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What it looks like

java.lang.NumberFormatException: For input string: "12a"

Extends IllegalArgumentException in java.lang.

What it means

The parse methods of Integer, Long, Double, BigDecimal and friends throw NumberFormatException when the text is not a valid number in the expected format. The message quotes the offending input, which makes empty strings and stray whitespace easy to spot: For input string: "" means the string was empty, and Cannot parse null string means it was null.

Common causes

• Whitespace, a thousands separator or a currency symbol in the text: " 42", "1,000", "$5".
• An empty or null value from a form field, query parameter or CSV cell.
• A value too large for the type: Integer.parseInt("3000000000") overflows an int.
• A decimal comma in a locale that does not use the dot, or a decimal point passed to parseInt.

How to fix it

1. Trim the input and check for empty before parsing.
2. Use Long.parseLong or BigInteger when values can exceed the int range.
3. Use NumberFormat.getInstance(locale) to parse locale-formatted numbers, and BigDecimal for money.
4. Catch the exception at the boundary where the text enters the system and turn it into a validation error.

Broken and fixed

int qty = Integer.parseInt(request.getParameter("qty")); // null, "" or " 3" fail

String raw = request.getParameter("qty");
int qty = (raw == null || raw.isBlank()) ? 1 : Integer.parseInt(raw.trim());

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In Spring and enterprise code

In Spring MVC, a bad number in a @RequestParam int becomes a MethodArgumentTypeMismatchException and a 400 response before your code runs.

Debug it faster

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