StringIndexOutOfBoundsException in Java: charAt and substring (AP CSA)

A StringIndexOutOfBoundsException means you asked for a character position that does not exist in a String. Like arrays, String positions run from 0 to length() - 1, and methods like charAt and substring throw when you go past that.

Why students see this error

A String is an indexed sequence of characters numbered from 0. The string "cat" has length 3 and valid indexes 0, 1, and 2. Calling charAt(3) asks for a fourth character that does not exist, so Java throws the exception. The same applies to substring when the start or end goes past the end of the String.

• A loop uses i <= s.length() instead of i < s.length().
• Calling charAt(s.length()) instead of charAt(s.length() - 1) for the last character.
• A substring call whose end index is greater than the String's length.

Broken code that triggers it

public class Spell {
  public static void main(String[] args) {
    String word = "cat";   // length 3, indexes 0..2

    for (int i = 0; i <= word.length(); i++) {  // BUG: <=
      System.out.println(word.charAt(i));        // crashes at i = 3
    }
  }
}

The loop allows i to reach 3 because of <=, but "cat" has no character at index 3 (valid indexes are 0, 1, 2). On the pass where i is 3, charAt(3) throws the exception.

Trace: watch where it breaks

Trace the index against the valid range of the String:

Each existing character prints fine. The fault is the loop letting i reach length(), one past the last valid index of length() - 1.

The fix

public class Spell {
  public static void main(String[] args) {
    String word = "cat";

    for (int i = 0; i < word.length(); i++) {   // < stops at length() - 1
      System.out.println(word.charAt(i));
    }
  }
}

Using i < word.length() stops the loop after the last valid index. The same rule applies to substring(a, b): b may equal length() (it is exclusive), but no index may exceed it.

How College Board tests this concept

Watch for items that traverse a String with a <= loop, or that mix up substring's inclusive start and exclusive end. The exam often asks for the exact output of a String loop, where one off-by-one step changes everything.

Common MCQ traps (practice set)

Question 1: Predict the result

String s = "hi";
System.out.println(s.charAt(2));

Question 2: substring boundary

Consider these two calls on String t = "code"; (length 4).

I.    t.substring(0, 4)
II.   t.charAt(4)

Question 3: Find the last character safely

String w has at least one character.