Java TreeMap Class

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The TreeMap class belongs to the Java Collections Framework and resides in the java.util package. The TreeMap structure maintains key-value pairs in ascending order through a Red-Black Tree and implements both NavigableMap and SortedMap interfaces.

Key Features

  • Stores key-value pairs, just like HashMap
  • This collection orders keys based on their natural sequence or through a custom comparator function.
  • The TreeMap does not permit null keys but allows multiple null values.
  • The storage structure of TreeMap uses a Red-Black Tree which keeps balance as a binary search tree.
  • Not synchronized (thread-unsafe)

Syntax

TreeMap<KeyType, ValueType> map = new TreeMap<>(); 
TreeMap<Integer, String> students = new TreeMap<>();

Example

import java.util.TreeMap;
import java.util.Map;

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

        // Adding entries
        employeeMap.put(3, "Charlie");
        employeeMap.put(1, "Alice");
        employeeMap.put(2, "Bob");

        // Sorted by key
        System.out.println("Employee Map: " + employeeMap);

        // First and last
        System.out.println("First Key: " + employeeMap.firstKey());
        System.out.println("Last Entry: " + employeeMap.lastEntry());

        // Iterating over map
        for (Map.Entry<Integer, String> entry : employeeMap.entrySet()) {
            System.out.println("ID: " + entry.getKey() + ", Name: " + entry.getValue());
        }

        // Submap example
        System.out.println("Head Map (keys < 3): " + employeeMap.headMap(3));
    }
}
Run ›

Note

  • Either the keys must implement the Comparable interface or a Comparator needs to be supplied.
  • Inserting null keys is prohibited because they result in a NullPointerException.
  • This data structure serves best when you require access to sorted keys along with high-performance range queries.
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