How to reverse a string in Java without using String.reverse()

Reverse a string in Java without using String.reverse()

We can use the for loop and the StringBuilder class to reverse a string.

Example

public class Main{
    public static String reverse(String input) {
        if (input == null) return null;
        StringBuilder output = new StringBuilder();
        for (int i = input.length() - 1; i >= 0; i--) {
            output.append(input.charAt(i));
        }
        return output.toString();
    }
    public static void main(String[] args){
        String text = "educative";
        System.out.println("Original string - " + text);
        System.out.println("Reversed string - " + reverse(text));
    }
}

Explanation

  • Line 3: We define the reverse() method.
  • Line 5: We perform a Null check.
  • Line 7: We create an instance of the StringBuilder class.
  • Lines 9–11: We traverse the input string from the last index to the first and the characters are appended to the StringBuilder instance.
  • Line 17: We define the text.
  • Line 19: We invoke the reverse() method.

Reverse the character array

First, the string is converted to a character array. Then, the array is reversed. A new string is formed from the reversed array.

Example

public class Main{
    public static String reverse(String input) {
        char[] characters = input.toCharArray();
        int n = characters.length;
        for (int i = 0; i < n / 2; i++) {
            char c = characters[i];
            characters[i] = characters[n - i - 1];
            characters[n - i - 1] = c;
        }
        return new String(characters);
    }
    public static void main(String[] args){
        String text = "educative";
        System.out.println("Original string - " + text);
        System.out.println("Reversed string - " + reverse(text));
    }
}

Explanation

  • Line 3: We define the reverse() method.
  • Line 4: We convert the input string to a character array.
  • Line 5: We assign the length of the input string/text to a variable.
  • Lines 7–11: We reverse the character array.
  • Line 13: We return a new string from the reverse character array.
  • Line 17: We define the text.
  • Line 19: We invoke the reverse() method.

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