StringBuilder ⚒️

The StringBuffer / StringBuilder classes are primarily used to modify string values without having to initialize a new String object every time. They simply create a resizable array of all the strings, copying them back to a string only when necessary.

Java and immutable strings

Example in Java without string builder

This function concatenate a list of strings.
The strings are all the same length (call this x) and that there are n strings.

String joinWords(String[] words) {
	String sentence = "";
	
	for (String w : words) 
		sentence = sentence + w;
	
	return sentence;
}

Since in Java strings are immutable, in this piece of code a new copy of the string is created on each concatenation, and the two strings are copied over, character by character. The first iteration requires us to copy x characters, the second 2x characters, and so on.

Time complexity: $O (x + 2 x + . . . + n x) = O (x \cdot n^{2})$ (sum of 1 through n equals $\frac{n (n + 1)}{2}$ , or $O (n^{2})$ )

Example in Java with string builder

As said before, StringBuilder simply creates a resizable array of all the strings, copying them back only when necessary.

String joinWords(String[] words) {
	StringBuilder sentence = new StringBuilder();
	
	for (String w: words)
		sentence.append(w);
		
	return sentence.toString();
}

Time complexity: $O (x \cdot n)$

C++ and mutable strings

C++ strings are mutable so the performance considerations of concatenation are less of a concern. However, you can decide to use std::stringstream.