Creating Subsets and Headsets from TreeMap with Java

Last Updated : 23 Jul, 2025

A Java TreeMap is a sorted collection that is a component of the Java Collections Framework. Subsets are parts of the map, and headsets are made up of items that are less than or equal to a specified key.

In this article, we will learn how to create subsets and headsets from a TreeMap based on specific criteria in Java.

How to Create Subsets and Headsets from a TreeMap Based on Specific Criteria in Java?

Approaches to create subsets and headsets from a TreeMap

  • SubMap(): TreeMap's subMap(K fromKey, K toKey) function yields a view of the area of the map whose keys fall between fromKey and toKey.
  • HeadMap(): The part of the map whose keys are strictly smaller than toKey is shown in the view returned by the TreeMap headMap(K toKey) function.

1. Creating a Subset using subMap()

Below is the implementation of Creating a Subset using subMap() :

Java 
// Java program to create subsets from a TreeMap
import java.util.TreeMap;

public class SubSetExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> treeMap = new TreeMap<>();

        treeMap.put(1, "A");
        treeMap.put(2, "B");
        treeMap.put(3, "C");
        treeMap.put(4, "D");
        treeMap.put(5, "E");

        // creating a subset from key 3 (inclusive) to key 5 (exclusive)
        TreeMap<Integer, String> subMap = new TreeMap<>(treeMap.subMap(3, 5));

        System.out.println("Original TreeMap: " + treeMap);
        System.out.println("Subset (inclusive of key 3, exclusive of key 5): " + subMap);
    }
}

Output
Original TreeMap: {1=A, 2=B, 3=C, 4=D, 5=E}
Subset (inclusive of key 3, exclusive of key 5): {3=C, 4=D}

Explanation of the Program:

  • In the above program, a TreeMap is created.
  • A subset is created from the original TreeMap using the subMap() method. This specifies the start (inclusive) and end (exclusive) keys.
  • The original TreeMap and the subset are printed to the console.

2. Creating a Headset using headMap

Below is the implementation of Creating a Headset using headMap:

Java 
// Java program to create headsets from a TreeMap
import java.util.TreeMap;

public class HeadSetExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> treeMap = new TreeMap<>();

        treeMap.put(1, "A");
        treeMap.put(2, "B");
        treeMap.put(3, "C");
        treeMap.put(4, "D");
        treeMap.put(5, "E");

        // Creating a headset up to key 4 (exclusive)
        TreeMap<Integer, String> headMap = new TreeMap<>(treeMap.headMap(4));

        System.out.println("Original TreeMap: " + treeMap);
        System.out.println("Headset (up to key 4, exclusive): " + headMap);
    }
}

Output
Original TreeMap: {1=A, 2=B, 3=C, 4=D, 5=E}
Headset (up to key 4, exclusive): {1=A, 2=B, 3=C}

Explanation of the Program:

  • In the above program, a TreeMap is created.
  • A headset is created from the original TreeMap using the headMap() method, specifying the end key (exclusive).
  • The original TreeMap and the headset are printed to the console.
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