The String Toolbox
Immutability as the key idea, the everyday methods, split's regex gotcha.
A String is an immutable object — a value that can never be changed.
Every method that "modifies" a string actually returns a new one:
String name = " Ada Lovelace ";
String clean = name.trim(); // "Ada Lovelace" — name itself unchanged!
String upper = name.toUpperCase();
String both = name.trim().toUpperCase(); // chain: method calls compose
The everyday toolbox:
| Method | Job | Example |
|---|---|---|
| length() | character count | "hi".length() → 2 |
| charAt(i) | char at index | "hi".charAt(1) → 'i' |
| substring(a, b) | from a up to b-1 | "hello".substring(1, 3) → "el" |
| indexOf(s) | first position or -1 | "banana".indexOf("na") → 2 |
| contains(s) | does it appear | "banana".contains("ana") → true |
| replace(a, b) | all a become b | "aa".replace("a","b") → "bb" |
| split(regex) | cut into an array | "a,b".split(",") → ["a","b"] |
| strip()/trim() | drop surrounding whitespace | " x ".strip() → "x" |
| isEmpty()/isBlank() | empty / whitespace-only | |
Immutability is why string equality is .equals (Module 2) and why
substring is cheap — the new string shares nothing mutable with the old.
Splitting gotcha: split takes a regex. To split on a literal dot,
escape it: "1.2".split("\\."). To split a sentence into words:
sentence.split("\\s+") — one or more whitespace characters.
Next: building strings efficiently.