Java Programs›Strings›Toggle String Case

Toggle String Case in Java

beginner· Strings · String Manipulation

Problem

Toggling case means flipping every letter's case — uppercase becomes lowercase and lowercase becomes uppercase — while leaving non-letters untouched.

Given a string, swap the case of every letter it contains.

Input

Hello World

Output

hELLO wORLD

Java Program

Java

public class ToggleCase {
    public static void main(String[] args) {
        String str = "Hello World";
        StringBuilder result = new StringBuilder();
        // Flip the case of every letter; anything else is appended unchanged
        for (char c : str.toCharArray()) {
            if (Character.isUpperCase(c)) {
                result.append(Character.toLowerCase(c));
            } else if (Character.isLowerCase(c)) {
                result.append(Character.toUpperCase(c));
            } else {
                result.append(c);
            }
        }
        System.out.println(result.toString());
    }
}

▶ Run Code

Output

hELLO wORLD

Core Logic

Checking each character's case and flipping it individually, one pass through the string, builds the toggled result.

How It Works

1Character.isUpperCase(c) checks whether the current character is an uppercase letter.
2If it is, Character.toLowerCase(c) converts it to lowercase before appending.
3Character.isLowerCase(c) catches lowercase letters, converting them to uppercase with Character.toUpperCase(c).
4Anything that's neither — spaces, digits, punctuation — is appended unchanged.

For "Hello World", 'H' becomes 'h', 'e' becomes 'E', and so on, producing "hELLO wORLD".

💡

Key Point: The space between the two words falls through both case checks and gets appended as-is — toggling only ever touches actual letters.

Complexity

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

Why: Each character is visited once and appended to a result buffer that grows to match the input length.

Key Concepts

Character.isUpperCase()Character.isLowerCase()StringBuilder

Approach 2: Java 8

Java

import java.util.stream.Collectors;

public class ToggleCaseStream {
    public static void main(String[] args) {
        String str = "Hello World";
        // Maps each character to its toggled form, then joins them back into a String
        String result = str.chars()
            .mapToObj(c -> Character.isUpperCase(c)
                ? String.valueOf(Character.toLowerCase((char) c))
                : String.valueOf(Character.toUpperCase((char) c)))
            .collect(Collectors.joining());
        System.out.println(result);
    }
}

▶ Run Code

Output

hELLO wORLD

Core Logic

The same per-character toggle can be expressed as a stream — map each character to its flipped form and join the results.

How It Works

1str.chars() returns an IntStream of the string's character codes.
2.mapToObj(...) maps each code to its toggled single-character String, using the same uppercase/lowercase checks as the loop version.
3.collect(Collectors.joining()) concatenates all the toggled characters back into one result string.

Mapping "Hello World" character by character flips each letter's case, and Collectors.joining() reassembles "hELLO wORLD".

💡

Key Point: The ternary inside mapToObj() has to check both cases explicitly, same as the loop — streams change how the logic is expressed, not the underlying character-by-character work.

Complexity

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

Why: Each character is mapped to its toggled form and Collectors.joining() rebuilds a result string holding all n characters.

Key Concepts

Streamchars()Collectors.joining()

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