Build it step by step

Every step below leaves a scenario that compiles and runs. If you stop halfway you will have something that works, not something broken. Follow along in Greenfoot rather than reading straight through.

1. Step 1

   On your screen An array diagram with each slot labeled with its index below it.

   int[] lanes = {3, 7, 2, 9};
   lanes[0]   // 3
   lanes[1]   // 7
   lanes[3]   // 9

   Square brackets after the name pick a slot.

   `lanes[0]` is the FIRST one. The index is not "which position counting from one", it is "how far along from the start", and the first element is zero steps along.

   That phrasing is worth adopting. It makes 0 feel correct rather than arbitrary.

2. Step 2

   On your screen An assignment into a specific array slot.

   lanes[2] = 5;
   // lanes is now {3, 7, 5, 9}

   An indexed slot behaves exactly like a variable. You can read from it and assign into it with `=`.

   Nothing new. `lanes[2]` is just a name for one particular box.

3. Step 3

   On your screen The length field being used, contrasted with the size method of a List.

   int[] lanes = {3, 7, 2, 9};
   lanes.length      // 4, NO parentheses
   
   List es = getObjects(Enemy.class);
   es.size()         // parentheses, because it is a method

   Arrays use `length` with NO parentheses. Lists use `size()` WITH parentheses.

   There is no logic to remember here that would help you. It is an inconsistency in Java, and everybody gets it wrong sometimes. If `length()` fails, drop the parentheses; if `size` fails, add them.

4. Step 4

   On your screen An ArrayIndexOutOfBoundsException with the offending index visible in the message.

   int[] lanes = {3, 7, 2, 9};   // length 4
   lanes[4];                     // THROWS: index 4 out of bounds for length 4

   Length 4 means the last valid index is 3. Asking for 4 throws.

   Read the message: it names the index you tried AND the length. When the index is exactly the length, as here, you have the classic off-by-one and the fix is almost always changing `<=` to `<` in a loop.

   You have met this rule three times already: world coordinates run 0 to width-1, getRandomNumber(n) never gives n, and `i < n` stops at n-1. Same rule, fourth costume.