Java Programs›Strings›Remove Extra Spaces from a String

Remove Extra Spaces from a String in Java

intermediate· Strings · String Manipulation

Problem

Extra spaces are the leading, trailing, and repeated spaces that can creep into text — collapsing them down to single spaces between words is called normalizing whitespace.

Given a string, remove its leading and trailing spaces, and collapse every run of spaces between words down to one.

Input

  Java   is   fun  

Output

Java is fun

Java Program

Java

public class RemoveExtraSpaces {
    public static void main(String[] args) {
        String str = "  Java   is   fun  ";
        String trimmed = str.trim();
        StringBuilder result = new StringBuilder();
        boolean lastWasSpace = false;

        for (char c : trimmed.toCharArray()) {
            if (c == ' ') {
                if (!lastWasSpace) result.append(c); // only the first space in a run gets through
                lastWasSpace = true;
            } else {
                result.append(c);
                lastWasSpace = false;
            }
        }

        System.out.println(result.toString());
    }
}

▶ Run Code

Output

Java is fun

Core Logic

Trimming the ends first, then tracking whether the previous character was already a space, lets a single pass collapse every run of spaces down to one.

How It Works

1str.trim() removes any leading or trailing spaces before the scan begins.
2A boolean lastWasSpace flag tracks whether the previous character appended was a space.
3For a space character, it's only appended if lastWasSpace is false — the first space in a run gets through, later ones in the same run are skipped.
4For any other character, it's always appended, and lastWasSpace is reset to false.

For " Java is fun ", trimming first removes the outer spaces, then the runs of three spaces between words each collapse down to one, producing "Java is fun".

💡

Key Point: Trimming has to happen before the scan, not after — trimming a string that still has internal double spaces wouldn't touch those, since trim() only strips from the very start and end.

Complexity

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

Why: Each character is visited once, and the result buffer holds the normalized string, whose length is at most the original's.

Key Concepts

String.trim()boolean flagStringBuilder

Approach 2: Regex Replace

Java

public class RemoveExtraSpacesRegex {
    public static void main(String[] args) {
        String str = "  Java   is   fun  ";
        // Trims the ends, then collapses every run of spaces down to one
        String result = str.trim().replaceAll(" +", " ");
        System.out.println(result);
    }
}

▶ Run Code

Output

Java is fun

Core Logic

A single regex can express 'one or more spaces' directly, replacing every run with exactly one space in one call.

How It Works

1str.trim() removes the leading and trailing spaces first, same as the manual version.
2.replaceAll(" +", " ") matches every run of one or more spaces and replaces each run with a single space.
3Runs of just one space match too, but replacing a single space with a single space leaves it unchanged.

Trimming " Java is fun " gives "Java is fun", and replaceAll(" +", " ") then collapses each triple-space run down to one, producing "Java is fun".

💡

Key Point: " +" only matches the literal space character — "\\s+" would be the version to reach for if tabs or newlines also needed collapsing.

Complexity

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

Why: trim() and replaceAll() each build a new string, so the final result string is what the extra space is spent on.

Key Concepts

regexreplaceAll()String.trim()

Approach 3: Java 8

Java

import java.util.Arrays;
import java.util.stream.Collectors;

public class RemoveExtraSpacesStream {
    public static void main(String[] args) {
        String str = "  Java   is   fun  ";
        // Splitting on space runs removes them; joining puts back exactly one
        String result = Arrays.stream(str.trim().split(" +"))
            .collect(Collectors.joining(" "));
        System.out.println(result);
    }
}

▶ Run Code

Output

Java is fun

Core Logic

Splitting the trimmed string on runs of spaces produces exactly the individual words, so joining them back with single spaces rebuilds the string with every run collapsed.

How It Works

1str.trim() removes the leading and trailing spaces first, same as the other two approaches.
2.split(" +") splits on every run of one or more spaces, producing an array of just the words — the runs themselves never appear in the output.
3Arrays.stream(...) turns that array into a Stream<String>.
4.collect(Collectors.joining(" ")) rejoins the words with exactly one space between each, regardless of how many spaces originally separated them.

Trimming " Java is fun " gives "Java is fun", which splits into ["Java", "is", "fun"] and joins back as "Java is fun".

💡

Key Point: This sidesteps run-collapsing entirely — instead of detecting and skipping repeated spaces, splitting removes them completely and joining() puts back exactly the separator that's wanted.

Complexity

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

Why: split() scans the whole string once to build the word array, and joining() builds a new string holding those words with single-space separators.

Key Concepts

StreamArrays.stream()Collectors.joining()

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