06 — Concatenation and performance

The one piece of Java text advice everybody has heard — "don't use + in a loop, use StringBuilder" — is still correct, and almost every explanation of why has been out of date since Java 9.

This chapter separates the part that is true (the loop is quadratic, and always will be) from the part that changed (+ has not compiled to a StringBuilder for years), and is careful about what can honestly be measured without a benchmarking harness. Which is less than you would like.

Lesson	The one thing
+ in a loop is quadratic	proved by counting copies, not by timing
+ is not StringBuilder	it is invokedynamic, and constants fold away entirely
Capacity, and the right tool	the buffer doubles; String.join is usually the answer