Kamil owns a chain of bookstores all over Malaysia. Each of his employees have ID numbers that consists of numbers and c...

HashMap/Hashtable

• HashMap has implementation based on a hash table. (Use this class instead of Hashtable which is legacy class) .
• The HashMap gives you an unsorted, unordered Map.
• When you need a Map and you don't care about the order (when you iterate through it), then HashMap is the right choice.
• Keys of HashMap is like Set means no duplicates allowed and unordered while values can be any object even null or duplicate is also allowed.
• HashMap is very much similar to Hashtable only difference is Hashtable has all method synchronized for thread safety while HashMap has non-synchronized methods for better performance.
• We can visualize HashMap as below diagram where we have keys as per hash-code and corresponding values.

• HashMap provides constant-time performance for inserting and locating pairs.
• Performance can be adjusted via constructors that allow you to set the capacity and load factor of the hash table.

import java.util.*;
public class EmployeeSalaryStoring {
 public static void main(String[] args) {
  //Below Line will create HashMap with initial size 10 and 0.5 load factor
  Map<String, Integer>empSal = new HashMap<String, Integer>(10, 0.5f);
  //Adding employee name and salary to map
  empSal.put("Ramesh", 10000);
  empSal.put("Suresh", 20000);
  empSal.put("Mahesh", 30000);
  empSal.put("Naresh", 1000);
  empSal.put("Nainesh", 15000);
  empSal.put("Rakesh", 10000); // Duplicate Value also allowed but Keys should not be duplicate
  empSal.put("Nilesh", null); //Value can be null as well
  System.out.println("Original Map: "+ empSal);// Printing full Map 
  //Adding new employee the Map to see ordering of object changes
  empSal.put("Rohit", 23000);
  //Removing one key-value pair
  empSal.remove("Nilesh");
  System.out.println("Updated Map: "+empSal);// Printing full Map 
  //Printing all Keys
  System.out.println(empSal.keySet());
  //Printing all Values
  System.out.println(empSal.values());
 }
}

Sample Output

package linkedhashmap;
import java.util.LinkedHashMap;
import java.util.Map;
public class LinkedHashMapDemo {
 public static void main (String args[]){
  //Here Insertion order maintains 
  Map<Integer, String>lmap = new LinkedHashMap<Integer, String>();
  lmap.put(12, "Mahesh");
  lmap.put(5, "Naresh");
  lmap.put(23, "Suresh");
  lmap.put(9, "Sachin");
  System.out.println("LinkedHashMap before modification" + lmap);
  System.out.println("Is Employee ID 12 exists: " +lmap.containsKey(12));
  System.out.println("Is Employee name Amit Exists: "+lmap.containsValue("Amit"));
  System.out.println("Total number of employees: "+ lmap.size());
  System.out.println("Removing Employee with ID 5: " + lmap.remove(5));
  System.out.println("Removing Employee with ID 3 (which does not exist): " + lmap.remove(3));
  System.out.println("LinkedHashMap After modification" + lmap);
 }
}

Sample Output