HashMap

Key–value storage with constant-time average lookups using hashing.

Intermediate

Map<String, Integer> count = new HashMap<>();
count.put("LED", 3);
count.merge("LED", 1, Integer::sum);       // 4
int n = count.getOrDefault("MOTOR", 0);
for (var e : count.entrySet()) System.out.println(e.getKey() + "=" + e.getValue());

Keys must implement hashCode() and equals() consistently. Use TreeMap for sorted keys and LinkedHashMap to keep insertion order.

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