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This tutorial explains the key differences between Mapand in Java HashMap.

In Java, Mapis an interface for storing data in key-value pairs, and HashMapis Mapan implementation class of the interface. Java has several classes ( TreeHashMap, LinkedHashMap) that implement Mapthe interface to store data into key-value pairs. Let's look at some examples.

MapInterfaces in Java

You cannot use the interface alone Mapto save data, but we can create an object of its implementation class and then use Mapthe reference to save the object. Here, we use HashMapthe class to store the data and the Mapinterface to save the reference of that class. See the example below.

import java.util.HashMap;
import java.util.Map;

public class SimpleTesting {
  public static void main(String[] args) {
    Map<String, Integer> map = new HashMap<>();
    map.put("One", 1);
    map.put("Two", 2);
    map.put("Three", 3);
    System.out.println(map);
  }
}

Output:

{One=1, Two=2, Three=3}

HashMapClasses in Java

HashMapIt is Mapan implementation class of the interface. Therefore, we can use it to create a collection of key-value pairs. See the example below.

import java.util.HashMap;

public class SimpleTesting {
  public static void main(String[] args) {
    HashMap<String, Integer> map = new HashMap<>();
    map.put("One", 1);
    map.put("Two", 2);
    map.put("Three", 3);
    System.out.println(map);
  }
}

Output:

{One=1, Two=2, Three=3}

MapUsing references to store objects in Java

Since Mapis an interface, we can use it to save references to its implementing classes, such as HashMap, TreeMapetc. We can save TreeMapa or HashMapobject Mapin an interface. See the example below.

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

public class SimpleTesting {
  public static void main(String[] args) {
    Map<String, Integer> map = new HashMap<>();
    map.put("One", 1);
    map.put("Two", 2);
    map.put("Three", 3);
    System.out.println(map);
    Map<String, Integer> tmap = new TreeMap<>(map);
    System.out.println(tmap);
  }
}

Output:

{One=1, Two=2, Three=3}
{One=1, Three=3, Two=2}

MapUsing references to store objects in Java

This is an important example of using reference when working with implementation classes Map. Look, we have a method that takes Mapan object as a parameter. So, while calling, we can pass an object of any class like HashMapor HashTable. See the example below.

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.TreeMap;

public class SimpleTesting {
  static void printMap(Map<String, Integer> map) {
    for (String key : map.keySet()) {
      System.out.println(key + ":" + map.get(key));
    }
  }
  public static void main(String[] args) {
    HashMap<String, Integer> hashmap = new HashMap<>();
    hashmap.put("One", 1);
    hashmap.put("Two", 2);
    hashmap.put("Three", 3);
    printMap(hashmap);
    TreeMap<String, Integer> tmap = new TreeMap<>(hashmap);
    printMap(tmap);
    LinkedHashMap<String, Integer> lmap = new LinkedHashMap<>(hashmap);
    printMap(lmap);
  }
}

Run the following code.

Java SimpleTesting 10 20

Output:

10
20