Java Academy · Lesson

TreeMap: Sorted Key-Value Pairs

Use TreeMap to maintain sorted order and navigate with firstKey, lastKey, floorKey, and ceilingKey.

Lesson 1 of 413 steps

TreeMap: Sorted Key-Value Pairs is a free Java Academy lesson on CoddyKit. This is lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Java Academy learning path, and your progress syncs across the web and the CoddyKit app. The Java Academy course includes 4 lessons in total.

What is TreeMap?

TreeMap is a sorted map implementation backed by a Red-Black tree. Keys are maintained in ascending natural order (or custom comparator order). All basic operations are O(log n).

import java.util.TreeMap;

TreeMap<String, Integer> scores = new TreeMap<>();
scores.put("Charlie", 85);
scores.put("Alice", 92);
scores.put("Bob", 78);

// Iteration is in key order: Alice, Bob, Charlie
for (var entry : scores.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

firstKey, lastKey, floorKey, ceilingKey

TreeMap's NavigableMap interface exposes navigation methods for finding keys relative to a given value:

TreeMap<Integer, String> map = new TreeMap<>();
map.put(10, "ten"); map.put(20, "twenty"); map.put(30, "thirty"); map.put(40, "forty");

System.out.println(map.firstKey());       // 10
System.out.println(map.lastKey());        // 40
System.out.println(map.floorKey(25));     // 20 (largest key ≤ 25)
System.out.println(map.ceilingKey(25));   // 30 (smallest key ≥ 25)
System.out.println(map.lowerKey(20));     // 10 (strictly less)
System.out.println(map.higherKey(20));    // 30 (strictly greater)

Navigating Entries

floorEntry, ceilingEntry, firstEntry, lastEntry return the full Map.Entry rather than just the key:

TreeMap<Integer, String> prices = new TreeMap<>();
prices.put(100, "Budget"); prices.put(300, "Standard"); prices.put(700, "Premium");

var entry = prices.floorEntry(350);
System.out.println(entry.getKey() + ": " + entry.getValue()); // 300: Standard

var top = prices.lastEntry();
System.out.println(top.getValue()); // Premium

subMap, headMap, tailMap

Extract range views from a TreeMap. These views are backed by the original map — changes in one reflect in the other.

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 1; i <= 10; i++) map.put(i, "v"+i);

// Keys from 3 (inclusive) to 7 (exclusive)
System.out.println(map.subMap(3, 7));   // {3=v3, 4=v4, 5=v5, 6=v6}

// Keys strictly less than 5
System.out.println(map.headMap(5));     // {1=v1, 2=v2, 3=v3, 4=v4}

// Keys >= 7
System.out.println(map.tailMap(7));     // {7=v7, 8=v8, 9=v9, 10=v10}

Inclusive/Exclusive Boundaries

Use the overloaded variants for fine-grained boundary control:

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 1; i <= 10; i++) map.put(i*10, "v"+i);

// From 30 (inclusive) to 60 (inclusive)
System.out.println(map.subMap(30, true, 60, true));
// {30=v3, 40=v4, 50=v5, 60=v6}

descending Order

Use descendingMap() or descendingKeySet() to iterate keys in reverse order:

TreeMap<String, Integer> tm = new TreeMap<>();
tm.put("A", 1); tm.put("C", 3); tm.put("B", 2);

for (String key : tm.descendingKeySet()) {
    System.out.print(key + " "); // C B A
}

pollFirstEntry and pollLastEntry

Remove and return the first or last entry atomically — useful for building priority maps:

TreeMap<Integer, String> events = new TreeMap<>();
events.put(8, "Breakfast");
events.put(12, "Lunch");
events.put(18, "Dinner");

var first = events.pollFirstEntry(); // removes 8=Breakfast
System.out.println(first.getValue() + " removed");
System.out.println(events.firstKey()); // 12

Use Case: Leaderboard

A leaderboard needs players sorted by score. TreeMap sorts by key automatically:

TreeMap<Integer, String> leaderboard = new TreeMap<>(Comparator.reverseOrder());
leaderboard.put(1200, "Alice");
leaderboard.put(1500, "Bob");
leaderboard.put(900, "Carol");

int rank = 1;
for (var e : leaderboard.entrySet()) {
    System.out.println(rank++ + ". " + e.getValue() + " (" + e.getKey() + ")");
}
// 1. Bob (1500)
// 2. Alice (1200)
// 3. Carol (900)

Use Case: Event Scheduler

Map timestamps to events — use ceilingEntry to find the next scheduled event after a given time:

import java.time.LocalTime;
TreeMap<LocalTime, String> schedule = new TreeMap<>();
schedule.put(LocalTime.of(9,0), "Standup");
schedule.put(LocalTime.of(14,0), "Review");
schedule.put(LocalTime.of(17,0), "Retro");

LocalTime now = LocalTime.of(11, 30);
var next = schedule.ceilingEntry(now);
System.out.println("Next: " + next.getValue()); // Review

TreeMap vs HashMap Performance

Key comparison:

HashMap: O(1) average get/put; unordered
TreeMap: O(log n) get/put; sorted by key
LinkedHashMap: O(1) average; insertion-ordered

Use TreeMap when you need sorted keys or range queries. HashMap is faster for simple key lookup.

Thread Safety

TreeMap is NOT thread-safe. For concurrent access, use ConcurrentSkipListMap which also maintains sorted order with O(log n) operations and supports concurrent reads/writes.

Quick Check

You have a TreeMap<Integer, String> with keys {10, 20, 30, 40}. What does map.floorKey(25) return?

Answer it in the interactive lesson

Recap: TreeMap

Key takeaways:

TreeMap maintains keys in sorted (ascending) order via Red-Black tree
All operations are O(log n)
Navigation: firstKey, lastKey, floorKey, ceilingKey, lowerKey, higherKey
Range views: subMap, headMap, tailMap (backed views)
Use ConcurrentSkipListMap for thread-safe sorted maps

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Frequently Asked Questions

Is the “TreeMap: Sorted Key-Value Pairs” lesson free?

Yes — the full text of “TreeMap: Sorted Key-Value Pairs” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Java Academy course, upgrade to CoddyKit PRO. The Java Academy course includes 4 lessons in total.

What will I learn in “TreeMap: Sorted Key-Value Pairs”?

Use TreeMap to maintain sorted order and navigate with firstKey, lastKey, floorKey, and ceilingKey. You practise Java Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Java Academy?

No prior experience is required. Java Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “TreeMap: Sorted Key-Value Pairs” lesson take?

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