How to Check if a String Is Numeric in Java

java

prompt-engineering

best-practices

functional-programming

byAlex Kataev·Feb 15, 2025

To check if a String is numeric in Java, you can use the str.matches("\\d+") method. This will return true if the string contains only digits.

String str = "12345";
boolean isNumeric = str.matches("\\d+"); // True. That's a Bingo!
System.out.println("Is numeric: " + isNumeric); // Will print: Is numeric: true

Drill-Down Methods

The Try-Catch Approach with Parse Methods

A common way of checking numericality is by parsing the string to a number and observing if it throws an exception. The Double.parseDouble() method is quite handy for this.

public boolean isNumeric(String str) {
    try {
        double d = Double.parseDouble(str);
        return true; // Well, that escalated quickly!
    } catch (NumberFormatException nfe) {
        return false; // Caught 'em, red-handed!
    }
}

Remember: Use this approach sparingly if errors are likely to be frequent - exception handling can be performance-costly.

Getting Fancy with Regex

If we want to include negative numbers and decimals, we employ regex (regular expressions) to our arsenal.

boolean isNumeric = someString.matches("-?\\d+(\\.\\d+)?");

This checks for numbers, with or without a decimal point. Be aware, it doesn’t account for scientific notations or internationalization issues.

Strong Validation with NumberFormat

We can leverage Java's NumberFormat for a more robust validation.

public boolean isFullyNumeric(String str) {
    ParsePosition pos = new ParsePosition(0);
    NumberFormat.getInstance().parse(str, pos);
    return str.length() == pos.getIndex(); // Ensuring full string was numeric.
}

Smart Work with Apache Commons Lang

If you're a fan of the Apache Commons Lang library, you have a few handy helpers:

NumberUtils.isCreatable(str) (for checks including exponentials)
StringUtils.isNumeric(str) (for digit-only checks)
NumberUtils.isNumber(str) (for earlier versions prior to 3.5)

Streamline with Java 8 Streams

Java 8 introduced us to streams, which can make this process more efficient.

boolean isNumeric = someString.chars().allMatch(Character::isDigit); // Java 8, like a boss.

Navigating Edge Cases

When Spaces & Locale Come into Play

Sometimes a string might contain spaces or it might be dependent on the locale (decimal point and minus sign rules differ among regions).

For spaces: StringUtils.isNumericSpace(str)
For locale-specific checks: DecimalFormatSymbols

Handling non-Latin Digits

When working with non-Latin digits, Character.isDigit() is a lifesaver as it recognizes digits from multiple scripts:

boolean isNumeric = someString.codePoints().allMatch(Character::isDigit); // Not everyone speaks English ya'know!

In the Android Realm

For our Android kin, the TextUtils.isDigitsOnly() method works wonders.

boolean isNumeric = TextUtils.isDigitsOnly(someCharSequence); // Android for the win!

Embracing Functional Programming

Java 8 introduced us to functional programming, which led to the lambda expressions that are as efficient as they are expressive:

boolean isNumeric = someString.chars().allMatch(Character::isDigit); // Even Java like lambdas.

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