How to check if a String is numeric in Java

Question

How would you check if a String was a number before parsing it?

Answer 1

Based off of other answers I wrote my own and it doesn't use patterns or parsing with exception checking.

It checks for a maximum of one minus sign and checks for a maximum of one decimal point.

Here are some examples and their results:

"1", "-1", "-1.5" and "-1.556" return true

"1..5", "1A.5", "1.5D", "-" and "--1" return false

Note: If needed you can modify this to accept a Locale parameter and pass that into the DecimalFormatSymbols.getInstance() calls to use a specific Locale instead of the current one.

 public static boolean isNumeric(final String input) {
    //Check for null or blank string
    if(input == null || input.isBlank()) return false;

    //Retrieve the minus sign and decimal separator characters from the current Locale
    final var localeMinusSign = DecimalFormatSymbols.getInstance().getMinusSign();
    final var localeDecimalSeparator = DecimalFormatSymbols.getInstance().getDecimalSeparator();

    //Check if first character is a minus sign
    final var isNegative = input.charAt(0) == localeMinusSign;
    //Check if string is not just a minus sign
    if (isNegative && input.length() == 1) return false;

    var isDecimalSeparatorFound = false;

    //If the string has a minus sign ignore the first character
    final var startCharIndex = isNegative ? 1 : 0;

    //Check if each character is a number or a decimal separator
    //and make sure string only has a maximum of one decimal separator
    for (var i = startCharIndex; i < input.length(); i++) {
        if(!Character.isDigit(input.charAt(i))) {
            if(input.charAt(i) == localeDecimalSeparator && !isDecimalSeparatorFound) {
                isDecimalSeparatorFound = true;
            } else return false;
        }
    }
    return true;
}