Map and Deque

Map

A Map stores key-value pairs. Keys are unique; values may repeat.

Common Implementations

Creating Maps

java
// mutable
Map<String, Integer> scores = new HashMap<>();
scores.put("Alice", 90);
scores.put("Bob", 85);

// factory — unmodifiable
Map<String, Integer> fixed = Map.of("Alice", 90, "Bob", 85);

// Map.entry / Map.ofEntries for more than 10 entries
Map<String, Integer> big = Map.ofEntries(
    Map.entry("Alice", 90),
    Map.entry("Bob", 85)
);

Key Map Methods

java
Map<String, Integer> map = new HashMap<>(Map.of("A", 1, "B", 2, "C", 3));

map.get("A");                          // 1
map.getOrDefault("Z", 0);             // 0 — key not found
map.put("D", 4);                       // insert or replace
map.putIfAbsent("A", 99);             // ignored — "A" already exists
map.remove("B");                       // removes entry
map.containsKey("C");                  // true
map.containsValue(3);                  // true
map.size();                            // number of entries

// iterating entries
for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + "=" + entry.getValue());
}

// compute helpers
map.merge("A", 10, Integer::sum);       // A → 1+10 = 11
map.computeIfAbsent("E", k -> k.length()); // E → 1
map.replaceAll((k, v) -> v * 2);        // doubles all values

Deque

A Deque (double-ended queue) supports insertion and removal at both ends. It is also used as a stack.

Common Implementations

Creating a Deque

java
Deque<String> deque = new ArrayDeque<>();

Deque as a Queue (FIFO)

java
deque.offer("first");    // add to tail
deque.offer("second");
deque.offer("third");

deque.peek();            // "first" — view head, no remove
deque.poll();            // "first" — remove head

Deque as a Stack (LIFO)

java
deque.push("a");   // add to head
deque.push("b");
deque.push("c");

deque.peek();      // "c" — top of stack
deque.pop();       // "c" — remove from head

Deque Method Reference

Map.of() Restrictions

Choosing Map vs Deque