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

You can also use StringUtils.isNumericSpace which returns true for empty strings and ignores internal spaces in the string. Another way is to use NumberUtils.is parsable which basically checks the number is parsable according to Java. (The linked javadocs contain detailed examples for each method.)

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Answer 2

Perhaps the easiest and the most reliable way to check whether a String is numeric or not is by parsing it using Java's built-in methods:

1. Integer.parseInt(String)
2. Float.parseFloat(String)
3. Double.parseDouble(String)
4. Long.parseLong(String)
5. new BigInteger(String)

If these methods don't throw any NumberFormatException, then it means that the parsing was successful and the String is numeric:

public static boolean isNumeric(String strNum) {
    if (strNum == null) {
        return false;
    }
    try {
        double d = Double.parseDouble(strNum);
    } catch (NumberFormatException nfe) {
        return false;
    }
    return true;
}

Answer 3

Check if a string is numeric

public class Numeric {

    public static void main(String[] args) {

        String string = "12345.15";
        boolean numeric = true;

        try {
            Double num = Double.parseDouble(string);
        } catch (NumberFormatException e) {
            numeric = false;
        }

        if(numeric)
            System.out.println(string + " is a number");
        else
            System.out.println(string + " is not a number");
    }
}

Output

12345.15 is a number

In the above program, we have a String named string that contains the string to be checked. We also have a boolean value numeric which stores if the final result is numeric or not.