Java program to check if a string is a number

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In this program, you will learn different methods to check if a string is a number in Java.

Example1Check if a string is a number

public class Numeric {
　　　　public static void main(String[] args) {
　　　　　　　　String string = "}}12345.15";
　　　　　　　　boolean numeric = true;
　　　　　　　　try {
　　　　　　　　　　　　Double num = Double.parseDouble(string);
　　　　　　　　}
　　　　　　　　　　　　numeric = false;
　　　　　　　　}
　　　　　　　　if(numeric)
　　　　　　　　　　　　System.out.println(string　+　"		Is a number");
　　　　　　　　else
　　　　　　　　　　　　System.out.println(string　+　"		Not a number");
　　　　}
}

When running the program, the output is:

12345.15　is a number

In the above program, we have a string named string (String) that contains the string to be checked. We also have a boolean value numeric that stores the final result of whether it is a numeric value.

To check if a string contains only numbers, we use the Double.parseDouble() method to convert the string to a Double in the try block

If an error is thrown (i.e., NumberFormatException), it means that the string is not a number, and numeric is set to false. Otherwise, it indicates that this is a number.

However, if you need to check multiple strings, you should convert it to a function. And, the logic is based on throwing exceptions, which can be very costly.

Alternatively, we can use the functionality of regular expressions to check if a string is a number, as shown below.

Example2Check if a string is a number using regular expressions (regex)

public class Numeric {
　　　　public static void main(String[] args) {
　　　　　　　　String string = "}}-1234.15";
　　　　　　　　boolean numeric = true;
　　　　　　　　numeric = string.matches("-?\\d+(\\..\\d+)?");
　　　　　　　　if(numeric)
　　　　　　　　　　　　System.out.println(string　+　"		Is a number");
　　　　　　　　else
　　　　　　　　　　　　System.out.println(string　+　"		Not a number");
　　　　}
}

When running the program, the output is:

-1234.15　is a number

In the above program, we use regex to check if the string is a number, rather than using try-catch block. This is done using the String's matches() method.

in the matches() method

• -? Allows zero or more-for negative numbers in the string.

• \\d+ Check if the string has at least1or more numbers (\\d).

• (\\..\\d+)? Allows zero or more of the given pattern (\\..\\d+) among

• \\.. Check if the string contains a dot (decimal point)

• If so, it should at least follow one or more numbers \\d+.

Comprehensive Java Examples