List & ArrayList

The Collections Framework is Java's toolbox of growable data structures. List is the everyday one: an ordered, growable sequence.

import java.util.ArrayList;
import java.util.List;

List<String> names = new ArrayList<>();   // interface left, implementation right
names.add("Ada");
names.add("Grace");
names.add(1, "Linus");                    // insert at index 1
System.out.println(names.get(0));         // Ada
System.out.println(names.size());         // 3
System.out.println(names.remove("Ada"));  // true (removed)

Declare the interface type (List), construct the implementation (ArrayList). All your calling code sees the interface; swapping to LinkedList tomorrow touches exactly one line. The <String> is a generic type parameter — more on it in a moment, but it already pays: names.get(0) is statically known to be a String.

The core List methods: add(e), add(i, e), get(i), set(i, e), remove(i), remove(Object), size(), contains(e), indexOf(e), isEmpty(), clear(). Iteration uses for-each like arrays:

for (String n : names) { System.out.println(n); }

ArrayList vs LinkedList — the interview staple, kept honest: ArrayList is a resizable array; get(i) is instant, middle inserts shift elements. LinkedList is a chain; ends are cheap, get(i) walks from the nearest end. For beginners the honest summary: use ArrayList until you have measured a real problem (and in a Beginner course you almost certainly have not).

Next: sets, maps, and the shape of lookups.