What's the difference between StringBuilder and StringBuffer, and when does capacity tuning matter?

They have identical APIs, but StringBuffer's methods are synchronized and StringBuilder's are not, so StringBuilder is the default choice for single-threaded use (almost always) and StringBuffer only when one builder is genuinely shared across threads — which is rare and usually better solved another way. Both grow by reallocating and copying when capacity is exceeded; presizing with new StringBuilder(expectedSize) avoids repeated array copies when you're appending a large, known volume of text in a hot path.