How to Reverse a String in Java

"Reverse a string" is often the first coding question in a fresher interview. The interviewer usually wants to see two things: that you know the built-in way, and that you can do it by hand with a loop, which shows you understand indexes. This lesson covers both, plus recursion, and explains one trap that even experienced developers miss.

The Built-In Way

public class BuiltIn {
    public static void main(String[] args) {
        String reversed = new StringBuilder("hello").reverse().toString();
        System.out.println(reversed);
    }
}

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Strings can't change, so you copy the text into a StringBuilder, reverse it there, and turn it back into a String. This is the version to use in real code.

By Hand: Swapping from Both Ends

public class TwoPointer {
    static String reverse(String s) {
        char[] c = s.toCharArray();
        for (int i = 0, j = c.length - 1; i < j; i++, j--) {
            char temp = c[i];
            c[i] = c[j];
            c[j] = temp;
        }
        return new String(c);
    }
 
    public static void main(String[] args) {
        System.out.println(reverse("Navika"));
        System.out.println(reverse("racecar"));
        System.out.println("[" + reverse("") + "]");
    }
}

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How it works

1. toCharArray() gives an array you can change.
2. i starts at the first position and j at the last. Each pass swaps those two characters, then moves both pointers inward.
3. The loop stops when they meet (i < j is false), so each character moves exactly once: about n/2 swaps for n characters.
4. A palindrome like "racecar" reads the same both ways, and an empty string stays empty.

Two More Ways

public class OtherWays {
    static String backward(String s) {
        StringBuilder sb = new StringBuilder();
        for (int i = s.length() - 1; i >= 0; i--) {
            sb.append(s.charAt(i));
        }
        return sb.toString();
    }
 
    static String recursive(String s) {
        if (s.isEmpty()) {
            return s;
        }
        return recursive(s.substring(1)) + s.charAt(0);
    }
 
    public static void main(String[] args) {
        System.out.println(backward("Srihitha"));
        System.out.println(recursive("Advitha"));
    }
}

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The Emoji Trap

Emoji take two chars in Java (see "Character Encoding and Charsets in Java"). A hand-written swap reverses those two halves too, which breaks the emoji into two invalid pieces. StringBuilder.reverse() knows about these pairs and keeps them together:

Common Mistakes

Building the result with += in the loop. It's slow for long strings; use a StringBuilder or a char[].

Looping with i <= j and swapping the middle character with itself. Harmless, but i < j is cleaner.

Recursion on very long text. Each character adds a stack frame and a new string.

Interview Corner

Practice

1. Easy: Reverse each word in "Java is fun" separately, giving "avaJ si nuf".
2. Harder: Write static boolean isPalindrome(String s) with the two-pointer idea, without building a reversed copy, ignoring case.

Key Takeaways