beginnerString ProblemsJava

Check Whether a String Contains Only Digits

Return true only if the string is non-empty and every character is a digit from 0 to 9.

Quick Answer

Scan the string once. If it is empty, return false. For each character, check whether it lies between '0' and '9'; the moment you find one that does not, return false. If the loop finishes without a non-digit, return true.

Problem Statement

Given a string, determine whether it consists only of decimal digits ('0' through '9'). Return true if every character is a digit and the string is not empty; otherwise return false.

A leading +/-, a decimal point, or any letter or space makes the answer false. For example, "12345" returns true, while "12a45" and "-99" return false.

Input: A single string s.

Output: A boolean: true if s is non-empty and all characters are digits, else false.

Examples

Example 1
Input:  "12345"
Output: true

Every character is between '0' and '9', and the string is not empty.

Example 2
Input:  "12a45"
Output: false

The character 'a' is not a digit, so the whole check fails.

Constraints

  • 0 <= s.length() <= 10000
  • Only characters '0'-'9' count as digits; no sign or decimal point allowed

Think Before You Code

Reveal the questions to ask yourself first
  • How do you test whether a single character is a digit without a library call?
  • As soon as you find one non-digit, do you need to keep scanning?
  • What should an empty string return, and why?

Hints

Open them one at a time — try after each before revealing the next.

Hint 1
A character `c` is a digit exactly when `c >= '0' && c <= '9'`.
Hint 2
You can return `false` immediately on the first non-digit — no need to look further.
Hint 3
Treat the empty string as `false` up front, since it contains no digits at all.

Approach

Reveal the step-by-step approach

Do a single left-to-right pass with an early exit.

  1. If the string is empty, return false.
  2. For each character c:
    • If c < '0' or c > '9', return false.
  3. If the loop completes, every character was a digit, so return true.

Comparing against the characters '0' and '9' works because the digit characters are contiguous and in order in ASCII.

Dry Run

Walk through the example step by step

Checking "12a45":

index | char | '0'<=c<='9'? | action
------+------+--------------+------------------
0     | 1    | yes          | continue
1     | 2    | yes          | continue
2     | a    | no           | return false
result: false

Solution

Reveal the full Java solution
public class OnlyDigits {
    public static boolean isAllDigits(String s) {
        if (s.isEmpty()) {
            return false;
        }
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c < '0' || c > '9') {
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        System.out.println(isAllDigits("12345")); // true
        System.out.println(isAllDigits("12a45")); // false
    }
}

The check succeeds only if the loop never triggers an early return false, which means every character passed the '0'-to-'9' range test. The explicit empty-string guard reflects the common convention that "only digits" implies at least one digit is present.

Returning as soon as a non-digit is seen keeps the work at worst O(n) and often far less, since a single bad character ends the scan immediately.

Time: O(n) where n is the length of the stringSpace: O(1)

Common Mistakes

  • Returning true for an empty string when the requirement is at least one digit.
  • Accepting signs or a decimal point (like "-99" or "3.14") as numeric when only 0-9 are allowed.

Edge Cases to Test

  • The empty string returns false.
  • A single-digit string like "7" returns true.
  • A string with a trailing space, such as "123 ", returns false.

Interview Follow-Ups

  • How would you extend this to allow an optional leading sign and one decimal point?
  • How would using Character.isDigit change behaviour for non-ASCII digit characters?

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