List and Set

Collection Hierarchy (excerpt)

plaintext
Iterable
└── Collection
    ├── List      — ordered, allows duplicates
    ├── Set       — no duplicates
    └── Queue

List

A List maintains insertion order and allows duplicates and null values (in mutable implementations).

Common Implementations

Creating Lists

java
// mutable
List<String> mutable = new ArrayList<>();
mutable.add("Alice");
mutable.add("Bob");

// factory — unmodifiable (no add/remove/set)
List<String> fixed = List.of("Alice", "Bob", "Carol");

// mutable copy of a fixed list
List<String> copy = new ArrayList<>(List.of("A", "B", "C"));

Key List Methods

java
List<String> list = new ArrayList<>(List.of("A", "B", "C", "B"));

list.get(1);                   // "B"
list.set(0, "Z");              // replace index 0 → ["Z","B","C","B"]
list.add(1, "X");              // insert at index 1
list.remove(0);                // remove by index
list.remove("B");              // remove first occurrence by value
list.indexOf("B");             // first index of "B"
list.lastIndexOf("B");         // last index of "B"
list.subList(1, 3);            // view, not copy
list.size();                   // number of elements
list.contains("C");            // true
list.isEmpty();                // false
Collections.sort(list);        // sorts in place

Set

A Set contains no duplicates. Most implementations do not guarantee order.

Common Implementations

Creating Sets

java
// mutable
Set<String> mutable = new HashSet<>();
mutable.add("Alice");
mutable.add("Alice");   // duplicate ignored
System.out.println(mutable.size());   // 1

// factory — unmodifiable, no null allowed
Set<String> fixed = Set.of("Alice", "Bob", "Carol");

Key Set Methods

java
Set<Integer> set = new HashSet<>(Set.of(1, 2, 3, 4, 5));

set.add(6);          // true — added
set.add(3);          // false — already present
set.remove(2);       // true
set.contains(4);     // true
set.size();          // 5

// set operations
Set<Integer> other = new HashSet<>(Set.of(3, 4, 5, 6, 7));
set.retainAll(other);  // intersection — mutates set
set.addAll(other);     // union — mutates set
set.removeAll(other);  // difference — mutates set

List.of() and Set.of() — Important Restrictions

Choosing List vs Set