Hashtable , HashMap, Set , List differences java

What is difference between HashMap and Hashtable?
Ans:

HashMap= HashMap is not synchronized.In case of single thread, using HashMap is faster than than the Hashtable. HashMap allows null keys and null values to be stored.Iterator in the HashMap is fail-fast.This means Iterator will produce exception if concurrent modifications are made to the HashMap.

Hashtable=Hashtable is synchronized by default.In case of multiple threads,using Hashtable is advisable.With a single thread ,Hashtable becomes slow. Hashtable does not allow null keys or values.Enumeration for the Hashtable is not fail-fast.This means even if concurrent modifications are done to Hashtable ,there will not be any incorrect results produced by Enumeration.

Can you make HashMap synchronized?
Ans:
Yes,we can make it using synchronizedMap() methods as shown:

Collections.synchronizedMap(new HashMap());

What is difference between a Set and a List?
Ans:

Set= A set represent a collection of elements.Order of the element may change in the set.Set will not allow duplicate values to be stored.Accessing elements by their index is not possible. Sets will not allow null elements.

Lists= A List represents ordered collection of elements.List preserves the order of elements in which they are entered.List will allow duplicate values.Accessing elements by index is possible.Lists allow null elements to be stored.

HashMap Class Java example

Hash Map Class

 HashMap is a collection that stores elements in the form of key-value pairs.If key is provided,its corresponding value can be retrieve easily.Keys should be unique .This means we cannot use duplicate data for keys in the HashMap.
HashMap is not synchronized and hence in case of multi threading we can get unreliable result.

Default Initial Capacity=16
Load Factor=0.75

HashMap Class Methods

• value put(key,value)
• value get(Object key)
• Set<K> keySet()
• Collection<V> values
• value remove(Object key)
• void clear()
• boolean isEmpty()
• int size()

Program Sample:

/**

 * 

 */

package com.collectionpack;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.util.HashMap;

import java.util.HashSet;

import java.util.Set;

/**

 * @author Abhinaw.Tripathi

 *

 */

public class HashMapDemo {

/**

* @param args

* @throws IOException 

* @throws NumberFormatException 

*/

public static void main(String[] args) throws NumberFormatException, IOException

{

HashMap<String , Long> hm=new HashMap<>();

        String name,str;

        Long phone;

        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

        while(true)

        {

        System.out.println("1. enter phone entries");

        System.out.println("2. lookup in the phone book");

        System.out.println("3. display name in book");

        System.out.println("4. exit");

       

        System.out.println("your choice: ");

        int n=Integer.parseInt(br.readLine());

        switch (n) 

        {

case 1:

System.out.println("Enter name: ");

name=br.readLine();

System.out.println("Enter phone number: ");

str=br.readLine();

phone=new Long(str);

hm.put(name, phone);

break;

case 2:

System.out.println("Enter name: ");

name=br.readLine();

name=name.trim();

phone=hm.get(name);

System.out.println("Phone :"+phone);

break;

case 3:

Set<String> set=new HashSet<>();

set=hm.keySet();

System.out.println(set);

break;

default:

return;

}

        }

}

}

Output:

1. enter phone entries

2. lookup in the phone book

3. display name in book

4. exit

your choice: 

1

Enter name: 

abhinaw

Enter phone number: 

8130752107

1. enter phone entries

2. lookup in the phone book

3. display name in book

4. exit

your choice: 

2

Enter name: 

abhinaw

Phone :8130752107

1. enter phone entries

2. lookup in the phone book

3. display name in book

4. exit

your choice: