Core Java Interview Questions:- What are Constructors in Java?

In this video we will learn about Constructors in Java.


What is difference between decorator and adapter, template and strategy, factory and abstract factory?

Decorator and Adapter.

Decorator Adaptar
Used to deal with new behavior or responsibilities without affecting existing code Mainly involved in converting interfaces as per the requirement of client
Provides a wrapper over objects to apply same method which yields different results based on whether the object invoked is granular or composite Allows use of new libraries without changing existing methods
Change of interface does not happen Change of interface happens in this case

Template and Strategy.

Template Strategy
Defines outline for algorithms and allows sub class to override some steps Uses object composition to provide variety of algorithm implementation
Efficient in terms of less redundant code Flexible in terms of allowing users to change algorithm at run time
Depends on super class for methods All the algorithms can be itself implemented

Factory and Abstract Factory.

Factory Abstract factory
Specifies creation method which will be  used by sub class for object creation Used to create group of related objects while hiding actual classes
Deals with single product Deals with number of products
Each concrete factory implements abstract factory in case more than one products needed to be supported Makes uses of factory pattern provided by each family of product


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Java J2ee interview questions and answers: – From the below java code which constructor will fire first?

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Ok, the first thing, let’s try to understand the above java interview question.  Let’s say you have two classes one is parent class and the other is a child class which extends (inherits) from the parent class as shown in the below code. Now if you create the object of “Child” class which constructor will fire first.

 class Parent


public Parent()


System.out.println(“This is parent”);



class Child extends Parent


public Child()


System.out.println(“This is Child”);



The answer is first the Parent class constructor will get executed and then the child class constructor will fire.

One more  Java j2ee interview question which was recently asked  to one of my friend was :-  Does Stringbuffer always give better performance as compared to string.

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Java Training: -How will you explain Bootstrap, Extension and System Class loader?

There three types of class loaders:-

  • Bootstrap Class loader also called as primordial class loader.
  • Extension Class loader.
  • System Class loader.

Let’s now try to get the fundamentals of these class loaders.

Bootstrap Class loader

Bootstrap class loader loads those classes those which are essential for JVM to function properly. Bootstrap class loader is responsible for loading all core java classes for instance java.lang.*,* etc. Bootstrap class loader finds these necessary classes from “jdk/jre/lib/rt.jar”. Bootstrap class loader cannot be instantiated from JAVA code and is implemented natively inside JVM.

Extension Class loader

The extension class loader also termed as the standard extensions class loader is a child of the bootstrap class loader. Its primary responsibility is to load classes from the extension directories, normally located the “jre/lib/ext” directory. This provides the ability to simply drop in new extensions, such as various security extensions, without requiring modification to the user’s class path.

System Class loader

The system class loader also termed application class loader is the class loader responsible for loading code from the path specified by the CLASSPATH environment variable. It is also used to load an application’s entry point class that is the “static void main ()” method in a class.

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Java Training: – Explain the concept of local interfaces?

One of the biggest issues of creating objects using home interface is performance. Below are the steps which follow when you call the EJB object:-

  • JAVA client calls the local stub.
  • Stub marshal the values in to some other form which the network understands and sends it to the skeleton.
  • Skeleton then de-marshals it back to a form which is suitable for JAVA.
  • Skeleton then calls the EJB object and methods.
  • EJB object then does object creation, connection pooling, transaction etc.
  • Once EJB object calls the bean and the bean completes its functionalities. All the above steps must again be repeated to reach back to the JAVA client.

So you can easily guess from the above step that its lot of work. But this has been improved in EJB 2.0 using Local objects. Local objects implement local interface rather than using remote interface. Just to have a comparison below are the steps how the local object works.

  • JAVA client calls the local object.
  • Local object does connection pooling, transactions and security.
  • It then passes calls the bean and when bean completes its work it returns the data to the Local object who then passes the same to the end client.

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Java Training: – Elaborate different types of resultset?

“ResultSet” is an object that contains the results of SQL query. That means it contains the rows that satisfy the conditions of the query.

There are three basic types of resultsets:-

Forward-only results set

These types of resultset are non-scrollable and moves only forward.

Scroll-insensitive set

Resultset is scrollable so the cursor can move forward, backward, to a particular row etc.

While the resultset is open it does not show any changes done to the underlying database.

Scroll-sensitive set

Resultset is scrollable so the cursor can move forward, backward, to a particular row etc.

Resultset is sensitive to changes when it is open or used. That means if any values are modified in the database it’s propagated to the resultset.

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Java Training: – Show “ResultSet”, “RowSet”, “CachedRowset”, “JdbcRowset” and “WebRowSet” relation ship?

Below are the major points of difference:-

  • “ResultSet” is a connected architecture while “RowSet” is a disconnected architecture.
  • As “ResultSet” is a connected architecture we cannot serialize the object while “RowSet” can be serialized.
  • “RowSet” object is a Java bean while “ResultSet” is not. In the below diagram you can see “RowSet” interface also derives from “BaseRowSet” interface. “BaseRowSet” interface has all the ingredients to make it a Java bean.

Below diagram shows the complete relationship between all the interface and classes.


Figure: – Interface diagram for “Resultset” and “RowSet”

As “RowSet” is a disconnected from database it need to maintain Meta data for columns. “RowSet” can also provide scrollable resultsets or updatable resultsets even if the JDBC driver is not supporting the same. Java client can get a “RowSet” manipulate the same and finally send all the results to the database at one go. Sun has provided three implementation of “RowSet” as below:-

CachedRowSet: – Disconnected rowset always keeps the data in memory. It is scrollable and can also be serialized. It’s an ideal choice where we want to pass data between tiers or to do batch updates. “CachedRowSet” are disconnected so can be used for huge number of updates. Get the “CachedRowSet” object manipulate all your data and send the whole bunch of manipulated data in on go to the Database.

JDBCRowSet: – It’s opposite to “CachedRowSet”. It maintains a connection to the database while the client has reference to it. So it’s a connected architecture as compared to CachedRowSet.

WebRowSet :- “WebRowSet” is a extension of “CachedRowSet”. But the added feature is it can produce XML presentation of the data cached. If you are thinking of exposing your data through web services or to provide data to thin client which are written in different language other than JAVA. Best bet if you want to pass data in XML format over HTTP protocol.

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