Reverse String in Java: Solution, Explanation & Practice

Reverse a string

Problem summary

Write a program that reverses any given string.

Starter code

public class Main {
    public static void main(String[] args) {
        String text = "Hello"; // Test case 1
        
        // Reverse the string
        // Print: Reversed: <result>
    }
}

Expected output and test cases

Reverse Hello
```
Reversed: olleH
```
Reverse Java
```
Reversed: avaJ
```
Single character
```
Reversed: a
```

Hints

Use StringBuilder's reverse() method
Or iterate from end to start
Build new string character by character

Validated solution

Reveal Java solution

public class Main {
    static String reverse(String text) {
        StringBuilder result = new StringBuilder(text.length());
        for (int index = text.length() - 1; index >= 0; index--) result.append(text.charAt(index));
        return result.toString();
    }
    public static void main(String[] args) {
        System.out.println("Reversed: " + reverse("Hello"));
    }
}

How to approach the problem

Read characters from the final index down to zero and append them to one StringBuilder. The builder avoids creating a new immutable String after every character append.

Approach

Allocate a builder with the known final length.
Visit indices from length - 1 down to zero.
Convert the builder once at the boundary.

Time and space complexity

Time: O(n). Space: O(n).

Edge cases to test

An empty string stays empty.
charAt works on UTF-16 code units; code-point-aware reversal is a different requirement.

Common mistakes

Starting at text.length(), which is outside the valid index range.
Repeatedly concatenating with + in a loop.

Follow-up challenge

Reverse Unicode code points without splitting an emoji surrogate pair.

Related Strings exercises

Practice all Strings exercises · Run this idea in the Java compiler