HashMap Custom implementation - put, get, remove Employee object in java

In this post i will be explaining how to put, get, remove Employee object in custom HashMap.

Employee object overrides:
>equals method - helps in checking equality of employee objects used as key in entry objects.
>hashCode method - helps in finding bucket’s index on which data will be stored.


      @Override
   public boolean equals(Object o){
         
          if(o==null)
                 return false;
          if(this.getClass()!=o.getClass())
                 return false;
  
          Employee e=(Employee)o;
          return e.id.equals(this.id) && e.name.equals(this.name);         
   }
         
   @Override
   public int hashCode(){
          return id.hashCode() + name.hashCode();     
   }



We will maintain bucket (ArrayList) which will store Entry (LinkedList).







Full Program/SourceCode to put, get, remove Employee object in custom HashMap>
package com.ankit;
/** Copyright (c), AnkitMittal  JavaMadeSoEasy.com */
/**
* @author AnkitMittal
* Copyright (c), AnkitMittal . All Contents are copyrighted and must not be reproduced in any form.
* Employee class- to be used as key in HashMapCustom.
*/
class Employee {
   private String id;
   private String name;
  
   /**
   * Employee constructor
   */
   public Employee(String id, String name) { // constructor
          this.id = id;
          this.name = name;
   }
   @Override
   public String toString() {
          return "Employee[id=" + id + ", name=" + name + "] ";
   }
   @Override
   public boolean equals(Object o){
         
          if(o==null)
                 return false;
          if(this.getClass()!=o.getClass())
                 return false;
  
          Employee e=(Employee)o;
          return e.id.equals(this.id) && e.name.equals(this.name);         
   }
         
   @Override
   public int hashCode(){
          return id.hashCode() + name.hashCode();     
   }
}

/** Copyright (c), AnkitMittal  JavaMadeSoEasy.com */
/**
* @author AnkitMittal
* Copyright (c), AnkitMittal . All Contents are copyrighted and must not be reproduced in any form.
* This class provides custom implementation of HashMap(without using java api's)- which allows us to store data in key-value pair form.
* insertion order of key-value pairs is not maintained.
* @param <K>
* @param <V>
*/
class HashMapCustom<K, V> {
  
   private Entry<K,V>[] table;   //Array of Entry.
   private int capacity= 4;  //Initial capacity of HashMap
  
  
   static class Entry<K, V> {
       K key;
       V value;
       Entry<K,V> next;
  
       public Entry(K key, V value, Entry<K,V> next){
           this.key = key;
           this.value = value;
           this.next = next;
       }
   }
  
   @SuppressWarnings("unchecked")
   public HashMapCustom(){
      table = new Entry[capacity];
   }
  
   /**
    * Method allows you put key-value pair in HashMapCustom.
    * If the map already contains a mapping for the key, the old value is replaced.
    * Note: method does not allows you to put null key thought it allows null values.
    * Implementation allows you to put custom objects as a key as well.
    * Key Features: implementation provides you with following features:-
    *     >provide complete functionality how to override equals method.
    *  >provide complete functionality how to override hashCode method.
    * @param newKey
    * @param data
    */
   public void put(K newKey, V data){
      if(newKey==null)
          return;    //does not allow to store null.
     
      int hash=hash(newKey);
      Entry<K,V> newEntry = new Entry<K,V>(newKey, data, null);
     
       if(table[hash] == null){
        table[hash] = newEntry;
       }else{
          Entry<K,V> previous = null;
          Entry<K,V> current = table[hash];
         
          while(current != null){ //we have reached last entry of bucket.
         if(current.key.equals(newKey)){                
             if(previous==null){  //node has to be insert on first of bucket.
                   newEntry.next=current.next;
                   table[hash]=newEntry;
                   return;
             }
             else{
             newEntry.next=current.next;
             previous.next=newEntry;
             return;
             }
         }
         previous=current;
            current = current.next;
        }
        previous.next = newEntry;
       }
   }
   /**
    * Method returns value corresponding to key.
    * @param key
    */
   public V get(K key){
       int hash = hash(key);
       if(table[hash] == null){
        return null;
       }else{
        Entry<K,V> temp = table[hash];
        while(temp!= null){
            if(temp.key.equals(key))
                return temp.value;
            temp = temp.next; //return value corresponding to key.
        }        
        return null;   //returns null if key is not found.
       }
   }
   /**
    * Method removes key-value pair from HashMapCustom.
    * @param key
    */
   public boolean remove(K deleteKey){
      
      int hash=hash(deleteKey);
            
     if(table[hash] == null){
          return false;
     }else{
     Entry<K,V> previous = null;
     Entry<K,V> current = table[hash];
    
     while(current != null){ //we have reached last entry node of bucket.
         if(current.key.equals(deleteKey)){             
             if(previous==null){  //delete first entry node.
                   table[hash]=table[hash].next;
                       return true;
             }
             else{
                   previous.next=current.next;
                return true;
             }
         }
         previous=current;
            current = current.next;
         }
     return false;
     }
   
   }
  
   /**
    * Method displays all key-value pairs present in HashMapCustom.,
    * insertion order is not guaranteed, for maintaining insertion order refer LinkedHashMapCustom.
    * @param key
    */
   public void display(){
      
      for(int i=0;i<capacity;i++){
          if(table[i]!=null){
                 Entry<K, V> entry=table[i];
                 while(entry!=null){
                       System.out.print("{"+entry.key+"="+entry.value+"}" +"   ");
                       entry=entry.next;
                 }
          }
      }            
   
   }
   /**
    * Method implements hashing functionality, which helps in finding the appropriate bucket location to store our data.
    * This is very important method, as performance of HashMapCustom is very much dependent on  this method's implementation.
    * @param key
    */
   private int hash(K key){
       return Math.abs(key.hashCode()) % capacity;
   }
}
/**
* Main class- to test HashMap functionality.
*/
public class HashMapCustomEmp {
  
   public static void main(String[] args) {
      HashMapCustom<Employee, Integer> hashMapCustom = new HashMapCustom<Employee, Integer>();
      hashMapCustom.put(new Employee("10", "sam"), 12);
      hashMapCustom.put(new Employee("21", "amy"), 121);
      hashMapCustom.put(new Employee("31", "rob"), 151);
      hashMapCustom.put(new Employee("41", "sam"), 15);
      hashMapCustom.put(new Employee("42", "wil"), 89);
     
          System.out.println("Display values corresponding to keys>");
          System.out.println("value corresponding to employee with id=21 & name='amy' : "+hashMapCustom.get(new Employee("21", "amy")));
          System.out.println("value corresponding to employee with id=51 & name='pat' : "+hashMapCustom.get(new Employee("51", "pat")));
         
       System.out.print("Displaying : ");
       hashMapCustom.display();
      
       System.out.println("\n\nvalue corresponding to employee with id=21 & name='amy' removed: "+hashMapCustom.remove(new Employee("21", "amy")));
       System.out.println("value corresponding to employee with id=51 & name='pat' removed: "+hashMapCustom.remove(new Employee("51", "pat")));
         
          System.out.print("Displaying : ");
       hashMapCustom.display();
  
   }
}
/*Output
Display values corresponding to keys>
value corresponding to employee with id=21 & name='amy' : 121
value corresponding to employee with id=51 & name='pat' : null
Displaying : {Employee[id=21, name=amy] =121}   {Employee[id=41, name=sam] =15}   {Employee[id=42, name=wil] =89}   {Employee[id=10, name=sam] =12}   {Employee[id=31, name=rob] =151}  
value corresponding to employee with id=21 & name='amy' removed: true
value corresponding to employee with id=51 & name='pat' removed: false
Displaying : {Employee[id=41, name=sam] =15}   {Employee[id=42, name=wil] =89}   {Employee[id=10, name=sam] =12}   {Employee[id=31, name=rob] =151}  
*/




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