Most frequently asking Java Interview questions

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          Most of the people and specially interview facing people looking interview related Question. Now I am try to show interview question on collection related and Database question. Hey common I am also facing these following questions one of interview.

Q) What is the Difference between ArrayList and Vector in Java?

R)

Vector	ArrayList
1. Vector came along with the first version of java development kit (JDK)	1. java.util.ArrayList was introduced in java version1.2
2. All the methods of Vector is synchronized	2.The methods of ArrayList is not synchronized
3. Vector uses Array internally as data structure	3. ArrayList uses Array internally as data structure
4. Vector doubles the size of its array when its size is increased	4. ArrayList increases by half of its size when its size is increased
5. Vector’s methods are synchronized	5.ArrayList’s methods are not synchronized.

                

Q) Vector or ArrayList? Which is better to use in java?

R) In general, executing a ‘synchronized’ method results in costlier performance than a unsynchronized method. Keeping the difference in mind, using Vector will incur a performance hit than the ArrayList. But, when there is a certain need for thread-safe operation Vector needs to be used

Q) Is there an alternate available in java for Vector?

R)

ArrayList can be synchronized using the java collections framework utility class and then ArrayList itself can be used in place of Vector.

When there is no need for synchronized operation and you still look for better performance ‘Array’ can be used instead of ArrayList. But the development is tedious, since it doesn’t provide user friendly methods.

When you use Vector or ArrayList, always initialize to the largest capacity that the java program will need. Since incrementing the size is a costlier operation.

Q) What is the difference between Collection and Collections?

Collection	Collections
1. Collection is the root interface  in collection hierarchy	1. Collections is a class which  extends Object class & it consists exclusively static  methods
2. it allows duplicate & non-duplicate elements  which may be ordered or unordered.	2. Collections are used to store,  retrieve, manipulate, and  communicate aggregate data
3. A collection (sometimes called  a container) is an object that groups  multiple elements into a single unit.	3. Collections are used to store,  retrieve and manipulate data,  and to transmit data from one  method to another. Collections typically  represent data items  that form a natural group, like  an order (a collection of order lines), a mail folder (a collection of messages), or a telephone  directory (a collection of name/ phone-number pairs).
4. Collection is an interface which can be implemented the List, Set, Sorted  Set, and Queue.	4. Collections is an utility class  which contain the utility methods  such as Sorting etc.
5. Collection(Uppercase C)represents  the interface which is extended by  three other interfaces(Set, List and ueue,Map doesn’t extend Collection  interface). Guys, its extended not Implemented  (hope you all know the difference between the two,interface can  extend interface, remember?)	5. Collections(upper case C with  at the end) this the java.util. Collections class that holds a pile of static methods  for use with  collections

Note: after reading above difference you are able to understand which one is used when right…. Here I am mentioned some of the point while I am writing this time my mind is cached these points you people great you may know many things this post only for fast recollecting its not a measurement to  know this much okay friends.

Q)What is the difference between Hashtable and Hashmap ?

Hashtable	Hashmap
1. Hashtable was part of the original java.util and is a concreate   implementation of a Dictionary class.	1. HashMap implements Map interface.
2. Hashtable is synchronised	2. HashMap is not synchronised
3. Hashtable does not allow "null" key	3. HashMap allows "null" Key.
4. In HashTable you can change  the iteration	4. HashMap you can change the iteration  but you will get a java.util.ConcurrentModificationException.

Note: HashMap allows 1 null key and multiple null values but why? What was 

the basic idea behind it making such feature by JAVA builder?

R) HashMap allows only one null key, basic idea behind it is that key can have unique value 

only if one null is assigned to a key next null will be duplicate. You can try the following code 

and mail me what are your opinion friends.

HashMap hm=new HashMap();

hm.put("1","11");

hm.put("2","22");

hm.put("3","33");

hm.put("4","242");

hm.put(null,null);

hm.put(null,null);

HashMap also allow more null values or not check once and mail me.

Hashtable:

Hashtable is a data structure that retains values of key-value pair. It doesn’t allow null for both the keys and the values. You will get a NullPointerException if you add null value. It is synchronized. So it comes with its cost. Only one thread can access HashTable at a particular time.

import java.util.Map;

import java.util.Hashtable;

public class TestClass {

    public static void main(String args[ ]) {

    Map<Integer,String> states = new Hashtable<Integer,String>();

    states.put(1, "Andhra Pradesh");

    states.put(2, "Karnatka");

    states.put(3, null);    //will throw NullPointerEcxeption at runtime

    System.out.println(states.get(1));

    System.out.println(states.get(2));

//  System.out.println(states.get(3));

    }

}

HashMap:

HashMap is like Hashtable but it also accepts key value pair. It allows null for both the keys and the values. Its performance better is better than HashTable, because it is unsynchronized.

import java.util.HashMap;

import java.util.Map;

public class TestClass {

    public static void main(String args[ ]) {

    Map<Integer,String> states = new HashMap<Integer,String>();

    states.put(1, "Andhra Pradesh");

    states.put(2, "Karnatka");

    states.put(3, null);    // Okay

    states.put(null,"UK");

    System.out.println(states.get(1));

    System.out.println(states.get(2));

    System.out.println(states.get(3));

    }

}

Q) What is the difference between comparator and comparable?

R)

Comparator	Comparable
1.Comparator in Java is defined in java.util package	1. Comparable interface in Java is definedin java.lang package
2. Comparator interface in Java has method public int compare (Object o1, Object o2) which returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.	2. Comparable interface has method public int compareTo(Object o) which returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.
3. Comparator in Java compare two objects provided to him	3. Comparable interface compares "this" reference with the object specified.
4.Comparator in Java is used to implement custom ordering of object. public int compare(Object o1, Object o2)	4. Comparable in Java is used to implement natural ordering of object. In Java API String, Date and wrapper classes implements Comparable interface.Its always good practice to override compareTo() for value objects.

Example Program:

1. public class Customer implements Comparable {
    private int customer_id;
    private String name;
    
    /**
     * Compare current customer with specified person
     * return zero if customer _id for both person is same 
     * return negative if current customer _id is less than specified one
     * return positive if specified customer _id is greater than specified one
     */
    @Override 
    public int compareTo(Object o) {
        Customer c = (Customer) o; 
        return this. customer _id - o. customer _id ;
    }
    ….
}

2. /**
 * Comparator implementation which sorts Customer objects on Customer _id field
 */
public class SortByCustomer _ID implements Comparator{

    public int compare(Object o1, Object o2) {
        Customer c1 = (Customer) o;
       Customer c2 = (Customer) o; 
        return p1.getCustomerId() - p2.get CustomerId();
    }
}

 Most frequently asking Java Interview question in telephonic or face to face code writing ?

Q) What is the difference between Observer and Observable in Java?

 R)

Observer is an object that wishes to be notified when the state of other object changes.(Listener,registered with the source to be notified of any change in the source) 

Observable is an object whose state may be of interest and in which other objects may register an interest(Source of an event/action)

import java.util.Observable;
import java.util.Observer;

/**
 * A simple demo of Observable->Observer
 */
public class ObservDemo extends Object {
  MyView view;

  MyModel model;

  public ObservDemo() {

    view = new MyView();

    model = new MyModel();
    model.addObserver(view);

  }

  public static void main(String[] av) {
    ObservDemo me = new ObservDemo();
    me.demo();
  }

  public void demo() {
    model.changeSomething();
  }

  /** The Observer normally maintains a view on the data */
  class MyView implements Observer {
    /** For now, we just print the fact that we got notified. */
    public void update(Observable obs, Object x) {
      System.out.println("update(" + obs + "," + x + ");");
    }
  }

  /** The Observable normally maintains the data */
  class MyModel extends Observable {
    public void changeSomething() {
      // Notify observers of change
      setChanged();
      notifyObservers();
    }
  }
}