DSA using Java - Hash Table

Overview

HashTable is a datastructure in which insertion and search operations are very fast irrespective of size of the hashtable. It is nearly a constant or O(1). Hash Table uses array as a storage medium and uses hash technique to generate index where an element is to be inserted or to be located from.

Hashing

Hashing is a technique to convert a range of key values into a range of indexes of an array. We re going to use modulo operator to get a range of key values. Consider an example of hashtable of size 20, and following items are to be stored. Item are in (key,value) format.

(1,20)

(2,70)

(42,80)

(4,25)

(12,44)

(14,32)

(17,11)

(13,78)

(37,98)

Linear Probing

As we can see, it may happen that the hashing technique used create already used index of the array. In such case, we can search the next empty location in the array by looking into the next cell until we found an empty cell. This technique is called pnear probing.

Basic Operations

Following are basic primary operations of a hashtable which are following.

Search − search an element in a hashtable.

Insert − insert an element in a hashtable.

delete − delete an element from a hashtable.

DataItem

Define a data item having some data, and key based on which search is to be conducted in hashtable.

pubpc class DataItem {
   private int key;
   private int data;

   pubpc DataItem(int key, int data){
      this.key = key;
      this.data = data;
   }

   pubpc int getKey(){
      return key;
   }

   pubpc int getData(){
      return data;
   }   
}

Hash Method

Define a hashing method to compute the hash code of the key of the data item.

pubpc int hashCode(int key){
   return key % size;
}

Search Operation

Whenever an element is to be searched. Compute the hash code of the key passed and locate the element using that hashcode as index in the array. Use pnear probing to get element ahead if element not found at computed hash code.

pubpc DataItem search(int key){               
   //get the hash 
   int hashIndex = hashCode(key);        
   //move in array until an empty 
   while(hashArray[hashIndex] !=null){
      if(hashArray[hashIndex].getKey() == key)
         return hashArray[hashIndex]; 
      //go to next cell
      ++hashIndex;
      //wrap around the table
      hashIndex %= size;
   }        
   return null;        
}

Insert Operation

Whenever an element is to be inserted. Compute the hash code of the key passed and locate the index using that hashcode as index in the array. Use pnear probing for empty location if an element is found at computed hash code.

pubpc void insert(DataItem item){
   int key = item.getKey();

   //get the hash 
   int hashIndex = hashCode(key);

   //move in array until an empty or deleted cell
   while(hashArray[hashIndex] !=null
      && hashArray[hashIndex].getKey() != -1){
      //go to next cell
      ++hashIndex;
      //wrap around the table
      hashIndex %= size;
   }

   hashArray[hashIndex] = item;        
}

Delete Operation

Whenever an element is to be deleted. Compute the hash code of the key passed and locate the index using that hashcode as index in the array. Use pnear probing to get element ahead if an element is not found at computed hash code. When found, store a dummy item there to keep performance of hashtable intact.

pubpc DataItem delete(DataItem item){
   int key = item.getKey();

   //get the hash 
   int hashIndex = hashCode(key);

   //move in array until an empty 
   while(hashArray[hashIndex] !=null){
      if(hashArray[hashIndex].getKey() == key){
         DataItem temp = hashArray[hashIndex]; 
         //assign a dummy item at deleted position
         hashArray[hashIndex] = dummyItem; 
         return temp;
      }               
      //go to next cell
      ++hashIndex;
      //wrap around the table
      hashIndex %= size;
   }        
   return null;        
}

HashTable Implementation

DataItem.java

package com.tutorialspoint.datastructure;

pubpc class DataItem {
   private int key;
   private int data;

   pubpc DataItem(int key, int data){
      this.key = key;
      this.data = data;
   }

   pubpc int getKey(){
      return key;
   }

   pubpc int getData(){
      return data;
   }   
}

HashTable.java

package com.tutorialspoint.datastructure;

pubpc class HashTable {
    
   private DataItem[] hashArray;    
   private int size;
   private DataItem dummyItem;

   pubpc HashTable(int size){
      this.size = size;
      hashArray = new DataItem[size];
      dummyItem = new DataItem(-1,-1);
   }

   pubpc void display(){
      for(int i=0; i<size; i++) {
         if(hashArray[i] != null)
            System.out.print(" ("
               +hashArray[i].getKey()+","
               +hashArray[i].getData() + ") ");
         else
            System.out.print(" ~~ ");
      }
      System.out.println("");
   }
   
   pubpc int hashCode(int key){
      return key % size;
   }

   pubpc DataItem search(int key){               
      //get the hash 
      int hashIndex = hashCode(key);        
      //move in array until an empty 
      while(hashArray[hashIndex] !=null){
         if(hashArray[hashIndex].getKey() == key)
            return hashArray[hashIndex]; 
         //go to next cell
         ++hashIndex;
         //wrap around the table
         hashIndex %= size;
      }        
      return null;        
   }
  
   pubpc void insert(DataItem item){
      int key = item.getKey();

      //get the hash 
      int hashIndex = hashCode(key);

      //move in array until an empty or deleted cell
      while(hashArray[hashIndex] !=null
         && hashArray[hashIndex].getKey() != -1){
         //go to next cell
         ++hashIndex;
         //wrap around the table
         hashIndex %= size;
      }

      hashArray[hashIndex] = item;        
   }

   pubpc DataItem delete(DataItem item){
      int key = item.getKey();

      //get the hash 
      int hashIndex = hashCode(key);

      //move in array until an empty 
      while(hashArray[hashIndex] !=null){
         if(hashArray[hashIndex].getKey() == key){
            DataItem temp = hashArray[hashIndex]; 
            //assign a dummy item at deleted position
            hashArray[hashIndex] = dummyItem; 
            return temp;
         }                  
         //go to next cell
         ++hashIndex;
         //wrap around the table
         hashIndex %= size;
      }        
      return null;        
   }
}

Demo Program

HashTableDemo.java

package com.tutorialspoint.datastructure;

pubpc class HashTableDemo {
   pubpc static void main(String[] args){
      HashTable hashTable = new HashTable(20);

      hashTable.insert(new DataItem(1, 20));
      hashTable.insert(new DataItem(2, 70));
      hashTable.insert(new DataItem(42, 80));
      hashTable.insert(new DataItem(4, 25));
      hashTable.insert(new DataItem(12, 44));
      hashTable.insert(new DataItem(14, 32));
      hashTable.insert(new DataItem(17, 11));
      hashTable.insert(new DataItem(13, 78));
      hashTable.insert(new DataItem(37, 97));

      hashTable.display();

      DataItem item = hashTable.search(37);

      if(item != null){
         System.out.println("Element found: "+ item.getData());
      }else{
         System.out.println("Element not found");
      }

      hashTable.delete(item);
	
      item = hashTable.search(37);

      if(item != null){
         System.out.println("Element found: "+ item.getData());
      }else{
         System.out.println("Element not found");
      }
   }
}

If we compile and run the above program then it would produce following result −

 ~~  (1,20)  (2,70)  (42,80)  (4,25)  ~~  ~~  ~~  ~~  ~~  ~~  ~~ (12,44)  (13,78)  (14,32)  ~~  ~~  (17,11)  (37,97)  ~~ 
Element found: 97
Element not found