Java Collections

Lists, Arrays & Collections

Collections let you store multiple values in one variable. Loops, filters, and maps operate on collections in every Java program.

• Ordered lists — arrays, lists, vectors
• Key-value maps — objects, dictionaries, hashes
• Immutable vs mutable collections
• Iteration with for loops and higher-order methods

int[] nums = {1, 2, 3, 4, 5};
Map<String, Integer> user = Map.of("score", 95);

1. Choose the right structure — array for ordered data, map for lookups
2. Prefer built-in methods over manual index loops when available
3. Watch for off-by-one errors in index-based loops