TreeMap in Java

TreeMap stores key-value pairs sorted by key. By default it uses the natural ordering of keys (alphabetical for Strings, numeric for numbers). You can also supply a custom Comparator to define any order.

Operations like get, put, remove are O(log n) — slower than HashMap's O(1), but you get sorted keys and powerful range-query methods in return.

Basic Usage — Automatic Sort

Run this example to see its output.

Line by line:

• TreeMap maintains keys sorted at all times — even after additions.
• firstKey() / lastKey() — smallest and largest key in O(log n).
• headMap("Priya") — all entries with key strictly less than "Priya".
• tailMap("Priya") — all entries with key ≥ "Priya".
• subMap("Asha", "Ravi") — keys from "Asha" (inclusive) to "Ravi" (exclusive).

Navigable Methods

Run this example to see its output.

These range-navigation methods are unique to TreeMap — HashMap does not have them.

Custom Order with Comparator

Run this example to see its output.

TreeMap Does Not Allow null Keys

TreeMap needs to compare keys for ordering. null cannot be compared → NPE.

When to Choose TreeMap

Use TreeMap when:

• You need keys in sorted order
• You need range queries (all keys between X and Y)
• You need first/last key quickly

Use HashMap when:

• You only need lookup by key (faster, O(1))
• Order does not matter

Practice Task

Task: Create a TreeMap<String, Integer> storing city population (in lakhs): Mumbai: 200, Delhi: 190, Bangalore: 130, Chennai: 100, Hyderabad: 100. Print all cities alphabetically, the first and last alphabetically, and all cities whose name starts before "D" (headMap).

Solution:

Run this example to see its output.

What to observe: Keys print in alphabetical order regardless of insertion order. headMap("D") returns only "Bangalore" and "Chennai" since both come before "D" alphabetically. Try tailMap("H") — it returns cities from "H" onwards.