String immutability and key methods: substring, indexOf, charAt, strip, repeat, isBlank

A String in Java is immutable: once created, its characters never change. Every method that seems to modify a string, such as toUpperCase or concat, actually returns a new String and leaves the original alone. The most common exam trap is a line like s.toUpperCase(); whose result is thrown away, so s is unchanged when printed. Immutability lets strings be shared safely, cached in the string pool, and used as reliable HashMap keys.