Java Program To Check If the given String Is A Number or Not

In this tutorial, we will see a Java program to check if the given String is a number or not.

We can check it using the regex expression.

Java Program To Check If the Given String is Number or Not Using Regex Expression

In the below program, we have simply used the regex expression to check whether the given string is numeric or not.

The regex expression to check this is -?\\d+

Here,

-? –> negative sign, could have none or one

\\d+ –> one or more digits

Java Program

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import java.util.List;

/**
 *  A Java program to check if the given String is a number or not.
 *  @author coderolls.com
 */
public class CheckIfNumber {

    public static void main(String[] args) {
        List<String> strings = List.of("95965", "Jack485", "-9859", "784Will", "056", "Google");
        for(String str : strings) {
            boolean isNumber = isNumberOrNot(str);
            if (isNumber) {
                System.out.println("The String '" + str + "' is a Number.");
            } else {
                System.out.println("The String '" + str + "' is not a Number.");
            }
        }

    }

    /**
     * A method checks if the input param string is numeric or not
     * It uses a regex Expression
     * -?     --> negative sign, could have none or one
     * \\d+   --> one or more digits
     * @param str
     * @return
     */
    private static boolean isNumberOrNot(String str) {
        return str.matches("-?\\d+");
    }
}

Output

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The String '95965' is a Number.
The String 'Jack485' is not a Number.
The String '-9859' is a Number.
The String '784Will' is not a Number.
The String '056' is a Number.
The String 'Google' is not a Number.

Java Program To Check If the Given String is Number or Not Using the Character.isDigit() method

In this method, we are iterating over an array of characters of the String and checking if any character is a digit or not.

If we find one or more digits in the string, the given string is not a number string or a number.

import java.util.List;

/**
 *  A Java program to check if the given String is a number or not.
 *  using the Character.isDigit() method.
 *  @author coderolls.com
 */
public class CheckIfNumber2 {

    public static void main(String[] args) {
        List<String> strings = List.of("95965", "Jack485", "-9859", "784Will", "056", "Google");
        for(String str : strings) {
            boolean isNumber = isNumberOrNot(str);
            if (isNumber) {
                System.out.println("The String '" + str + "' is a Number.");
            } else {
                System.out.println("The String '" + str + "' is not a Number.");
            }
        }

    }

    /**
     * A method checks if the input param string is numeric or not
     * It uses a regex Expression
     * -?     --> negative sign, could have none or one
     * \\d+   --> one or more digits
     * @param str
     * @return
     */
    private static boolean isNumberOrNot(String str) {
        char[] chars = str.toCharArray();
        for(int i=0; i<str.length(); i++) {

            char ch = str.charAt(i);
            if(i==0 && ch == '-') {
                continue;
            }
            
            if (! (Character.isDigit(ch))) {
                return false;
            }
        }
        //If not a single ch is a digit, the string is numeric
        return true;
    }
}

Output

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The String '95965' is a Number.
The String 'Jack485' is not a Number.
The String '-9859' is a Number.
The String '784Will' is not a Number.
The String '056' is a Number.
The String 'Google' is not a Number.

Java Program To Check If the Given String is Number or Not NumberFormatExpression way.

We can also check if it using the NumberFormatExpression way.

Let’s See how. (Not recommended to use, Just for Info)

We can try to parse the given String using the Integer.parseInt(str) method.

If the given string is not a number the method will throw NumberFormatExpression and we can return false by catching this expression. Otherwise true.

See the sample method below.

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public static boolean isInteger(String s) {
    try { 
        Integer.parseInt(s); 
    } catch(NumberFormatException e) { 
        return false; 
    } catch(NullPointerException e) {
        return false;
    }
    // only got here if we didn't return false
    return true;
}

But this way is not recommended as it causes expensive exceptions in the code.