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 A grid with row and column numbers labeled on the edges, and one cell highlighted at row 2 column 1.

   int[][] map = {
       {0, 0, 0, 0, 0},
       {0, 0, 0, 0, 0},
       {0, 9, 0, 0, 0}
   };
   
   map[2][1]   // 9. Row 2, column 1.

   ROW first, then COLUMN. Always.

   `map[2][1]` means: go to row 2 (the third row down), then along to column 1 (the second across). That is where the 9 is.

   Read it aloud as "row two, column one". Saying it that way every time is the single most effective way to stop getting it backwards.

2. Step 2

   On your screen A diagram contrasting x-then-y screen coordinates with row-then-column array indexing.

   setLocation(x, y);   // x FIRST, across then down
   map[row][col];       // row FIRST, down then across

   And here is why everybody gets this wrong.

   Every coordinate in this course so far has been x first: setLocation, addObject, super. Across, then down.

   A 2D array is the OPPOSITE. Row first, which is the down one.

   These two orders sit next to each other in the same line of code in 6.4, so being clear now saves a great deal of confusion later.

3. Step 3

   On your screen A non-square grid rendered correctly beside the same grid rendered transposed, with the difference obvious.

   // A 2 by 4 map. NOT square, on purpose.
   int[][] m = {
       {1, 2, 3, 4},
       {5, 6, 7, 8}
   };
   
   m[1][3];   // 8, correct
   m[3][1];   // THROWS: there is no row 3

   This map is deliberately not square, and that is the lesson.

   Swapping the indexes here throws immediately, because there is no row 3. That is the kind way to find out.

   On a SQUARE map, swapping them does not throw. It quietly reads the wrong cell and your map renders transposed, flipped along the diagonal. If your test map is square, the bug is invisible.

   Always test with a non-square map. Always.