How to Reverse a String in Java

The Challenge

Reverse a string - turn "Hello" into "olleH"!

Solutions

Method 1: Using StringBuilder

java

String text = "Hello World";
String reversed = new StringBuilder(text).reverse().toString();

System.out.println("Original: " + text);
System.out.println("Reversed: " + reversed);
// Output: Original: Hello World
//        Reversed: dlroW olleH

Method 2: Using Loop

java

String text = "Hello World";
String reversed = "";

for (int i = text.length() - 1; i >= 0; i--) {
    reversed += text.charAt(i);
}

System.out.println("Reversed: " + reversed);
// Output: Reversed: dlroW olleH

Method 3: Using char Array

java

String text = "Hello World";
char[] chars = text.toCharArray();
int left = 0, right = chars.length - 1;

while (left < right) {
    char temp = chars[left];
    chars[left] = chars[right];
    chars[right] = temp;
    left++;
    right--;
}

String reversed = new String(chars);
System.out.println("Reversed: " + reversed);
// Output: Reversed: dlroW olleH

Method 4: Using Recursion

java

public static String reverse(String str) {
    if (str.isEmpty()) {
        return str;
    }
    return reverse(str.substring(1)) + str.charAt(0);
}

// Usage
System.out.println(reverse("Hello")); // olleH

Did You Know?

The StringBuilder.reverse() method is the most efficient way to reverse a string in Java. It modifies the string in-place with O(n) time complexity!

Watch Out!

Using string concatenation in a loop (reversed += char) is inefficient because strings are immutable. Each concatenation creates a new string object!

Real-World Use Cases

Palindrome checking: Compare string with its reverse
Data processing: Reverse order of data
Text effects: Create mirror text
Algorithms: String manipulation challenges

›Short versionOpen if you only have a minute.

new StringBuilder(text).reverse().toString() reverses UTF-16 char values.
A character outside the basic multilingual plane is two chars. Reversing those chars breaks the pair.
Building the reverse with += is O(n^2). The builder is linear.
Null has no reverse. Decide that at the boundary.

Reverse a string

Produce a string with the char sequence in the opposite order.

Working approach

Use StringBuilder.reverse for ordinary BMP text.
Use a builder in a backward loop when you want the loop to be visible.
Do not reverse char by char if the text contains characters that need surrogate pairs.

Walk the sample

Input "code"
Chars from the end are e, d, o, c
The result is "edoc"

The trap

reverse() swaps char values, not visible characters. An emoji stored as a surrogate pair comes out with the two halves exchanged.

java

static String reverseBmp(String text) {
    return new StringBuilder(text).reverse().toString();
}

// A manual loop has the same char-level behavior:
static String reverseByHand(String text) {
    StringBuilder out = new StringBuilder(text.length());
    for (int i = text.length() - 1; i >= 0; i--) {
        out.append(text.charAt(i));
    }
    return out.toString();
}

What does StringBuilder.reverse do to a single emoji?

The emoji is one code point stored as two chars, a high surrogate and a low surrogate.

reverse swaps those chars. The result is not a well-formed copy of that character. For text that stays inside one char per character, the builder reverse is the right one-liner.

Time O(n), space O(n).

Easy to get wrong

Appending with += in the backward loop.
Assuming one char is one character the reader sees.