Java Program to Count Number of Digits in a String

How to Check if a String Contains only Digits in Java?

Last Updated : 28 Nov, 2024

In Java, to check if a string contains only digits, we can use various methods such as simple iteration, regular expressions, streams, etc.

Example:

The example below uses the Character.isDigit() method to check each character of the string to ensure all characters are digits. This is the most simple method among all others.

Java

// Java program to check if a string 
// contains only digits using Character.isDigit()
public class CheckDigits {
  
    public static boolean onlyDigits(String s) {
      
        // Traverse each character in the string
        for (int i = 0; i < s.length(); i++) {
          
            // Check if the character is not a digit
            if (!Character.isDigit(s.charAt(i))) {
              
                // If any character is not a digit, return false
                return false; 
            }
        }
        return true;  // If all characters are digits, return true
    }

    public static void main(String[] args) {
      
        System.out.println(onlyDigits("1234"));       
        System.out.println(onlyDigits("123s"));       
    }
}

Output

true
false

Table of Content

Other Methods to Check if a String Contains only Digits

Using Regular Expressions
Using Traversal
Using Arraylist.contains() Method

Other Methods to Check if a String Contains only Digits

1. Using Regular Expressions

In this method, we will use the regular expressions to check if the string matches the pattern of only digits. This method is more complex than Character.isDigit() but still very readable and concise.

Java

// Java program to check if a string 
// contains only digits using regex
import java.util.regex.*;

public class CheckDigits {
  
    public static boolean onlyDigits(String s) {
      
        // Regex to check if the string 
        // contains only digits
        return s.matches("[0-9]+");
    }

    public static void main(String[] args) {
      
        System.out.println(onlyDigits("1234"));       
        System.out.println(onlyDigits("123s"));       
    }
}

Explanation: In the above example, the regular expression "[0-9]+" matches strings that contain only digits. The matches() method returns true if the string matches the regex.

2. Using Traversal

In this method, the idea is to traverse each character in the string and check if the character of the string contains only digits from 0 to 9. If all the character of the string contains only digits then return true, otherwise, return false.

Java

// Java program to check if a string 
// contains only digits using traversal
class CheckDigits { 
 
    public static boolean onlyDigits(String s, int n) 
    { 
        // Traverse the string from 
        // start to end 
        for (int i = 0; i < n; i++) { 

            // Check if character is 
            // not a digit between 0-9 
            // then return false
            if (s.charAt(i) < '0'
                || s.charAt(i) > '9') { 
                return false; 
            } 
        } 
          // If we reach here, that means 
          // all characters were digits
        return true; 
    } 
 
    public static void main(String args[]) { 
      
        String s = "1s234"; 
        int l = s.length(); 
 
        System.out.println(onlyDigits(s, l)); 
    } 
}

Output

false

Note: This approach works but is less readable than Character.isDigit(), as it requires checking ASCII values manually.

3. Using Arraylist.contains() Method

In this method, it creates an ArrayList of digits (0 to 9) and checks if each character in the string is present in this list. It is less efficient and more complex than other methods.

Java

// Java program to check a string
// contains only digits using Arraylist.contains()
import java.util.*;

class CheckDigits {

    public static boolean onlyDigits(String s) {
      
        // String containing all digits
        String d = "0123456789";
        ArrayList<Character> n = new ArrayList<Character>();

        // Add all digits to ArrayList
        for (int i = 0; i < d.length(); i++) {
            n.add(d.charAt(i));
        }

        // Traverse the string and check 
        // if each character is a digit
        for (int i = 0; i < s.length(); i++) {
          
            // Check if the character is not a digit
            if (!n.contains(s.charAt(i))) {
                return false;
            }
        }

        // If we reach here, 
        // all characters are digits
        return true;
    }

    public static void main(String args[]) {
      
        String s = "1234"; 
        System.out.println(onlyDigits(s)); 
    }
}

Output

true

Java Program to Count Number of Digits in a String

prashant_srivastava

Improve

Article Tags :

Practice Tags :

Similar Reads

How to Check if a String contains only ASCII in Java?

The full form of ASCII is the American Standard Code for Information Interchange. It is the numeric representation of Character. As Java supports multiple languages, and it follows the Unicode system. In simple terms for better understanding, it converts the Character to a specific unique number, an

How to Check if a String Contains Only Lowercase Letters in Java?

A string is a data structure that contains a set of characters. It can be a word or can be a sentence. Also, it can be empty or can have a single letter. A string can contain Uppercase letters, Lowercase letters, or numbers of special characters. In this article, we will learn how to check if a Stri

Check if a String Contains Only Alphabets in Java using ASCII Values

Given a string, now we all know that the task is to check whether a string contains only alphabets. Now we will be iterating character by character and checking the corresponding ASCII value attached to it. If not found means there is some other character other than "a-z" or "A-Z". If we traverse th

Java Program to Count Number of Digits in a String

The string is a sequence of characters. In java, objects of String are immutable. Immutable means that once an object is created, it's content can't change. Complete traversal in the string is required to find the total number of digits in a string. Examples: Input : string = "GeeksforGeeks password

How to Convert a String to an Numeric in Java?

In Java programming, there are situations in which we must convert a string of numeric characters into one of the following data types: double, float, long, or int. To execute arithmetic operations and other numerical calculations, this conversion is necessary. We will look at how to convert a strin

Check if a given string is Pangram in Java

Given string str, the task is to write Java Program check whether the given string is a pangram or not. A string is a pangram string if it contains all the character of the alphabets ignoring the case of the alphabets. Examples: Input: str = "Abcdefghijklmnopqrstuvwxyz"Output: YesExplanation: The gi

Capitalize the First Letter of a String in Java

In Java, to check if a String contains only uppercase letters, we can use many approaches. These approaches verify each character in the String to ensure they are all in uppercase. A simple method, such as iterating through the characters and checking their case. In this article, we will discuss how

Extract all integers from the given String in Java

In Java, we can extract integers from a given string using several approaches such as regular expressions, character iteration, or the split() method. In this article, we will learn how to extract all integers from the given String in Java.Example:We will use regular expressions to extract all integ

Java Program to Check Whether the String Consists of Special Characters

In Java, special characters refer to symbols other than letters and digits, such as @, #, !, etc. To check whether the String consists of special characters, there are multiple ways, including using the Character class, regular expressions, or simple string checks.Example:In this example, we will us

Java Program to Check If a Number is Neon Number or Not

A neon number is a number where the sum of digits of the square of the number is equal to the number. The task is to check and print neon numbers in a range. Illustration: Case 1: Input : 9 Output : Given number 9 is Neon number Explanation : square of 9=9*9=81; sum of digit of square : 8+1=9(which