Count Non-Space Characters in a String - Java

Category: Easy | Concepts used: String iteration, character checks, Unicode whitespace detection

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Problem Statement

Given a string, count how many characters are not whitespace (spaces, tabs, newlines, line breaks).

Input : "Hello World"     Output: 10   (11 characters total - 1 space)
Input : "   "              Output: 0    (all spaces)

Examples (with edge scenarios)

#	Input	Output	Why
1	"Hello World"	10	11 total characters, 1 space
2	"" (empty)	0	Nothing to count
3	" " (only spaces)	0	All characters are spaces
4	"a b c"	3	3 letters, 2 spaces
5	"Hi\tThere"	7	Tab (\t) counts as whitespace

⚠️ Common Beginner Mistake

Mistake	Impact	Fix
Checking only plain spaces (ch == ' ')	Fails to detect tabs (\t) and line feeds (\n)	Use Character.isWhitespace(ch)
Returning str.length() directly	Counts space characters as letters	Loop and selectively count only non-whitespace characters

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Before You Code: Clarify the Contract

Before choosing an algorithm, confirm how null and empty strings should behave, whether comparison is case-sensitive, and whether spaces or punctuation count. Java char values are UTF-16 code units, not always complete human-visible Unicode characters, so international text may require code points or grapheme-aware libraries.

Analogy: Counting Apples in Packing Peanuts

Imagine you receive a package containing apples (visible characters) packed in foam packing peanuts (whitespace characters).

• To find out how many apples you got, you reach into the box and count them.
• As you unpack, you ignore the peanuts, cardboard inserts, and paper stuffing (various forms of whitespace).
• Every time your hand touches a real, solid apple, you count it. Your final count is the total number of apples, ignoring the packing material!

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Solution 1 — Loop + Manual Space Check (Basic)

This solution counts characters that are not equal to the plain space character ' '.

Intuition

Walk through the string character by character. If a character is not equal to ' ', increment our counter.

public class CountNonSpace {
    public static int countNonSpace(String str) {
        if (str == null || str.isEmpty()) {
            return 0;
        }

int count = 0;
        for (int i = 0; i < str.length(); i++) {
            if (str.charAt(i) != ' ') { // Only checks plain space ' '
                count++;
            }
        }
        return count;
    }

public static void main(String[] args) {
        System.out.println(countNonSpace("Hello World")); // 10
        System.out.println(countNonSpace("   "));          // 0
        System.out.println(countNonSpace(""));              // 0
    }
}

Output:

10
0
0

Dry Run (str = “a b”)

i = 0: 'a' != ' ' -> count = 1
i = 1: ' ' == ' ' -> skip
i = 2: 'b' != ' ' -> count = 2
Final count = 2

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Solution 2 — Using Character.isWhitespace() (Handles All Whitespace)

This is the preferred solution as it handles all standard whitespace characters (tabs, newlines, vertical tabs, Unicode spaces).

Intuition

Whitespace isn’t just the spacebar character. We delegate whitespace detection to Character.isWhitespace(), which handles tabs (\t), newlines (\n), carriage returns (\r), and more.

public class CountNonSpaceWhitespace {
    public static int countNonSpace(String str) {
        if (str == null || str.isEmpty()) {
            return 0;
        }

int count = 0;
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if (!Character.isWhitespace(ch)) { // Checks space, tab, newline, carriage return, etc.
                count++;
            }
        }
        return count;
    }

public static void main(String[] args) {
        System.out.println(countNonSpace("Hi\tThere"));   // 7
        System.out.println(countNonSpace("Hello World")); // 10
    }
}

Output:

7
10

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📊 Visual Flowchart

graph TD
    Start["Given String S"] --> Empty{"S is null or empty?"}
    Empty -->|Yes| RetZero["Return 0"]
    Empty -->|No| InitCount["Initialize count = 0"]
    InitCount --> Loop{"i < S.length()?"}
    Loop -->|Yes| Fetch["ch = S.charAt(i)"]
    Fetch --> Check{"Character.isWhitespace(ch)?"}
    Check -->|Yes| IncLoop["i++"]
    Check -->|No| IncCount["count++"]
    IncCount --> IncLoop
    IncLoop --> Loop
    Loop -->|No| End["Return count"]

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Interviewer Insights

This question determines if you consider real-world formatting characters beyond basic spacebars.

Follow-up questions you might get:

• “What about using replaceAll() to solve this in one line?” → You can write:

  public static int countNonSpaceOneLiner(String str) {
      if (str == null) return 0;
      return str.replaceAll("\\s", "").length();
  }

  Interview Tip: Proactively explain the trade-offs of this one-liner. While it’s concise, replaceAll() uses Regular Expressions (slower CPU-wise) and internally creates a brand-new string in memory, taking (O(N)) auxiliary space. The loop-based solution is much more memory efficient, operating in (O(1)) auxiliary space.
• “Why is Character.isWhitespace() better than checking a list of chars manually?” → It supports Unicode whitespace characters (like the non-breaking space \u00A0 or Ogham space mark), making the software internationalization-ready.

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Quick Recap

Approach	Handles tabs/newlines?	Space Complexity (Auxiliary)	Time Complexity	Interview Signal
Manual ' ' check	No	(O(1))	(O(N))	Basic string traversal
Character.isWhitespace()	Yes	(O(1))	(O(N))	Industry-standard, Unicode-compliant, robust
Regex (replaceAll)	Yes	(O(N))	(O(N))	Concise, but memory-intensive