Hash-Map

HashMap in Java

Overview

The HashMap class in Java is a part of the java.util package and implements the Map interface. It provides a collection of key-value pairs and does not guarantee the order of the elements.

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Characteristics of HashMap

Feature	Details
Package	java.util
Allows Duplicates	No (Allows duplicate values, but not keys)
Maintains Insertion Order	No
Thread-Safe	No
Initial Capacity	16 (default)

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Key Features of HashMap

• Key-Value Pairs: Stores data in key-value pairs.
• Fast Lookup: Provides constant time complexity (O(1)) for get() and put() operations under ideal conditions.
• No Guarantee of Order: Elements are not ordered according to their insertion sequence.
• Allows null values: Allows one null key and multiple null values.

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How to Use HashMap

Importing HashMap

import java.util.HashMap;

Creating a HashMap

HashMap<KeyType, ValueType> hashMap = new HashMap<>();

• Example

  HashMap<String, String> map = new HashMap<>();
  HashMap<Integer, String> mapWithIntegers = new HashMap<>();

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HashMap Specific Methods

The HashMap class provides several methods beyond those in the Map interface. Here are some of the most commonly used:

Method	Description	Return Type	Example
put(K key, V value)	Associates the specified value with the specified key.	V	map.put("Name", "Alice");
get(Object key)	Returns the value associated with the specified key.	V	map.get("Name");
remove(Object key)	Removes the mapping for the specified key from the map.	V	map.remove("Name");
containsKey(Object key)	Checks if the map contains a mapping for the specified key.	boolean	map.containsKey("Name");
containsValue(Object value)	Checks if the map contains the specified value.	boolean	map.containsValue("Alice");
size()	Returns the number of key-value mappings in the map.	int	map.size();
clear()	Removes all mappings from the map.	void	map.clear();
isEmpty()	Checks if the map is empty.	boolean	map.isEmpty();
keySet()	Returns a set view of the keys contained in the map.	Set	map.keySet();
values()	Returns a collection view of the values contained in the map.	Collection	map.values();
entrySet()	Returns a set view of the mappings contained in the map.	Set<Map.Entry<K, V>>	map.entrySet();
putAll(Map<? extends K, ? extends V> m)	Copies all of the mappings from the specified map to the current map.	void	map.putAll(anotherMap);
equals(Object o)	Compares the specified object with this map for equality.	boolean	map.equals(otherMap);

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Example Code

import java.util.HashMap;

public class HashMapDataStructure {
    public static void main(String[] args) {
        // Create a HashMap
        HashMap<String, String> map = new HashMap<>();

        // Add key-value pairs
        map.put("Name", "Alice");
        map.put("Age", "25");
        map.put("City", "New York");

        // Access value by key
        System.out.println("Value for 'Name': " + map.get("Name"));

        // Check size
        System.out.println("Size of the map: " + map.size());

        // Check if a key exists
        System.out.println("Contains key 'Age': " + map.containsKey("Age"));

        // Remove a key-value pair
        map.remove("City");
        System.out.println("Map after removal: " + map);

        // Iterate over key-value pairs
        System.out.println("Iterating over map:");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }

        // Clear the map
        map.clear();
        System.out.println("Is map empty? " + map.isEmpty());
    }
}

For more: HashMapDataStructure.java