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Check String Contains Only Digits in Java

beginner· Strings · String

Problem

A string contains only digits when every single character in it is one of 0 through 9, with nothing else mixed in.

Given a string, determine whether it consists entirely of digit characters.

Input

12345

Output

Contains only digits: true

Java Program

Java

public class OnlyDigitsCheck {
    public static void main(String[] args) {
        String str = "12345";
        boolean onlyDigits = true;

        for (char c : str.toCharArray()) {
            if (!Character.isDigit(c)) {
                onlyDigits = false;
                break; // found a non-digit, no need to keep checking
            }
        }

        System.out.println("Contains only digits: " + onlyDigits);
    }
}

▶ Run Code

Output

Contains only digits: true

Core Logic

Checking every character against Character.isDigit(), and stopping the instant one fails, confirms whether the whole string is made of digits.

How It Works

1onlyDigits starts as true, assuming the string qualifies until proven otherwise.
2A for-each loop visits each character of the string in turn.
3Character.isDigit(c) checks whether the current character is one of 0 through 9.
4The first character that fails the check sets onlyDigits to false and exits the loop immediately with break.

For "12345", every character passes Character.isDigit(), so the loop finishes with onlyDigits still true.

💡

Key Point: This checks each character individually, rather than trying to parse the whole string as a number — a string like "007" is correctly reported as all digits, without worrying about leading zeros.

Complexity

Time Complexity: O(n)Space Complexity: O(1)

Why: Each character is checked once, and the loop exits at the first non-digit found, without allocating anything beyond a boolean flag.

Key Concepts

Character.isDigit()for-each loopearly exit with break

Approach 2: Using matches()

Java

public class OnlyDigitsCheckRegex {
    public static void main(String[] args) {
        String str = "12345";
        // matches() requires the pattern to cover the entire string, not just part of it
        boolean onlyDigits = str.matches("[0-9]+");
        System.out.println("Contains only digits: " + onlyDigits);
    }
}

▶ Run Code

Output

Contains only digits: true

Core Logic

A regex pattern can express 'one or more digits, and nothing else' directly, checking the whole string in a single call.

How It Works

1str.matches("[0-9]+") checks the pattern against the string.
2matches(), unlike find() on a Matcher, requires the pattern to match the ENTIRE string, not just part of it.
3[0-9]+ means 'one or more digit characters', so any non-digit anywhere in the string causes the whole match to fail.

"12345".matches("[0-9]+") returns true, since every character is a digit and the pattern covers the whole string.

💡

Key Point: The trailing + matters — without it, an empty string would also match [0-9]*, which is usually not the intended result for 'contains only digits'.

Complexity

Time Complexity: O(n)Space Complexity: O(1)

Why: matches() still scans every character against the pattern, but compiling and running the regex adds overhead a plain loop doesn't have.

Key Concepts

regexString.matches()

Approach 3: Java 8

Java

public class OnlyDigitsCheckStream {
    public static void main(String[] args) {
        String str = "12345";
        // allMatch() short-circuits at the first character that fails the check
        boolean onlyDigits = str.chars().allMatch(Character::isDigit);
        System.out.println("Contains only digits: " + onlyDigits);
    }
}

▶ Run Code

Output

Contains only digits: true

Core Logic

The same per-character check can ask a stream directly — does every character satisfy Character.isDigit()?

How It Works

1str.chars() returns an IntStream of the string's character codes.
2.allMatch(Character::isDigit) checks that every single code passes the digit test.
3allMatch() returns true only if every character qualifies, and stops at the first one that doesn't.

For "12345", allMatch(Character::isDigit) checks all five characters and finds no non-digit, so it returns true.

💡

Key Point: allMatch() short-circuits the same way the loop's break did — it stops checking as soon as a non-digit is found.

Complexity

Time Complexity: O(n)Space Complexity: O(1)

Why: allMatch() stops as soon as it finds a non-digit, the same short-circuiting behavior as the loop's break.

Key Concepts

Streamchars()allMatch()

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