Collections and Generics

Arrays are powerful but fixed in size. Java's Collections Framework provides dynamic, resizable data structures — lists, sets, maps, and queues. Generics add type safety so collections only hold the types you specify.

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The Collections Hierarchy

Iterable
  └── Collection
        ├── List      (ordered, allows duplicates)
        ├── Set       (no duplicates)
        └── Queue     (FIFO ordering)

Map (key-value pairs — separate hierarchy)

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List

A List is an ordered collection that allows duplicates.

ArrayList

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

List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
names.add("Alice");  // duplicates allowed

System.out.println(names);        // [Alice, Bob, Alice]
System.out.println(names.get(0)); // Alice
System.out.println(names.size()); // 3

names.remove("Alice");            // removes first occurrence
System.out.println(names);        // [Bob, Alice]

Common List Methods