Strings, Immutability, substring, and indexOf

Strings are immutable: once "HELLO" exists in memory, it exists forever as "HELLO". Nothing you call on it edits its characters. When you write s = s.substring(1), Java builds a brand new String object from the pieces you asked for, then points s at that new object. The old String is still sitting on the heap, now with nothing pointing at it. This matters because it means every String method is really a question, not a command — it returns an answer, it never mutates the original.

substring(start, end) carves out characters starting at index start, which counts, up to but not including index end, which doesn't. That's why the length of the result is always end minus start. "COMPUTER".substring(3, 6) starts at index 3 (the 'P') and stops before index 6 (the 'T'), giving "PUT". With one argument, substring(start) just grabs everything from start to the end of the string. indexOf works similarly but searches: it scans left to right and returns the index where the first match begins — not every place it could match.

This immutability is the same idea you meet with references elsewhere in the unit: a variable holding a String is really holding a reference to an object, and reassigning that variable just points it somewhere new. Concatenating in a loop makes this expensive — each += quietly builds a fresh String and discards the old one, which is why StringBuilder shows up later for heavy string work.