Java: Check If String Is a Number

In Java, determining whether a string is a valid number is a common task. There are several ways to achieve this, each suited to different scenarios. This guide will cover various methods to check if a string is a number, including using regular expressions, Integer.parseInt, Double.parseDouble, NumberFormat, and Apache Commons Lang.

Table of Contents

1. Introduction
2. Using Regular Expressions
3. Using Integer.parseInt and Double.parseDouble
4. Using NumberFormat
5. Using Apache Commons Lang
6. Conclusion

Introduction

Java provides several ways to check if a string is a number. Depending on your requirements, such as whether you want to check for integers or floating-point numbers, you can use different methods.

Using Regular Expressions

Regular expressions (regex) provide a flexible way to check if a string matches a pattern. You can use regex to determine if a string is a number.

Example

public class NumberCheckExample {
    public static void main(String[] args) {
        String str1 = "123";
        String str2 = "123.45";
        String str3 = "abc";

        System.out.println(isNumeric(str1)); // true
        System.out.println(isNumeric(str2)); // true
        System.out.println(isNumeric(str3)); // false
    }

    public static boolean isNumeric(String str) {
        return str.matches("-?\\d+(\\.\\d+)?");
    }
}

Explanation

• str.matches("-?\\d+(\\.\\d+)?"): This regex matches both integers and floating-point numbers, including negative numbers.

Using Integer.parseInt and Double.parseDouble

You can use Integer.parseInt and Double.parseDouble methods to check if a string can be parsed as an integer or a double.

Example

public class NumberCheckExample {
    public static void main(String[] args) {
        String str1 = "123";
        String str2 = "123.45";
        String str3 = "abc";

        System.out.println(isInteger(str1)); // true
        System.out.println(isInteger(str2)); // false
        System.out.println(isInteger(str3)); // false

        System.out.println(isDouble(str1)); // true
        System.out.println(isDouble(str2)); // true
        System.out.println(isDouble(str3)); // false
    }

    public static boolean isInteger(String str) {
        try {
            Integer.parseInt(str);
            return true;
        } catch (NumberFormatException e) {
            return false;
        }
    }

    public static boolean isDouble(String str) {
        try {
            Double.parseDouble(str);
            return true;
        } catch (NumberFormatException e) {
            return false;
        }
    }
}

Explanation

• Integer.parseInt(str): Attempts to parse the string as an integer. If it fails, it throws a NumberFormatException.
• Double.parseDouble(str): Attempts to parse the string as a double. If it fails, it throws a NumberFormatException.

Using NumberFormat

The NumberFormat class provides methods to parse numbers and can be used to check if a string is a number.

Example

import java.text.NumberFormat;
import java.text.ParseException;

public class NumberCheckExample {
    public static void main(String[] args) {
        String str1 = "123";
        String str2 = "123.45";
        String str3 = "abc";

        System.out.println(isNumber(str1)); // true
        System.out.println(isNumber(str2)); // true
        System.out.println(isNumber(str3)); // false
    }

    public static boolean isNumber(String str) {
        NumberFormat format = NumberFormat.getInstance();
        try {
            format.parse(str);
            return true;
        } catch (ParseException e) {
            return false;
        }
    }
}

Explanation

• NumberFormat.getInstance().parse(str): Attempts to parse the string as a number. If it fails, it throws a ParseException.

Using Apache Commons Lang

The Apache Commons Lang library provides a utility class NumberUtils that simplifies checking if a string is a number.

Dependency

Add the following dependency to your pom.xml if you are using Maven:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.12.0</version>
</dependency>

Example

import org.apache.commons.lang3.math.NumberUtils;

public class NumberCheckExample {
    public static void main(String[] args) {
        String str1 = "123";
        String str2 = "123.45";
        String str3 = "abc";

        System.out.println(NumberUtils.isCreatable(str1)); // true
        System.out.println(NumberUtils.isCreatable(str2)); // true
        System.out.println(NumberUtils.isCreatable(str3)); // false
    }
}

Explanation

• NumberUtils.isCreatable(str): Checks if the string is a valid Java number.

Conclusion

Checking if a string is a number in Java can be accomplished using various methods, including regular expressions, Integer.parseInt, Double.parseDouble, NumberFormat, and the Apache Commons Lang library. Each method has its own advantages and specific use cases:

• Regular expressions provide a flexible way to match patterns.
• Integer.parseInt and Double.parseDouble are straightforward for parsing specific number types.
• NumberFormat offers a more general approach to parsing numbers.
• The Apache Commons Lang library provides utility methods that simplify the process.

By understanding these methods, you can choose the most appropriate one for your specific use case when working with strings in Java.