Improving performance with StringBuilder

Let’s say we have a String content and we want to concatenated a String line to it. Nothing more natural than doing:

content = content + line;

Or simply

content += line;

It works! Simple and obvious. However, Java Strings are immutable. It means that every re-assignment above creates a new instance of String on memory with the content of the previous string plus the concatenated one.

This simple solution works, but what if the String is used within a loop to concatenate hundred/thousand lines from a buffer?

// Holds the String content
String content = "";

String line = "";
while ((line = ContentBuffer.readLine()) != null)
	content += line;

The implementation above can impact the performance as a new instance is created for every new line of text. So, a solution could be to use a StringBuilder instead:

// Holds the String content
StringBuilder contentSB = "";

String line = "";
while ((line = ContentBuffer.readLine()) != null)
	contentSB.append(line);

String content = contentSB.toString();

StringBuilder is mutable, so it can be changed without new instantiation. That’s it!!

An interesting discussion can be found here and here.