Random.nextInt(int) documentation:

Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)

...

Parameters:
bound - the upper bound (exclusive). Must be positive.

Note the "exclusive" part. So for example, if you call index.nextInt(5) it will return 0, 1, 2, 3 or 4... which are precisely the valid indexes for an array of length 5.

people

See more on this question at Stackoverflow