Scenario #1

Loop Bound: <= vs < With .length

An array with 5 elements has indices 0–4. Using <= with .length attempts index 5.

int[] arr = {10, 20, 30, 40, 50};
for (int i = 0; i <= arr.length; i++)  // i goes to 5
{
    System.out.println(arr[i]);  // crashes at i=5
}

for (int i = 0; i < arr.length; i++)  // i goes to 4
{
    System.out.println(arr[i]);
}

Scenario #2

Starting at 1 Instead of 0

Java arrays are zero-indexed. Starting a loop at i = 1 skips the first element entirely.

int sum = 0;
for (int i = 1; i < arr.length; i++)
{
    sum += arr[i];
}
// Misses arr[0] entirely

Scenario #3

Adjacent Comparison: Stopping Too Late

When comparing each element with its neighbor, the loop must stop one element early. Otherwise arr[i+1] goes out of bounds.

for (int i = 0; i < arr.length; i++)
{
    if (arr[i] == arr[i + 1])  // i+1 out of bounds on last pass
    {
        System.out.println("Duplicate at " + i);
    }
}

for (int i = 0; i < arr.length - 1; i++)
{
    if (arr[i] == arr[i + 1])
    {
        System.out.println("Duplicate at " + i);
    }
}

Scenario #4

String substring() — End Index Is Exclusive

substring(start, end) returns characters from start up to but not including end. Students forget the end is exclusive and extract one character too few or too many.

String word = "HELLO";
System.out.println(word.substring(1, 3));
// Prints "EL" (indices 1 and 2, NOT 1, 2, and 3)

// To get single char at index i (2026 curriculum uses substring):
String ch = word.substring(i, i + 1);

Scenario #5

2D Arrays: Wrong Dimension for Loop Bound

Using grid.length for the column loop or grid[0].length for the row loop causes out-of-bounds on non-square grids.

int[][] grid = new int[3][5]; // 3 rows, 5 cols
// WRONG: using grid[0].length (5) for row bound
for (int r = 0; r < grid[0].length; r++)  // goes to 4, but only rows 0-2
{
    // crashes at r=3
}

Scenario #6

Fencepost: Counting Gaps vs Posts

If you have 5 fence posts, you have 4 gaps between them. This pattern shows up when counting pairs, intervals, or separators.

// Printing elements separated by commas (no trailing comma)
for (int i = 0; i < arr.length; i++)
{
    System.out.print(arr[i]);
    if (i < arr.length - 1)
    {
        System.out.print(", ");
    }
}
// For {1,2,3}: prints "1, 2, 3" not "1, 2, 3, "