Java With Us

HashMap

Updated on 2026-04-21 Reviewed by the Java With Us editorial team

HashMap is Java's default Map implementation. It stores key-value pairs in a hash table, giving average O(1) time for get, put, remove and containsKey.

Usage

import java.util.HashMap;
import java.util.Map;

Map<String, Integer> scores = new HashMap<>();
scores.put("Ada", 95);
scores.put("Grace", 98);
scores.get("Ada");                         // 95
scores.getOrDefault("Unknown", 0);         // 0
scores.putIfAbsent("Linus", 90);
scores.computeIfAbsent("Ken", k -> 0);
scores.merge("Ada", 5, Integer::sum);      // 95 + 5 = 100

for (var entry : scores.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

equals + hashCode

Keys must have consistent hashCode() and equals() methods. If you mutate a key after inserting it (changing its hash), you'll lose it in the map.

Not ordered, not thread-safe

  • Iteration order is unspecified and can change between JVM versions.
  • For insertion-order iteration: use LinkedHashMap.
  • For sorted-by-key iteration: use TreeMap.
  • For concurrent access: use ConcurrentHashMap (not Hashtable — that's the legacy version).

Hash collision handling

Since Java 8, HashMap uses a hybrid of linked lists and balanced trees per bucket. When a bucket accumulates too many collisions, it converts to a tree to keep worst-case lookup at O(log n) instead of O(n). Rarely relevant unless you intentionally induce collisions.

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