Reverse a string in Java

java

string-reversal

coding-interview

best-practices

byAlex Kataev·Aug 12, 2024

For fast and furious string reversal in Java, StringBuilder is your buddy:

String reversed = new StringBuilder("Original").reverse().toString();

Here, we construct a StringBuilder with the original string, cleverly use reverse(), and get the inverted string back with toString(). Effective, efficient, and straightforward.

Options for different Java versions

Java 5 onwards, StringBuilder utilizes less resources, making it faster than StringBuffer. Here's how you can reverse a string:

String reversed = new StringBuilder("Java 5+").reverse().toString();

In versions prior to Java 5, StringBuffer is your friend. Use its reverse() method:

String reversed = new StringBuffer("Pre Java 5").reverse().toString();

Keep in mind that StringBuffer is synchronised and therefore slower than StringBuilder, so use it only when you require thread safety.

Manual reversal for muscle flexing

What if you're at a coding interview and the interviewer forbids the use of StringBuilder or StringBuffer? Don't fret, you can reverse a string manually:

public String reverseManually(String input) {
    char[] chars = input.toCharArray();
    int left = 0;
    int right = chars.length - 1;
    while (left < right) {
        // The ol' switcheroo! 🔄
        char temp = chars[left];
        chars[left] = chars[right];
        chars[right] = temp;
        left++;
        right--;
    }
    return new String(chars);
}

This method performs the classic act of swapping characters from both ends to middle. Throw it in your coding utility belt!

Reverse for the 'Purist'

Are you a purist, looking to gain the same results with traditional techniques? Well, let's reverse without using reverse():

public String reverseWithStringBuilder(String input) {
    StringBuilder reversed = new StringBuilder();
    for (int i = input.length() - 1; i >= 0; i--) {
        reversed.append(input.charAt(i)); // Going once... going twice... going thrice... back to front!
    }
    return reversed.toString();
}

This method elegantly iterates through the characters backwards, appending them to StringBuilder.

Word-wise reversal for fun and function

Fancy an upside-down world where words are reversed and not the sentences? Voila!

public String reverseWords(String input) {
    String[] words = input.split(" ");
    StringBuilder reversed = new StringBuilder();
    for (String word : words) {
        reversed.append(new StringBuilder(word).reverse().toString()).append(" ");
    }
    return reversed.toString().trim(); // Clean and tidy with trim(). No trailing spaces here! 🧹
}

This method looks at sentences through a unique lens by splitting words scalarly, reversing them, and stitching them back together.

Beware the Unicode Beast

Unicode and emoji characters are a castle full of hidden passageways; tricky to manipulate. They may not be reversed correctly with basic char-swapping methods. Handle surrogate pairs appropriately if you foresee them in your string.

Additionally, right when you think you've got the treasure, a NullPointerException dragon might spring up if you neglect to check for null or empty string. Saber at the ready!

public String safeReverse(String input) {
    if (input == null) {
        return null; // You shall not pass! 🧙
    }
    return new StringBuilder(input).reverse().toString();
}