Introduction of HashMap Java in 2022
HashMap is one of the popular data structure in Java
➡️ HashMap stores data in key, value pair structure.
➡️ HashMap stores unique keys
Example
class Factance {
public static void main(String[] args) {
// create a hashmap
HashMap factNumbers = new HashMap<>();
System.out.println("Yello HashMap: " + factNumbers);
// put() method to add elements
factNumbers.put("One", 1);
factNumbers.put("Two", 2);
factNumbers.put("Three", 3);
System.out.println("HashMap after put(): " + factNumbers);
}
}
Yello HashMap: {}
HashMap after put(): {One=1, Two=2, Three=3}
Common HashMap Functions
✅ put(key,value)
Method to add key value into HashMap
class Factance {
public static void main(String[] args) {
// create a hashmap
HashMap factNumbers = new HashMap<>();
System.out.println("Yello HashMap: " + factNumbers);
// put() method to add elements
factNumbers.put("One", 1);
factNumbers.put("Two", 2);
factNumbers.put("Three", 3);
System.out.println("HashMap after put(): " + factNumbers);
}
}
Yello HashMap: {}
HashMap after put(): {One=1, Two=2, Three=3}
✅ remove(key)
Method to remove entry(key,value) by key from HashMap
import java.util.HashMap;
class Factance {
public static void main(String[] args) {
// create a hashmap
HashMap factNumbers = new HashMap<>();
System.out.println("Yello HashMap: " + factNumbers);
// put() method to add elements
factNumbers.put("One", 1);
factNumbers.put("Two", 2);
factNumbers.put("Three", 3);
System.out.println("HashMap after put(): " + factNumbers);
factNumbers.remove("One");
System.out.println("\nHashMap after remove(): " + factNumbers);
}
}
Yello HashMap: {}
HashMap after put(): {One=1, Two=2, Three=3}
HashMap after remove(): {Two=2, Three=3}
✅ get(key)
Method to get value by key
import java.util.HashMap;
class Factance {
public static void main(String[] args) {
// create a hashmap
HashMap factNumbers = new HashMap<>();
System.out.println("Yello HashMap: " + factNumbers);
// put() method to add elements
factNumbers.put("One", 1);
factNumbers.put("Two", 2);
factNumbers.put("Three", 3);
Integer twoValue = factNumbers.get("Two");
System.out.println("\ntwoValue get(): " + twoValue);
}
}
Yello HashMap: {}
twoValue get(): 2
Internal working of HashMap
Whenever you call put method
- It calculates hashcode of key by hashCode method and add hashcode into hash table(array)
- Add the value into Linked List after comparing value with equals method
- In case value is already there, it simply updates the value
Whenever you call get method
- It calculates hashcode of key by hashCode meothod
- Look up hashtable by hashcode
- Iteracte and checks the value in Linked List using equals method
Collision
- Two keys have same hashcode, it means collision
- HashMap handles collision by storing LinkedIn on top of hash table
Load Factor & Capacity
➡️ Capacity means number of entries it can store
➡️ Load Factor means after filling 60 percent (o.6 load factor) of entries, hash table double size of the original hash table
HashMap numbers = new HashMap<>(8, 0.6f);
capacity 8 means it can store 8 entries
load factor 0.6 means hash table after filling 60 percent, size of original hash table gets double
Important concepts of HashMap
➡️ HashMap put, remove and get happens at speed of O(1)
➡️ HashMap uses hashCode and equals method internally
➡️ In case of collision, if Linked List is filled with a default capacity, Java 8 converts Linked List to self balancing tree to have optimization.
Keywords
java collision hashing hashcode equals linked-list self-balancing-tree load-factor capacity