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Home Interviews How you would reverse a string in Java without using the StringBuilder or StringBuffer classes. – MindStick
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1 Answers

Ravi Vishwakarma
Ravi Vishwakarma 17 Jul 2024
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Reversing a string in Java without using StringBuilder or StringBuffer can be accomplished in several ways. Here, I'll explain one approach using character arrays:

Example 1-

public class ReverseString {
    public static String reverse(String input) {
        // Convert the string to a character array
        char[] charArray = input.toCharArray();
        
        // Initialize pointers for start and end of the array
        int start = 0;
        int end = charArray.length - 1;
        
        // Swap characters from start to end until middle is reached
        while (start < end) {
            // Swap characters
            char temp = charArray[start];
            charArray[start] = charArray[end];
            charArray[end] = temp;
            
            // Move pointers towards the center
            start++;
            end--;
        }
        
        // Convert the character array back to a string
        return new String(charArray);
    }

    public static void main(String[] args) {
        String original = "Hello, World!";
        String reversed = reverse(original);
        System.out.println("Original: " + original);
        System.out.println("Reversed: " + reversed);
    }
}

Example 2- Using Recursion

public class ReverseStringRecursive {
    public static String reverse(String input) {
        // Base case: if the string is empty or has only one character, return it as is
        if (input == null || input.length() <= 1) {
            return input;
        }
        
        // Recursive case: reverse the substring from the second character to the end, and add the first character at the end
        return reverse(input.substring(1)) + input.charAt(0);
    }

    public static void main(String[] args) {
        String original = "Hello, World!";
        String reversed = reverse(original);
        System.out.println("Original: " + original);
        System.out.println("Reversed: " + reversed);
    }
}

Example 3- Using a predefined function

import java.util.*;

public class Main {
    public static void main(String[] args) {
        String original = "Hello, World!";
        
        // Split the string into a list of characters
        List<String> characters = Arrays.asList(original.split(""));
        
        // Reverse the list
        Collections.reverse(characters);
        
        // Join the reversed list back into a string
        String reversed = String.join("", characters);
        
        // Print the original and reversed strings
        System.out.println("Original: " + original);
        System.out.println("Reversed: " + reversed);
    }
}

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