Java Academy · Lesson

Submaps and Range Views

Extract subMap, headMap, and tailMap views for range-based lookups in sorted maps.

Submaps and Range Views is a free Java Academy lesson on CoddyKit — lesson 2 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, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Range Views in TreeMap

TreeMap's subMap, headMap, and tailMap return backed views — they reflect changes in the underlying map and vice versa. Changes through the view are reflected in the original.

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

var view = map.subMap(30, 70); // [30, 70)
System.out.println(view); // {30=item3, 40=item4, 50=item5, 60=item6}

map.put(45, "new"); // also visible through view!
System.out.println(view.containsKey(45)); // true

headMap: Keys Below a Bound

headMap(toKey) returns all entries with keys strictly less than toKey. Use the inclusive variant headMap(toKey, true) to include the bound.

TreeMap<String, Integer> words = new TreeMap<>();
"banana cherry apple date elderberry".chars()
    .mapToObj(c -> String.valueOf((char)c)).distinct()
    .forEach(w -> words.put(w, w.length()));
// Actually let's use real words:
TreeMap<String, Integer> wc = new TreeMap<>();
wc.put("apple",5); wc.put("banana",6); wc.put("cherry",6); wc.put("date",4);

System.out.println(wc.headMap("cherry")); // {apple=5, banana=6}

tailMap: Keys From a Bound

tailMap(fromKey) returns all entries with keys ≥ fromKey.

TreeMap<Integer, String> grades = new TreeMap<>();
grades.put(50,"F"); grades.put(60,"D"); grades.put(70,"C"); grades.put(80,"B"); grades.put(90,"A");

// All passing grades (>= 60)
var passing = grades.tailMap(60);
System.out.println(passing); // {60=D, 70=C, 80=B, 90=A}

subMap with Inclusive Bounds

The 4-argument subMap(from, fromInclusive, to, toInclusive) gives full control over boundary inclusivity:

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 10; i <= 100; i += 10) map.put(i, "v"+i);

// [30, 60] — both inclusive
System.out.println(map.subMap(30, true, 60, true));
// {30=v30, 40=v40, 50=v50, 60=v60}

// (30, 60) — both exclusive
System.out.println(map.subMap(30, false, 60, false));
// {40=v40, 50=v50}

Modifying Through a View

Put/remove operations on a subMap view are reflected in the original map (and vice versa). Attempting to insert a key outside the view's range throws an exception.

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 1; i <= 5; i++) map.put(i * 10, "v" + i);

var view = map.subMap(20, 40); // [20, 40)
view.remove(20); // removes from both view and original map
System.out.println(map.containsKey(20)); // false

// This would throw IllegalArgumentException:
// view.put(50, "out of range");

Use Case: Log Range Query

Retrieve all log entries between two timestamps using a TreeMap range view:

import java.time.*;
TreeMap<LocalDateTime, String> logs = new TreeMap<>();
logs.put(LocalDateTime.of(2024,1,1,8,0), "Server start");
logs.put(LocalDateTime.of(2024,1,1,10,0), "Request spike");
logs.put(LocalDateTime.of(2024,1,1,14,0), "Maintenance");
logs.put(LocalDateTime.of(2024,1,1,18,0), "Server stop");

var morning = logs.subMap(
    LocalDateTime.of(2024,1,1,8,0), true,
    LocalDateTime.of(2024,1,1,12,0), false
);
morning.forEach((t,m) -> System.out.println(t+" : "+m));

Use Case: Price Range Lookup

Find all products in a price range using TreeMap keys as prices:

TreeMap<Double, String> products = new TreeMap<>();
products.put(9.99, "Pen");
products.put(24.99, "Book");
products.put(49.99, "Headphones");
products.put(299.99, "Tablet");

double min = 10.0, max = 100.0;
var affordable = products.subMap(min, true, max, true);
affordable.forEach((p,n) -> System.out.println(n+" $"+p));
// Book $24.99, Headphones $49.99

descending SubMap

Chain descendingMap() on a view for reverse-order navigation:

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 10; i <= 100; i += 10) map.put(i, "v"+i);

// Get [40, 80] in descending order
map.subMap(40, true, 80, true)
   .descendingMap()
   .forEach((k,v) -> System.out.println(k + "=" + v));
// 80=v80, 70=v70, 60=v60, 50=v50, 40=v40

NavigableMap Interface

NavigableMap extends SortedMap and adds ceiling/floor/higher/lower key navigation plus descending views. TreeMap is the most common implementation; ConcurrentSkipListMap is the thread-safe alternative.

Performance of Views

Submap view operations (get, put, containsKey) are the same O(log n) as the underlying TreeMap. Creating the view itself is O(1) — no copying occurs. Range scans over n keys in the view are O(log N + n) where N is the full map size.

Pitfall: Stale Views

Because views are backed by the original map, a view can become empty or throw if the original map is cleared. Always document that views are live and don't persist them beyond their intended lifecycle.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(10, "a"); map.put(20, "b"); map.put(30, "c");

var view = map.subMap(10, 30);
map.clear(); // view becomes empty
System.out.println(view.size()); // 0 — but no exception

Quick Check

You call map.subMap(30, false, 70, true) on a TreeMap with keys {10,20,30,40,50,60,70,80}. Which keys are included in the result?

Recap: Submaps and Range Views

Key takeaways:

subMap, headMap, tailMap return live backed views — no copying
Changes in view reflect in original map and vice versa
4-arg subMap(from, fromInclusive, to, toInclusive) for full boundary control
Out-of-range puts via a view throw IllegalArgumentException
Range scan complexity: O(log N + n)

Frequently asked questions

Is the “Submaps and Range Views” lesson free?

Yes — the full text of “Submaps and Range Views” is free to read here on the web, and the Java Academy course includes 4 lessons in total. 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.

What will I learn in “Submaps and Range Views”?

Extract subMap, headMap, and tailMap views for range-based lookups in sorted maps. 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; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Submaps and Range Views” lesson take?

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