Check if String Contains Only Digits in Java

In this tutorial, we’ll explore different ways to check whether a given string contains only numeric digits in Java. This is a common validation step in many applications and interview coding problems.

🧪 Problem Statement

Given a string, determine if it contains only digits from 0-9.

🔍 Example

  • Input: "123456" → Output: true
  • Input: "abc123" → Output: false
  • Input: "" → Output: false

✅ Approach 1: Using Character Check


public class DigitCheck {
    public static void main(String[] args) {
        String input = "123456";
        System.out.println("Contains only digits: " + containsOnlyDigits(input));
    }

    static boolean containsOnlyDigits(String str) {
        if(str == null || str.isEmpty()) return false;

        for(char c : str.toCharArray()) {
            if(!Character.isDigit(c)) return false;
        }
        return true;
    }
}

✅ Approach 2: Using Regular Expression


public class DigitCheckRegex {
    public static void main(String[] args) {
        String input = "123abc";
        System.out.println("Contains only digits: " + input.matches("\\d+"));
    }
}

✅ Output

Contains only digits: true

🧠 Interview Tip

Using Character.isDigit() is a safe way to handle each character. Regex is faster to implement but might not give the best performance in tight loops or large datasets.

📌 Conclusion

We have discussed multiple ways to check if a string contains only digits in Java. Choose the method based on your requirement — character-by-character for better control, or regex for simplicity.

This problem is common in coding tests like those listed in Java Coding Round Questions.

🔤 Master Java String Validation Problems

Checking whether a string contains only digits is a common coding round and input validation problem. Strengthen your Java string handling skills by practicing these related problems and interview-focused topics.

🧠 Java Coding Round Questions

Practice frequently asked string and validation problems.

🧵 Java String Pool & Interning

Understand how strings are stored and handled internally in Java.

🚨 Exception Handling Best Practices

Learn how to safely validate user input and handle errors.

📦 Java Collections Interview Questions

Explore how strings and collections work together in real-world problems.

🔄 Palindrome String Program

Another classic string problem frequently asked in interviews.

🔁 Reverse String in Java

Strengthen your understanding of string traversal and manipulation.

🔍 First Non-Repeated Character

Learn frequency-based string processing techniques.

🧮 Check Anagram Strings

Practice character counting and comparison logic.

🎯 Java Interview Questions (Freshers)

Common interview questions related to strings and input validation.

🧪 Advanced Java Programs (Real-World)

Apply string validation logic in real-world Java programs.