# HashMap vs TreeMap vs LinkedHashMap — Core Java

Source: https://www.geekswithgeeks.com/en/core-java/java-map-implementations

> Choose the right Map for the job based on ordering guarantees and performance: hash-bucketed, sorted-by-key, or insertion-ordered.

## Three flavours

`HashMap` — no ordering guarantee, O(1) average get/put, the default choice. `LinkedHashMap` — preserves **insertion order** (or access order, configurable) at a small extra cost. `TreeMap` — keeps keys **sorted** (natural order or a `Comparator`), O(log n) operations, backed by a red-black tree.

## Side by side

Same keys inserted in the same order, three different iteration results.

```java
Map<String, Integer> h = new HashMap<>();
Map<String, Integer> l = new LinkedHashMap<>();
Map<String, Integer> t = new TreeMap<>();
for (var m : List.of(h, l, t)) { m.put("c", 3); m.put("a", 1); m.put("b", 2); }

System.out.println(l.keySet()); // [c, a, b]  (insertion order)
System.out.println(t.keySet()); // [a, b, c]  (sorted)
```

## Rule of thumb

Start with `HashMap`. Switch to `LinkedHashMap` only if predictable iteration order matters (e.g. an LRU cache). Switch to `TreeMap` only if you need the keys sorted or range queries like `firstKey()`/`headMap()`.