Why do String and StringBuilder/StringBuffer have different method sets?

Question

Because String and StringBuilder (or StringBuffer) are designed for different purposes, I can understand the reasons for half of the difference (such as String does not have append and delete). However, there are a few points still confusing me.

To name a few:

• String has both getBytes and getChars but StringBuilder has only getChars.
• String and StringBuilder have replace of different functionality respectively.
• String has toLowerCase and toUpperCase, whereas StringBuilder does not.
• String has trim, whereas StringBuilder does not.

I know they are by design, but why?

Answer 1

As of release JDK 5, StringBuffer has been supplemented with an equivalent class designed for use by a single thread, StringBuilder. The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization.

Answer 2

String and StringBuilder were designed and implements more than a decade apart. What was a good idea in Java 1.0 wasn't considered a good idea in Java 5.0.

e.g. trim() trims characters <= space. This include graphical characters, but not all whitespace e.g. (char) 160 is a whitespace, but not trimmed.