TreeMap and HashMap in Java

What is HashMap in Java?

HashMap in Java is a collection class that implements the Map interface. It stores elements as key-value pairs. Each key is unique, and a key maps to exactly one value. HashMap allows one null key and multiple null values. It does not maintain any specific order of elements because it uses a hashing mechanism for fast retrieval.

Example of HashMap in Java

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1, "Apple");
        map.put(2, "Banana");
        map.put(3, "Cherry");
        map.put(null, "Grapes");

        System.out.println("HashMap Elements:");
        for (Integer key : map.keySet()) {
            System.out.println(key + " => " + map.get(key));
        }
    }
}
  

Output:

HashMap Elements:
null => Grapes
1 => Apple
2 => Banana
3 => Cherry
  

What is TreeMap in Java?

TreeMap in Java is another implementation of the Map interface. Unlike HashMap, TreeMap stores elements in a sorted order of keys, either natural ordering or defined by a comparator. It does not allow null keys (throws NullPointerException if used) but allows multiple null values.

Example of TreeMap in Java

import java.util.TreeMap;

public class TreeMapExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(3, "Cherry");
        map.put(1, "Apple");
        map.put(2, "Banana");

        System.out.println("TreeMap Elements:");
        for (Integer key : map.keySet()) {
            System.out.println(key + " => " + map.get(key));
        }
    }
}
  

Output:

TreeMap Elements:
1 => Apple
2 => Banana
3 => Cherry
  

Differences Between HashMap and TreeMap

Aspect	HashMap	TreeMap
Order	No guaranteed order of elements.	Maintains sorted order of keys.
Null Keys	Allows one null key.	Does not allow null keys.
Performance	Faster for insertion, search, and delete operations.	Slower due to sorting overhead.
Implementation	Based on Hash Table.	Based on Red-Black Tree.
Sorting	Unsorted collection.	Automatically sorted by key.

Advantages

HashMap provides faster data access, making it ideal for performance-critical applications where ordering is not required. TreeMap is useful when you need to maintain a sorted map or perform range-based operations. Both classes offer efficient ways to store key-value pairs and are part of the Java Collections Framework.