When string concatenation is performed using the "+" operator, the compiler translates this operation to a suitable manipulation, possibly constructing several intermediate strings. In general, because strings are immutable, at least one new string has to be constructed to hold the result.

Building up a string one piece at a time in a loop requires a new string on every iteration, repeatedly copying longer and longer prefixes to fresh string objects. As a result, performance can be severely degraded.

Whenever a string is constructed using a loop that iterates more than just a few times, it is usually better to create a StringBuffer or StringBuilder object and append to that. Because such buffers are based on mutable character arrays, which do not require a new string to be created for each concatenation, they can reduce the cost of repeatedly growing the string.

To choose between StringBuffer and StringBuilder, check if the new buffer object can possibly be accessed by several different threads while in use. If multi-thread safety is required, use a StringBuffer. For purely local string buffers, you can avoid the overhead of synchronization by using a StringBuilder.

The following example shows a simple test that measures the time taken to construct a string. It constructs the same string of 65,536 binary digits, character-by-character, first by repeatedly appending to a string, and then by using a StringBuilder. The second method is three orders of magnitude faster.

J. Bloch, Effective Java (second edition), Item 51. Addison-Wesley, 2008.

Java API Specification: StringBuffer, StringBuilder.