Array index out of bounds: 30 (Java)

Question

I am writing a program to print out a user inputed integer into binary form. When I run it and input, say the number 5, it crashes and gives me the error:
java.lang.ArrayIndexOutOfBoundsException: 30 at PrintBinaryDigitsFixed.main(PrintBinaryDigitsFixed.java:27) i.e the line "digits[counter] = number % 2;"

Why am I getting an out of bounds exception? It should assign the remainder to the first element then move on to the second shouldn't it?

I feel like I'm making a glaringly obvious mistake but I can't tell what it is

 final int MIN = 0; final int MAX = (int) (Math.pow(2, 30) - 1); int[] digits = new int[30]; //array to hold the digits int number = readInput ("Enter an integer from " + MIN + " to " + MAX, MIN, MAX); int counter = 0; int modNumber = 2; while(modNumber / 2 != 0) { digits[counter] = number % 2; modNumber = number / 2; counter++; } System.out.print(number + " in binary form is "); listBackwardsFrom(digits, counter);

Thanks

If number = 5, number/2 returns 2 and hence modNumber is never equals to 0. — Alexis C.
– Alexis C., Commented Nov 3, 2013 at 17:33
BTW, you may want to use bit masks and shift operators instead of mod and division. Will be way faster and cleaner. — billc.cn
– billc.cn, Commented Nov 3, 2013 at 17:35
Thanks for the reply, I've never heard of those so I'll have a look. They sound like something to do with cryptography? — Callum.W
– Callum.W, Commented Nov 3, 2013 at 17:47
Does this answer your question? ArrayList index out of bounds — tripleee
– tripleee, Commented Oct 3, 2022 at 4:22

T.J. Crowder · Accepted Answer · 2013-11-03 17:32:36Z

You never change number in your loop, and you assign modNumber = number / 2 in the loop, so from the second iteration onward modNumber is a constant (for most of the first iteration it's 2, but then you assign number / 2 to it); if you reach that point at all, you'll stay there. So the loop continues until counter reaches 30, at which point digits[counter] throws the exception.

Thanks! You can imagine my inward groan when I realised that I completely missed that

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