JavaKitz

Friday, 29 June 2012

Q. How will you configure Hibernate?

Answer:

The configuration files hibernate.cfg.xml (or hibernate.properties) and mapping files *.hbm.xml are used by the Configuration class to create (i.e. configure and bootstrap hibernate) the SessionFactory, which in turn creates the Session instances. Session instances are the primary interface for the persistence service.

" hibernate.cfg.xml (alternatively can use hibernate.properties): These two files are used to configure the hibernate sevice (connection driver class, connection URL, connection username, connection password, dialect etc). If both files are present in the classpath then hibernate.cfg.xml file overrides the settings found in the hibernate.properties file.

" Mapping files (*.hbm.xml): These files are used to map persistent objects to a relational database. It is the best practice to store each object in an individual mapping file (i.e mapping file per class) because storing large number of persistent classes into one mapping file can be difficult to manage and maintain. The naming convention is to use the same name as the persistent (POJO) class name. For example Account.class will have a mapping file named Account.hbm.xml. Alternatively hibernate annotations can be used as part of your persistent class code instead of the *.hbm.xml files.

Q. What is a SessionFactory? Is it a thread-safe object?

Answer:

SessionFactory is Hibernates concept of a single datastore and is threadsafe so that many threads can access it concurrently and request for sessions and immutable cache of compiled mappings for a single database. A SessionFactory is usually only built once at startup. SessionFactory should be wrapped in some kind of singleton so that it can be easily accessed in an application code.

SessionFactory sessionFactory = new Configuration().configure().buildSessionfactory();

Q. What is a Session? Can you share a session object between different theads?

Answer:

Session is a light weight and a non-threadsafe object (No, you cannot share it between threads) that represents a single unit-of-work with the database. Sessions are opened by a SessionFactory and then are closed when all work is complete. Session is the primary interface for the persistence service. A session obtains a database connection lazily (i.e. only when required). To avoid creating too many sessions ThreadLocal class can be used as shown below to get the current session no matter how many times you make call to the currentSession() method.

&
public class HibernateUtil {
&
public static final ThreadLocal local = new ThreadLocal();

public static Session currentSession() throws HibernateException {
Session session = (Session) local.get();
//open a new session if this thread has no session
if(session == null) {
session = sessionFactory.openSession();
local.set(session);
}
return session;
}
}

It is also vital that you close your session after your unit of work completes. Note: Keep your Hibernate Session API handy.

Q. What are the benefits of detached objects?

Answer:

Detached objects can be passed across layers all the way up to the presentation layer without having to use any DTOs (Data Transfer Objects). You can later on re-attach the detached objects to another session.

Q. What are the pros and cons of detached objects?

Answer:

Pros:

" When long transactions are required due to user think-time, it is the best practice to break the long transaction up into two or more transactions. You can use detached objects from the first transaction to carry data all the way up to the presentation layer. These detached objects get modified outside a transaction and later on re-attached to a new transaction via another session.

Cons

" In general, working with detached objects is quite cumbersome, and better to not clutter up the session with them if possible. It is better to discard them and re-fetch them on subsequent requests. This approach is not only more portable but also more efficient because - the objects hang around in Hibernate's cache anyway.

" Also from pure rich domain driven design perspective it is recommended to use DTOs (DataTransferObjects) and DOs (DomainObjects) to maintain the separation between Service and UI tiers.

Q. How does Hibernate distinguish between transient (i.e. newly instantiated) and detached objects?

Answer

" Hibernate uses the version property, if there is one.
" If not uses the identifier value. No identifier value means a new object. This does work only for Hibernate managed surrogate keys. Does not work for natural keys and assigned (i.e. not managed by Hibernate) surrogate keys.
" Write your own strategy with Interceptor.isUnsaved().

What is ORM ?

ORM stands for object/relational mapping. ORM is the automated persistence of objects in a Java application to the tables in a relational database.

What does ORM consists of ?

An ORM solution consists of the followig four pieces:

API for performing basic CRUD operations
API to express queries refering to classes
Facilities to specify metadata
Optimization facilities : dirty checking,lazy associations fetching

What are the ORM levels ?

The ORM levels are:

Pure relational (stored procedure.)
Light objects mapping (JDBC)
Medium object mapping
Full object Mapping (composition,inheritance, polymorphism, persistence by reachability)

What is Hibernate?

Hibernate is a pure Java object-relational mapping (ORM) and persistence framework that allows you to map plain old Java objects to relational database tables using (XML) configuration files.Its purpose is to relieve the developer from a significant amount of relational data persistence-related programming tasks.

Why do you need ORM tools like hibernate?

The main advantage of ORM like hibernate is that it shields developers from messy SQL. Apart from this, ORM provides following benefits:

Improved productivity

High-level object-oriented API
Less Java code to write
No SQL to write

Improved performance

Sophisticated caching
Lazy loading
Eager loading

Improved maintainability

A lot less code to write

Improved portability

ORM framework generates database-specific SQL for you

What Does Hibernate Simplify?

Hibernate simplifies:

Saving and retrieving your domain objects
Making database column and table name changes
Centralizing pre save and post retrieve logic
Complex joins for retrieving related items
Schema creation from object model

What is the need for Hibernate xml mapping file?

Hibernate mapping file tells Hibernate which tables and columns to use to load and store objects. Typical mapping file look as follows:

8.What are the most common methods of Hibernate configuration?

The most common methods of Hibernate configuration are:

Programmatic configuration
XML configuration (hibernate.cfg.xml)
9.What are the important tags of hibernate.cfg.xml?

Following are the important tags of hibernate.cfg.xml:

0.What are the Core interfaces are of Hibernate framework?

The five core interfaces are used in just about every Hibernate application. Using these interfaces, you can store and retrieve persistent objects and control transactions.

Session interface
SessionFactory interface
Configuration interface
Transaction interface
Query and Criteria interfaces

1.What role does the Session interface play in Hibernate?

The Session interface is the primary interface used by Hibernate applications. It is a single-threaded, short-lived object representing a conversation between the application and the persistent store. It allows you to create query objects to retrieve persistent objects.

Session session =
sessionFactory.openSession();

Session interface role:

Wraps a JDBC connection
Factory for Transaction
Holds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier

What role does the SessionFactory interface play in Hibernate?

The application obtains Session instances from a SessionFactory. There is typically a single SessionFactory for the whole applicationå¹¼reated during application initialization. The SessionFactory caches generate SQL statements and other mapping metadata that Hibernate uses at runtime. It also holds cached data that has been read in one unit of work and may be reused in a future unit of work

SessionFactory
sessionFactory = configuration.buildSessionFactory();

13.What is the general flow of Hibernate communication with RDBMS?

The general flow of Hibernate communication with RDBMS is :

Load the Hibernate configuration file and create configuration object. It will automatically load all hbm mapping files
Create session factory from configuration object
Get one session from this session factory
Create HQL Query
Execute query to get list containing Java objects

14.What is Hibernate Query Language (HQL)?

Hibernate offers a query language that embodies a very powerful and flexible mechanism to query, store, update, and retrieve objects from a database. This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL.

15.How do you map Java Objects with Database tables?

First we need to write Java domain objects (beans with setter and getter).
Write hbm.xml, where we map java class to table and database columns to Java class variables.

Example :

<hibernate-mapping>

  <class name="com.test.User"  table="user">

   <property  column="USER_NAME" length="255" 

      name="userName" not-null="true"  type="java.lang.String"/>

   <property  column="USER_PASSWORD" length="255"

     name="userPassword" not-null="true"  type="java.lang.String"/>

 </class>

</hibernate-mapping>

What’s the difference between load() and get()?

load() vs. get() :-

load()	get()
Only use the `load()` method if you are sure that the object exists.	If you are not sure that the object exists, then use one of the `get()` methods.
`load()` method will throw an exception if the unique id is not found in the database.	`get()` method will return null if the unique id is not found in the database.
`load()` just returns a proxy by default and database won’t be hit until the proxy is first invoked.	`get()` will hit the database immediately.

17.What is the difference between and merge and update ?

Use update() if you are sure that the session does not contain an already persistent instance with the same identifier, and merge() if you want to merge your modifications at any time without consideration of the state of the session.

18.How do you define sequence generated primary key in hibernate?

Using <generator> tag.
Example:-

<id column="USER_ID" name="id" type="java.lang.Long"> 

   <generator class="sequence"> 

     <param name="table">SEQUENCE_NAME</param>

   <generator>

</id>

19.Define cascade and inverse option in one-many mapping?

cascade - enable operations to cascade to child entities.
cascade="all|none|save-update|delete|all-delete-orphan"

inverse - mark this collection as the "inverse" end of a bidirectional association.
inverse="true|false"
Essentially "inverse" indicates which end of a relationship should be ignored, so when persisting a parent who has a collection of children, should you ask the parent for its list of children, or ask the children who the parents are?

20.What do you mean by Named – SQL query?

Named SQL queries are defined in the mapping xml document and called wherever required.
Example:

<sql-query name = "empdetails">

   <return alias="emp" class="com.test.Employee"/>

      SELECT emp.EMP_ID AS {emp.empid},

                 emp.EMP_ADDRESS AS {emp.address},

                 emp.EMP_NAME AS {emp.name} 

      FROM Employee EMP WHERE emp.NAME LIKE :name

</sql-query>

Invoke Named Query :

List people = session.getNamedQuery("empdetails")

                    .setString("TomBrady", name)

                    .setMaxResults(50)

                    .list();

21.How do you invoke Stored Procedures?

<sql-query name="selectAllEmployees_SP" callable="true">

 <return alias="emp" class="employee">

   <return-property name="empid" column="EMP_ID"/>



   <return-property name="name" column="EMP_NAME"/>       

   <return-property name="address" column="EMP_ADDRESS"/>

    { ? = call selectAllEmployees() }

 </return>

</sql-query>

22.Explain Criteria API

Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.
Example :

List employees = session.createCriteria(Employee.class)

                        .add(Restrictions.like("name", "a%") )

                        .add(Restrictions.like("address", "Boston"))

                        .addOrder(Order.asc("name") )

                        .list();

23.Define HibernateTemplate?

org.springframework.orm.hibernate.HibernateTemplate is a helper class which provides different methods for querying/retrieving data from the database. It also converts checked HibernateExceptions into unchecked DataAccessExceptions.

24.What are the benefits does HibernateTemplate provide?

The benefits of HibernateTemplate are :

HibernateTemplate, a Spring Template class simplifies interactions with Hibernate Session.
Common functions are simplified to single method calls.
Sessions are automatically closed.
Exceptions are automatically caught and converted to runtime exceptions.

5.How do you switch between relational databases without code changes?

Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined.

26.If you want to see the Hibernate generated SQL statements on console, what should we do?

In Hibernate configuration file set as follows:
<property name="show_sql">true</property>

27.What are derived properties?

The properties that are not mapped to a column, but calculated at runtime by evaluation of an expression are called derived properties. The expression can be defined using the formula attribute of the element.

28.What is component mapping in Hibernate?

A component is an object saved as a value, not as a reference
A component can be saved directly without needing to declare interfaces or identifier properties
Required to define an empty constructor
Shared references not supported

Example:

29.What is the difference between sorted and ordered collection in hibernate?

sorted collection vs. order collection :-

sorted collection	order collection
A sorted collection is sorting a collection by utilizing the sorting features provided by the Java collections framework. The sorting occurs in the memory of JVM which running Hibernate, after the data being read from database using java comparator.	Order collection is sorting a collection by specifying the order-by clause for sorting this collection when retrieval.
If your collection is not large, it will be more efficient way to sort it.	If your collection is very large, it will be more efficient way to sort it .

hat is the advantage of Hibernate over jdbc?

Hibernate Vs. JDBC :-

JDBC	Hibernate
With JDBC, developer has to write code to map an object model's data representation to a relational data model and its corresponding database schema.	Hibernate is flexible and powerful ORM solution to map Java classes to database tables. Hibernate itself takes care of this mapping using XML files so developer does not need to write code for this.
With JDBC, the automatic mapping of Java objects with database tables and vice versa conversion is to be taken care of by the developer manually with lines of code.	Hibernate provides transparent persistence and developer does not need to write code explicitly to map database tables tuples to application objects during interaction with RDBMS.
JDBC supports only native Structured Query Language (SQL). Developer has to find out the efficient way to access database, i.e. to select effective query from a number of queries to perform same task.	Hibernate provides a powerful query language Hibernate Query Language (independent from type of database) that is expressed in a familiar SQL like syntax and includes full support for polymorphic queries. Hibernate also supports native SQL statements. It also selects an effective way to perform a database manipulation task for an application.
Application using JDBC to handle persistent data (database tables) having database specific code in large amount. The code written to map table data to application objects and vice versa is actually to map table fields to object properties. As table changed or database changed then it’s essential to change object structure as well as to change code written to map table-to-object/object-to-table.	Hibernate provides this mapping itself. The actual mapping between tables and application objects is done in XML files. If there is change in Database or in any table then the only need to change XML file properties.
With JDBC, it is developer’s responsibility to handle JDBC result set and convert it to Java objects through code to use this persistent data in application. So with JDBC, mapping between Java objects and database tables is done manually.	Hibernate reduces lines of code by maintaining object-table mapping itself and returns result to application in form of Java objects. It relieves programmer from manual handling of persistent data, hence reducing the development time and maintenance cost.
With JDBC, caching is maintained by hand-coding.	Hibernate, with Transparent Persistence, cache is set to application work space. Relational tuples are moved to this cache as a result of query. It improves performance if client application reads same data many times for same write. Automatic Transparent Persistence allows the developer to concentrate more on business logic rather than this application code.
In JDBC there is no check that always every user has updated data. This check has to be added by the developer.	Hibernate enables developer to define version type field to application, due to this defined field Hibernate updates version field of database table every time relational tuple is updated in form of Java class object to that table. So if two users retrieve same tuple and then modify it and one user save this modified tuple to database, version is automatically updated for this tuple by Hibernate. When other user tries to save updated tuple to database then it does not allow saving it because this user does not have updated data.

32.What are the Collection types in Hibernate ?

Bag
Set
List
Array
Map

33.What are the ways to express joins in HQL?

HQL provides four ways of expressing (inner and outer) joins:-

An implicit association join
An ordinary join in the FROM clause
A fetch join in the FROM clause.
A theta-style join in the WHERE clause.

34.Define cascade and inverse option in one-many mapping?

35.What is Hibernate proxy?

The proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies which implement the named interface. The actual persistent object will be loaded when a method of the proxy is invoked.

36.How can Hibernate be configured to access an instance variable directly and not through a setter method ?

By mapping the property with access="field" in Hibernate metadata. This forces hibernate to bypass the setter method and access the instance variable directly while initializing a newly loaded object.

37.How can a whole class be mapped as immutable?

Mark the class as mutable="false" (Default is true),. This specifies that instances of the class are (not) mutable. Immutable classes, may not be updated or deleted by the application.

38.What is the use of dynamic-insert and dynamic-update attributes in a class mapping?

dynamic-update (defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed
dynamic-insert (defaults to false): Specifies that INSERT SQL should be generated at whose values are not null.

39.What do you mean by fetching strategy ?

A fetching strategy is the strategy Hibernate will use for retrieving associated objects if the application needs to navigate the association. Fetch strategies may be declared in the O/R mapping metadata, or over-ridden by a particular HQL or Criteria query.

40.What is automatic dirty checking?

Automatic dirty checking is a feature that saves us the effort of explicitly asking Hibernate to update the database when we modify the state of an object inside a transaction.

41.What is transactional write-behind?

Hibernate uses a sophisticated algorithm to determine an efficient ordering that avoids database foreign key constraint violations but is still sufficiently predictable to the user. This feature is called transactional write-behind.

42.What are Callback interfaces?

Callback interfaces allow the application to receive a notification when something interesting happens to an object—for example, when an object is loaded, saved, or deleted. Hibernate applications don't need to implement these callbacks, but they're useful for implementing certain kinds of generic functionality.

43.What are the types of Hibernate instance states ?

Three types of instance states:

Transient -The instance is not associated with any persistence context
Persistent -The instance is associated with a persistence context
Detached -The instance was associated with a persistence context which has been closed – currently not associated

45.What are the types of inheritance models in Hibernate?

There are three types of inheritance models in Hibernate:

Table per class hierarchy
Table per subclass
Table per concrete class

Hibernate Interview Questions

1. What is Hibernate?

Hibernate is a powerful, high performance object/relational persistence and query service.

This lets the users to develop persistent classes following object-oriented principles such

as association, inheritance, polymorphism, composition, and collections.

2. What is ORM?

ORM stands for Object/Relational mapping. It is the programmed and translucent

perseverance of objects in a Java application in to the tables of a relational database using

the metadata that describes the mapping between the objects and the database. It works

by transforming the data from one representation to another.

3. What does an ORM solution comprises of?

• It should have an API for performing basic CRUD (Create, Read, Update, Delete)

operations on objects of persistent classes

• Should have a language or an API for specifying queries that refer to the classes

and the properties of classes

• An ability for specifying mapping metadata

• It should have a technique for ORM implementation to interact with transactional

objects to perform dirty checking, lazy association fetching, and other optimization

functions

4. What are the different levels of ORM quality?

There are four levels defined for ORM quality.

i. Pure relational

ii. Light object mapping

iii. Medium object mapping

iv. Full object mapping

5. What is a pure relational ORM?

The entire application, including the user interface, is designed around the relational

model and SQL-based relational operations.

6. What is a meant by light object mapping?

The entities are represented as classes that are mapped manually to the relational tables.

The code is hidden from the business logic using specific design patterns. This approach

is successful for applications with a less number of entities, or applications with common,

metadata-driven data models. This approach is most known to all.

7. What is a meant by medium object mapping?

The application is designed around an object model. The SQL code is generated at build

time. And the associations between objects are supported by the persistence mechanism,

and queries are specified using an object-oriented expression language. This is best suited for medium-sized applications with some complex transactions. Used when the mapping

exceeds 25 different database products at a time.

8. What is meant by full object mapping?

Full object mapping supports sophisticated object modeling: composition, inheritance,

polymorphism and persistence. The persistence layer implements transparent persistence;

persistent classes do not inherit any special base class or have to implement a special

interface. Efficient fetching strategies and caching strategies are implemented

transparently to the application.

9. What are the benefits of ORM and Hibernate?

There are many benefits from these. Out of which the following are the most important

one.

i. Productivity – Hibernate reduces the burden of developer by providing much of

the functionality and let the developer to concentrate on business logic.

ii. Maintainability – As hibernate provides most of the functionality, the LOC for the

application will be reduced and it is easy to maintain. By automated object/relational

persistence it even reduces the LOC.

iii. Performance – Hand-coded persistence provided greater performance than

automated one. But this is not true all the times. But in hibernate, it provides more

optimization that works all the time there by increasing the performance. If it is

automated persistence then it still increases the performance.

iv. Vendor independence – Irrespective of the different types of databases that are

there, hibernate provides a much easier way to develop a cross platform application.

10. How does hibernate code looks like?

Session session = getSessionFactory().openSession();

Transaction tx = session.beginTransaction();

MyPersistanceClass mpc = new MyPersistanceClass ("Sample App");

session.save(mpc);

tx.commit();

session.close();

The Session and Transaction are the interfaces provided by hibernate. There are many

other interfaces besides this.

11. What is a hibernate xml mapping document and how does it look like?

In order to make most of the things work in hibernate, usually the information is provided

in an xml document. This document is called as xml mapping document. The document

defines, among other things, how properties of the user defined persistence classes’ map

to the columns of the relative tables in database.

<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC

"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>

</id>

<many-to-one name="nxtPer" cascade="all" column="NxtPerId"/>

</class>

</hibernate-mapping>

Everything should be included under <hibernate-mapping> tag. This is the main tag for

an xml mapping document.

12. Show Hibernate overview?

13. What the Core interfaces are of hibernate framework?

There are many benefits from these. Out of which the following are the most important

one.

i. Session Interface – This is the primary interface used by hibernate applications.

The instances of this interface are lightweight and are inexpensive to create and destroy.

Hibernate sessions are not thread safe. ii. SessionFactory Interface – This is a factory that delivers the session objects to

hibernate application. Generally there will be a single SessionFactory for the whole

application and it will be shared among all the application threads.

iii. Configuration Interface – This interface is used to configure and bootstrap

hibernate. The instance of this interface is used by the application in order to specify the

location of hibernate specific mapping documents.

iv. Transaction Interface – This is an optional interface but the above three interfaces

are mandatory in each and every application. This interface abstracts the code from any

kind of transaction implementations such as JDBC transaction, JTA transaction.

v. Query and Criteria Interface – This interface allows the user to perform queries

and also control the flow of the query execution.

14. What are Callback interfaces?

These interfaces are used in the application to receive a notification when some object

events occur. Like when an object is loaded, saved or deleted. There is no need to

implement callbacks in hibernate applications, but they’re useful for implementing

certain kinds of generic functionality.

15. What are Extension interfaces?

When the built-in functionalities provided by hibernate is not sufficient enough, it

provides a way so that user can include other interfaces and implement those interfaces

for user desire functionality. These interfaces are called as Extension interfaces.

16. What are the Extension interfaces that are there in hibernate?

There are many extension interfaces provided by hibernate.

ProxyFactory interface - used to create proxies

ConnectionProvider interface – used for JDBC connection management

TransactionFactory interface – Used for transaction management

Transaction interface – Used for transaction management

TransactionManagementLookup interface – Used in transaction management.

Cahce interface – provides caching techniques and strategies

CacheProvider interface – same as Cache interface

ClassPersister interface – provides ORM strategies

IdentifierGenerator interface – used for primary key generation

Dialect abstract class – provides SQL support

17. What are different environments to configure hibernate?

There are mainly two types of environments in which the configuration of hibernate

application differs.

i. Managed environment – In this kind of environment everything from database

connections, transaction boundaries, security levels and all are defined. An example of

this kind of environment is environment provided by application servers such as JBoss,

Weblogic and WebSphere. ii. Non-managed environment – This kind of environment provides a basic

configuration template. Tomcat is one of the best examples that provide this kind of

environment.

18. What is the file extension you use for hibernate mapping file?

The name of the file should be like this : filename.hbm.xml

The filename varies here. The extension of these files should be “.hbm.xml”.

This is just a convention and it’s not mandatory. But this is the best practice to follow this

extension.

19. What do you create a SessionFactory?

Configuration cfg = new Configuration();

cfg.addResource("myinstance/MyConfig.hbm.xml");

cfg.setProperties( System.getProperties() );

SessionFactory sessions = cfg.buildSessionFactory();

First, we need to create an instance of Configuration and use that instance to refer to the

location of the configuration file. After configuring this instance is used to create the

SessionFactory by calling the method buildSessionFactory().

20. What is meant by Method chaining?

Method chaining is a programming technique that is supported by many hibernate

interfaces. This is less readable when compared to actual java code. And it is not

mandatory to use this format. Look how a SessionFactory is created when we use method

chaining.

SessionFactory sessions = new Configuration()

.addResource("myinstance/MyConfig.hbm.xml")

.setProperties( System.getProperties() )

.buildSessionFactory();

21. What does hibernate.properties file consist of?

This is a property file that should be placed in application class path. So when the

Configuration object is created, hibernate is first initialized. At this moment the

application will automatically detect and read this hibernate.properties file.

hibernate.connection.datasource = java:/comp/env/jdbc/AuctionDB

hibernate.transaction.factory_class =

net.sf.hibernate.transaction.JTATransactionFactory

hibernate.transaction.manager_lookup_class =

net.sf.hibernate.transaction.JBossTransactionManagerLookup

hibernate.dialect = net.sf.hibernate.dialect.PostgreSQLDialect

22. What should SessionFactory be placed so that it can be easily accessed?

As far as it is compared to J2EE environment, if the SessionFactory is placed in JNDI

then it can be easily accessed and shared between different threads and various

components that are hibernate aware. You can set the SessionFactory to a JNDI by

configuring a property hibernate.session_factory_name in the hibernate.properties file. 23. What are POJOs?

POJO stands for plain old java objects. These are just basic JavaBeans that have defined

setter and getter methods for all the properties that are there in that bean. Besides they

can also have some business logic related to that property. Hibernate applications works

efficiently with POJOs rather then simple java classes.

24. What is object/relational mapping metadata?

ORM tools require a metadata format for the application to specify the mapping between

classes and tables, properties and columns, associations and foreign keys, Java types and

SQL types. This information is called the object/relational mapping metadata. It defines

the transformation between the different data type systems and relationship

representations.

25. What is HQL?

HQL stands for Hibernate Query Language. Hibernate allows the user to express queries

in its own portable SQL extension and this is called as HQL. It also allows the user to

express in native SQL.

26. What are the different types of property and class mappings?

• Typical and most common property mapping

</property>

• Derived properties

<property name="averageBidAmount" formula="( select AVG(b.AMOUNT) from BID b

where b.ITEM_ID = ITEM_ID )" type="big_decimal"/>

• Typical and most common property mapping

• Controlling inserts and updates

<property name="name" column="NAME" type="string"

insert="false" update="false"/>

27. What is Attribute Oriented Programming?

XDoclet has brought the concept of attribute-oriented programming to Java. Until JDK

1.5, the Java language had no support for annotations; now XDoclet uses the Javadoc tag

format (@attribute) to specify class-, field-, or method-level metadata attributes. These

attributes are used to generate hibernate mapping file automatically when the application

is built. This kind of programming that works on attributes is called as Attribute Oriented

Programming. 28. What are the different methods of identifying an object?

There are three methods by which an object can be identified.

i. Object identity –Objects are identical if they reside in the same memory location

in the JVM. This can be checked by using the = = operator.

ii. Object equality – Objects are equal if they have the same value, as defined by the

equals( ) method. Classes that don’t explicitly override this method inherit the

implementation defined by java.lang.Object, which compares object identity.

iii. Database identity – Objects stored in a relational database are identical if they

represent the same row or, equivalently, share the same table and primary key value.

29. What are the different approaches to represent an inheritance hierarchy?

i. Table per concrete class.

ii. Table per class hierarchy.

iii. Table per subclass.

30. What are managed associations and hibernate associations?

Associations that are related to container management persistence are called managed

associations. These are bi-directional associations. Coming to hibernate associations,

these are unidirectional.

1) Explain Java Collections Framework?
Java Collections Framework provides a well designed set of interfaces and classes that support operations on a collections of objects.

2) Explain Iterator Interface.
An Iterator is similar to the Enumeration interface.With the Iterator interface methods, you can traverse a collection from start to end and safely remove elements from the underlying Collection. The iterator() method generally used in query operations.
Basic methods:
iterator.remove();
iterator.hasNext();
iterator.next();

3) Explain Enumeration Interface.
The Enumeration interface allows you to iterate through all the elements of a collection. Iterating through an Enumeration is similar to iterating through an Iterator. However, there is no removal support with Enumeration.
Basic methods:
boolean hasMoreElements();
Object nextElement();

4) What is the difference between Enumeration and Iterator interface?
The Enumeration interface allows you to iterate through all the elements of a collection. Iterating through an Enumeration is similar to iterating through an Iterator. However, there is no removal support with Enumeration.

5) Explain Set Interface.
In mathematical concept, a set is just a group of unique items, in the sense that the group contains no duplicates. The Set interface extends the Collection interface. Set does not allow duplicates in the collection. In Set implementations null is valid entry, but allowed only once.

6) What are the two types of Set implementations available in the Collections Framework?
HashSet and TreeSet are the two Set implementations available in the Collections Framework.

7) What is the difference between HashSet and TreeSet?
HashSet Class implements java.util.Set interface to eliminate the duplicate entries and uses hashing for storage. Hashing is nothing but mapping between a key value and a data item, this provides efficient searching.

The TreeSet Class implements java.util.Set interface provides an ordered set, eliminates duplicate entries and uses tree for storage.

What is a List?
List is a ordered and non duplicated collection of objects. The List interface extends the Collection interface.

9) What are the two types of List implementations available in the Collections Framework?
ArrayList and LinkedList are the two List implementations available in the Collections Framework.

10) What is the difference between ArrayList and LinkedList?
The ArrayList Class implements java.util.List interface and uses array for storage. An array storage's are generally faster but we cannot insert and delete entries in middle of the list.To achieve this kind of addition and deletion requires a new array constructed. You can access any element at randomly.

The LinkedList Class implements java.util.List interface and uses linked list for storage.A linked list allow elements to be added, removed from the collection at any location in the container by ordering the elements.With this implementation you can only access the elements in sequentially.

12) What collection will you use to implement a queue?
LinkedList

13) Explain Map Interface.
A map is a special kind of set with no duplicates.The key values are used to lookup, or index the stored data. The Map interface is not an extension of Collection interface, it has it's own hierarchy. Map does not allow duplicates in the collection. In Map implementations null is valid entry, but allowed only once.

14) What are the two types of Map implementations available in the Collections Framework?
HashMap and TreeMap are two types of Map implementations available in the Collections Framework.

15) What is the difference between HashMap and TreeMap?
The HashMap Class implements java.util.Map interface and uses hashing for storage. Indirectly Map uses Set functionality so, it does not permit duplicates. The TreeMap Class implements java.util.Map interface and uses tree for storage. It provides the ordered map.

16) Explain the functionality of Vector Class?
Once array size is set you cannot change size of the array. To deal with this kind of situations in Java uses Vector, it grows and shrink it's size automatically. Vector allows to store only objects not primitives. To store primitives, convert primitives in to objects using wrapper classes before adding them into Vector.The Vector reallocates and resizes itself automatically.

17) What does the following statement convey?
Vector vt = new Vector(3, 10);
vt is an instance of Vector class with an initial capacity of 3 and grows in increment of 10 in each relocation

18) How do you store a primitive data type within a Vector or other collections class?
You need to wrap the primitive data type into one of the wrapper classes found in the java.lang package, like Integer, Float, or Double, as in:
Integer in = new Integer(5);

19) What is the difference between Vector and ArrayList?
Vector and ArrayList are very similar. Both of them represent a growable array. The main difference is that Vector is synchronized while ArrayList is not.

20) What is the between Hashtable and HashMap?
Both provide key-value access to data. The key differences are :
a. Hashtable is synchronised but HasMap is not synchronised.
b. HashMap permits null values but Hashtable doent allow null values.
c. iterator in the HashMap is fail-safe while the enumerator for the Hashtable is not fail safe.

21) How do I make an array larger?
You cannot directly make an array larger. You must make a new (larger) array and copy the original elements into it, usually with System.arraycopy(). If you find yourself frequently doing this, the Vector class does this automatically for you, as long as your arrays are not of primitive data types.

22) Which is faster, synchronizing a HashMap or using a Hashtable for thread-safe access?
Because a synchronized HashMap requires an extra method call, a Hashtable is faster for synchronized access.

23) In which package would you find the interfaces amd claases defined in the Java Collection Framework?
java.util

24) What method in the System class allows you to copy eleemnts from one array to another?
System. arraycopy()

25) When using the System.arraycopy() method,What exception is thrown if the destination array is smaller than the souce array?
ArrayIndexOutofBoundsException

26) What is the use of Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region

27) What is the use of GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars

28) What is the use of SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar

29) What is the use of ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

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Friday, 29 June 2012

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