java applet - array checking

Question

OK so my code is here: http://www.so.pastebin.com/m7V8rQ2n

What I want to know... let's say I have an image which I can redraw on tiles... is there a way to check for future tiles so I DON'T go out of bounds of my already defined tile map?

Like if I were at the edge of a map... it would NOT let me go past it?

Thanks.

Answer 1

Generally speaking, preventing ArrayIndexOutOfBoundsException can be done by simply making sure that the index is within the bound.

JLS 10.4 Array Access

All arrays are 0-origin. An array with length n can be indexed by the integers 0 to n-1.

Thus, a simple check like this is quite typical:

if (i >= 0 && i < arr.length) {
    System.out.println(arr[i]);
}

Barring nasty things like arr getting reassigned between the check and the access, the above code will NEVER throw ArrayIndexOutOfBoundsException.

---

2D array "boards"

Often, you can be a lot more specific, e.g. when you have rectangular "boards" stored in a two-dimensional array (or rather, array of arrays in Java).

final int M = 10;  // height, i.e. number of rows
final int N = 8;   // width, i.e. number of columns
final int[][] board = new int[M][N];

Then you can have a method like the following:

boolean isInBound(int r, int c) {
    return (r >= 0) && (r < M) && (c >= 0) && (c < N);
}

The bound check is both easier to read and to write, since we know that we have an MxN board. If isInBound(r, c), then board[r][c] will NEVER throw ArrayIndexOutOfBoundsException.