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January 10, 2026•6 min read•Marina

Java Collections in Real Systems | Part 3: Map

JavaCollectionsMap

Java Collections in Real Systems | Part 3: Map

If List is about order and Set about uniqueness, Map is about relationships.

Everything revolves around key → value.

Map

  • No duplicate keys
  • Each key maps to one value
  • Fast lookups by key

    • HashMap

  • No guaranteed order
  • Very fast access
  • Use case: account balance lookup
    • Map<String, BigDecimal> balances = new HashMap<>();
balances.put("ACC1", new BigDecimal("1000"));
balances.put("ACC2", new BigDecimal("2500"));
balances.put("ACC3", new BigDecimal("500"));
balances.put("ACC2", new BigDecimal("3000")); // value overwritten

    LinkedHashMap

  • Preserves insertion order
  • Use case: ordered configuration rules
    • Map<String, String> rules = new LinkedHashMap<>();
rules.put("RULE1", "VALIDATE");
rules.put("RULE2", "AUTHORIZE");
rules.put("RULE3", "SETTLE");
rules.put("RULE2", "REAUTHORIZE"); // overwrites value, keeps order

    TreeMap

  • Keys stored in sorted order
  • Use case: tiered pricing or limits
    • Map<Integer, String> riskLevels = new TreeMap<>();
riskLevels.put(3, "HIGH");
riskLevels.put(1, "LOW");
riskLevels.put(2, "MEDIUM");
riskLevels.put(2, "MEDIUM_PLUS"); // overwrites value

    Think

  • HashMap → fastest key-based access, no ordering guarantees
  • LinkedHashMap → predictable iteration order with slight overhead
  • TreeMap → sorted keys using natural order or Comparator

    • If you want the big-picture comparison between List, Set, and Map, read the intro:

    List, Set, and Map: Key Differences in Java Collections.

    Previous in the series:

    Java Collections in Real Systems | Part 2: List.

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