How can we prepend strings with StringBuilder?

java

stringbuilder

performance-optimization

custom-solutions

byAlex Kataev·Feb 11, 2025

Here's your to-go solution: StringBuilder.insert(0, "string") for prepending strings.

StringBuilder sb = new StringBuilder("World!");
sb.insert(0, "Hello, "); // Jedi "Hello, " to "World!" using the mysterious 'Force' of Prepend.
System.out.println(sb); // Reveals... "Hello, World!"

The insert method, particularly at index 0, slips your string at the start, effectively delivering a cool prepend effect.

The cost of prepending

Though insert is our star player for prepending, it's not without costs. Prepending usually involves shifting of characters in the internal buffer, making it less efficient compared to appending. But again, we have StringBuilder being the Olympic weightlifter of Java objects, handling these shifty maneuvers efficiently.

Custom solutions for frequent prepending

For those dwellers in the specialized world where frequent prepends are a norm, a custom StringBuilder with a circular buffer could offer optimized performance. Boasting a runtime complexity reduction from O(n^2) to a groovy O(n log n), where n is the string length, we venture into the realm of mad StringBuilder enhancements.

When to extend and how

Does your code look like a string-prepending festival? Consider writing custom extension methods or even extending the StringBuilder class to make prepending more straightforward:

public static StringBuilder prepend(StringBuilder sb, String str) {
    return sb.insert(0, str); // Look, Ma! I'm prepending like a pro!
}

Enwrapped insert now can let your prepending operations becomes as expressive and charismatic as Freddie Mercury's performance at Live Aid!

Understanding trade-offs in string operations

Not all string manipulations are created equal. While StringBuilder is great for most scenarios, if your code is frequently messing around in front of the strings, it might be worth considering an alternative solution.

Custom solutions for performance gain

For highly demanding scenarios, consider implementing caching strategies or batch processing to minimize the overhead cost. Also, remember to actually measure the execution time and test it on various machines, as the highest of high performance can vary across contexts.

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