NumberFormatException in Java: parseInt on Non-Numeric Text (AP CSA)

A NumberFormatException happens when you try to convert a String to a number, but the String is not a valid number. The usual culprit is Integer.parseInt on text that contains letters, spaces, or symbols.

Why students see this error

Integer.parseInt reads the characters of a String and builds an int. If any character is not part of a valid integer, there is no number to build, so Java throws a NumberFormatException. Common offenders are spaces, letters, decimal points (for parseInt), and empty Strings.

• Parsing text that contains letters or symbols, like "12a".
• Leading or trailing spaces, like " 12".
• Parsing an empty String, or a decimal like "3.5" with parseInt.

Broken code that triggers it

String input = "12a";        // not a valid integer
int n = Integer.parseInt(input);   // NumberFormatException
System.out.println(n);

The String "12a" starts like a number but contains the letter a. Integer.parseInt cannot build an int from it, so it throws a NumberFormatException rather than guessing or partially parsing.

Trace: watch where it breaks

Trace what parseInt sees character by character:

parseInt does not stop at the valid prefix and return 12; any invalid character makes the entire String invalid, so the whole call throws.

The fix

String input = "12";
try {
  int n = Integer.parseInt(input.trim());
  System.out.println(n);
} catch (NumberFormatException e) {
  System.out.println("Not a valid number");
}

Two defenses: trim() removes stray spaces that would otherwise cause a failure, and the try/catch handles input that is genuinely not a number without crashing. For AP CSA, the core point is ensuring the String is a clean integer before parsing.

How College Board tests this concept

While exception handling with try/catch is light on the exam, the behavior of parseInt can appear: given a String with non-digit characters, the result is a thrown exception, not a truncated number.

Common MCQ traps (practice set)

Question 1: Predict the result

int x = Integer.parseInt("3.5");

Question 2: Which parse successfully?

Each call uses Integer.parseInt.

I.    Integer.parseInt("-42")
II.   Integer.parseInt(" 42")
III.  Integer.parseInt("42")

Question 3: Diagnose the cause

A program reads a String from the user and calls Integer.parseInt on it, and sometimes throws NumberFormatException.