Java Treemap — Free Java Tutorial

TL;DR: Learn Java Treemap in Java with a free, beginner-friendly tutorial, examples and practice for Indian students on Syllab.in.

Java Treemap in Java

TreeMap<K,V> is a Map implementation that stores key-value pairs in sorted key order (natural order or custom Comparator). It uses a Red-Black tree internally.

All operations (put, get, remove, containsKey) run in O(log n) time — slower than HashMap (O(1)) but always sorted.

TreeMap provides navigation methods: firstKey(), lastKey(), headMap(toKey), tailMap(fromKey), subMap(from, to), floorKey(), ceilingKey().

Use TreeMap when you need keys in sorted order, or when you need range queries on keys. Use HashMap when order does not matter and you want speed.

TreeMap<String, Integer> map = new TreeMap<>();
map.put("banana", 2);
map.put("apple", 5);
map.put("cherry", 1);
// Iterates in key order: apple, banana, cherry

map.firstKey();           // "apple"
map.lastKey();            // "cherry"
map.headMap("cherry");    // keys < "cherry"
map.floorKey("b");

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