The String Class and Immutability

Strings never change; String methods build new Strings

A String object is immutable. No method in the class edits the characters it already holds; every method that looks like an edit actually returns a brand-new String and leaves the original alone. So s.toUpperCase(); on its own does nothing useful, while s = s.toUpperCase(); does what you meant.

The methods in the AP subset, with the exact conventions they use:

• length() — number of characters; indices run from 0 to length() - 1.
• substring(from) — from that index to the end.
• substring(from, to) — includes from, excludes to, so its length is to - from.
• indexOf(str) — index of the first occurrence, or -1 if it is not there.
• equals(other) — same characters, case sensitive.
• compareTo(other) — negative, zero, or positive; it does not return −1 and 1 by contract.

Because a String variable is a reference, == asks “same object?” while equals asks “same characters?”. Two Strings built at run time can hold identical text and still fail ==. Comparing Strings with == is the single most common silent bug in this unit.