NumberFormatException in Java

Direct Answer

NumberFormatException occurs when a string cannot be parsed in the format required by a numeric method such as Integer.parseInt. Examples include an empty string, "12.5" passed to parseInt, or a value outside the int range. At an input boundary, trim only if the contract permits it, call the correct parser, catch NumberFormatException, and return a clear validation error or another result that represents invalid input.

Code Example

@RestController
@RequestMapping("/api/orders")
public class OrderController {
    
    @GetMapping("/{orderId}")
    public ResponseEntity<?> getOrder(@PathVariable String orderId) {
        try {
            Long id = Long.parseLong(orderId);
            var order = orderService.findById(id);
            return ResponseEntity.ok(order);
        } catch (NumberFormatException e) {
            // Return 400 Bad Request with helpful message
            return ResponseEntity
                .badRequest()
                .body(Map.of("error", "Order ID must be a valid number"));
        }
    }
    
    // Alternative: validation before parsing
    public Optional<Integer> parseQuantity(String input) {
        if (input == null || !input.matches("\\d+")) {
            return Optional.empty(); // invalid format
        }
        return Optional.of(Integer.parseInt(input)); // safe now
    }
}

Common Mistakes & Pitfalls

• Letting exceptions propagate: Freshers allow NumberFormatException to bubble up to the framework, resulting in generic 500 errors shown to users. Always catch it at the controller layer and return a proper 400 with a meaningful message.
• Not trimming whitespace: Calling parseInt on " 123 " (with spaces) throws NumberFormatException. Always trim input strings before parsing.
• Parsing without validation: Calling parseInt on every user input without checking format first makes your API fragile. Validate or wrap in try-catch to handle bad input gracefully.