The fundamentals of Hash tables?

Question

I\'m quite confused about the basic concepts of a Hash table. If I were to code a hash how would I even begin? What is the difference between a Hash table and just a normal

Answer 1

Hash table is a data structure that is created for quick look up.

The hash tables are not effective when the number of entries are very small.

reference

Some examples:

    import java.util.Collection;
    import java.util.Enumeration;
    import java.util.Hashtable;
    import java.util.Set;

    public class HashtableDemo {

    public static void main(String args[]) {

// Creating Hashtable for example

     Hashtable companies = new Hashtable();


// Java Hashtable example to put object into Hashtable
// put(key, value) is used to insert object into map

     companies.put("Google", "United States");
     companies.put("Nokia", "Finland");
     companies.put("Sony", "Japan");


// Java Hashtable example to get Object from Hashtable
// get(key) method is used to retrieve Objects from Hashtable

     companies.get("Google");


// Hashtable containsKey Example
// Use containsKey(Object) method to check if an Object exits as key in
// hashtable

     System.out.println("Does hashtable contains Google as key: "+companies.containsKey("Google"));


// Hashtable containsValue Example
// just like containsKey(), containsValue returns true if hashtable
// contains specified object as value

      System.out.println("Does hashtable contains Japan as value: "+companies.containsValue("Japan"));


// Hashtable enumeration Example
// hashtabl.elements() return enumeration of all hashtable values

      Enumeration enumeration = companies.elements();

      while (enumeration.hasMoreElements()) {
      System.out.println("hashtable values: "+enumeration.nextElement());
      }


// How to check if Hashtable is empty in Java
// use isEmpty method of hashtable to check emptiness of hashtable in
// Java

       System.out.println("Is companies hashtable empty: "+companies.isEmpty());


// How to find size of Hashtable in Java
// use hashtable.size() method to find size of hashtable in Java

      System.out.println("Size of hashtable in Java: " + companies.size());


// How to get all values form hashtable in Java
// you can use keySet() method to get a Set of all the keys of hashtable
// in Java

      Set hashtableKeys = companies.keySet();


// you can also get enumeration of all keys by using method keys()

      Enumeration hashtableKeysEnum = companies.keys();


// How to get all keys from hashtable in Java
// There are two ways to get all values form hashtalbe first by using
// Enumeration and second getting values ad Collection

      Enumeration hashtableValuesEnum = companies.elements();


      Collection hashtableValues = companies.values();


// Hashtable clear example
// by using clear() we can reuse an existing hashtable, it clears all
// mappings.

       companies.clear();
      }
     }

Output:

Does hashtable contains Google as key: true

Does hashtable contains Japan as value: true

hashtable values: Finland

hashtable values: United States

hashtable values: Japan

Is companies hashtable empty: false

Size of hashtable in Java: 3