Complete reference to core java interview questions basic to advanced level : SE to Team Lead level Part - 6

Enumeration elements() : Returns an enumeration of the values contained in the hashtable.

int size() : Returns the number of entries in the hashtable.

Properties

•Properties is a subclass of Hashtable

• Used to maintain lists of values in which the key is a String and the value is also a String

• Constructors

Properties()

Properties(Properties propDefault) : Creates an object that uses propDefault for its default value.

Methods  :

String getProperty(String key) : Returns the value associated with key.

Strng getProperty(String key, String defaultProperty) : Returns the value associated with key. defaultProperty is returned if key is neither in the list nor in the default property list .

Enumeration propertyNames() : Returns an enumeration of the keys. This includes those keys found in the default property list.

The Interfaces in Collections Framework

  Collection                    Map                    Iterator

 Set           List                  SortedMap           ListIterator

   |

SortedSet

  Collection :

§  A collection allows a group of objects to be treated as a single unit.

§  The Java collections library forms a framework for collection classes.

§  The CI is the root of collection hierarchy and is used for common functionality across all collections.

§  There is no direct implementation of Collection Interface.

§  Two fundamental interfaces for containers:

• Collection

boolean add(Object element) : Inserts element into a collection

Set Interface: extends Collection Interface.   The Class Hash set implements Set Interface.

§  Is used to represent the group of unique elements.

§  Set stores elements in an unordered way but does not contain duplicate elements.

§  identical to Collection interface, but doesn’t accept duplicates.

Sorted set : extends Set Interface.  The class Tree Set implements Sorted set Interface.

§  It provides the extra functionality of keeping  the elements sorted.

§  It represents the collection consisting  of Unique, sorted elements in ascending order.

expose the comparison object for sorting. 

List Interface :

§  ordered collection – Elements are added into a particular position.

§  Represents the sequence of numbers in a fixed order.

§  But may contain duplicate elements.

§  Elements can be inserted or retrieved by their position in the List using Zero based index.

§  List stores elements in an ordered way.

Map Interface: Basic Interface.The classes Hash Map & HashTable implements Map interface.

§  Used to represent the mapping of unique keys to values.

§  By using the key value we can retrive the values.

§  Two basic operations are get( ) & put( ) .

boolean put(Object key, Object value)  :  Inserts given value into map with key

Object get(Object key) :  Reads value for the given key.

Tree Map Class: Implements Sorted Set Interface.                

§  The elements are stored in sorted ascending order.         

§  Using key value we can retrieve the data.                            

§  Provides an efficient means of storing key/value pairs in sorted order and allows rapid retrivals.

      TreeMap tm=new TreeMap( );

      tm.put( “Prasad”,new Double(74.6));

The Classes in Collections Framework

                          Abstract Collection

       Abstract List                              Abstract Set                              Abstract Map

  Abstract       Array List         Hash Set          Tree Set           Hash Map          Tree Map

Sequential    

   List

Linked List

ArrayList

• Similar to Vector: it encapsulates a dynamically reallocated Object[] array

• Why use an ArrayList instead of a Vector?

• All methods of the Vector class are synchronized, It is safe to access a Vector object from two threads.

• ArrayList methods are not synchronized, use ArrayList in case of no synchronization

• Use get and set methods instead of elementAt and setElementAt methods of vector

HashSet

• Implements a set based on a hashtable

• The default constructor constructs a hashtable with 101 buckets and a load factor of 0.75

HashSet(int initialCapacity)

HashSet(int initialCapacity,float loadFactor)

loadFactor is a measure of how full the hashtable is allowed to get before its capacity is automatically increased

• Use Hashset if you don’t care about the ordering of the elements in the collection

TreeSet

• Similar to hash set, with one added improvement

• A tree set is a sorted collection

• Insert elements into the collection in any order, when it is iterated, the values are automatically presented in sorted order

• Maps :  Two implementations for maps:

HashMap

§  hashes the keys

§  The Elements may not in Order.

§  Hash Map is not synchronized and permits null values

§  Hash Map is not serialized.                                                                                    

§  Hash Map supports Iterators. 

TreeMap

• uses a total ordering on the keys to organize them in a search tree

• The hash or comparison function is applied only to the keys

• The values associated with the keys are not hashed or compared.

How are memory leaks possible in Java

If any object variable is still pointing to some object which is of no use, then JVM will not garbage collect that object  and object will remain in memory creating memory leak

What are the differences between EJB and Java beans

the main difference is Ejb componenets are distributed which means develop once and run anywhere.  java beans are not distributed. which means the beans cannot be shared . 

What would happen if you say this = null

this will give a compilation error as follows

cannot assign value to final variable this

Will there be a performance penalty if you make a method synchronized? If so, can you make any design changes to improve the performance

     yes.the performance will be down if we use synchronization.

one can minimise the penalty by including garbage collection algorithm, which reduces the cost of collecting large numbers of short- lived objects. and also by using Improved thread synchronization for invoking the synchronized methods.the invoking will be faster. 

How would you implement a thread pool

public class ThreadPool extends java.lang.Object implements ThreadPoolInt

 This class is an generic implementation of a thread pool, which takes the following input

a) Size of the pool to be constructed

b) Name of the class which implements Runnable (which has a visible default constructor) 

and constructs a thread pool with active threads that are waiting for activation. once the threads have finished processing they come back and wait once again in the pool.

This thread pool engine can be locked i.e. if some internal operation is performed on the pool then it is preferable that the thread engine be locked. Locking ensures that no new threads are issued by the engine. However, the currently executing threads are allowed to continue till they come back to the passivePool

How does serialization work

Its like FIFO method (first in first out) 

How does garbage collection work

      There are several basic strategies for garbage collection: reference counting, mark-sweep, mark-compact, and copying. In addition, some algorithms can do their job incrementally (the entire heap need not be collected at once, resulting in shorter collection pauses), and some can run while the user program runs (concurrent collectors). Others must perform an entire collection at once while the user program is suspended (so-called stop-the-world collectors). Finally, there are hybrid collectors, such as the generational collector employed by the 1.2 and later JDKs, which use different collection algorithms on different areas of the heap

 How would you pass a java integer by reference to another function

Passing by reference is impossible in JAVA but Java support the object reference so.

Object is the only way to pass the integer by refrence.

What is the sweep and paint algorithm 

The painting algorithm takes as input a source image and a list of brush sizes. sweep algo is that it computes the arrangement of n lines in the plane ... a correct algorithm,

Can a method be static and synchronized

no a static mettod can't be synchronised

Do multiple inheritance in Java

Its not possible directly. That means this feature is not provided by Java, but it can be achieved with the help of Interface. By implementing more than one interface.

What is data encapsulation? What does it buy you

      The most common example I can think of is a javabean. Encapsulation may be used by creating 'get' and 'set' methods in a class which are used to access the fields of the object. Typically the fields are made private while the get and set methods are public.

        dEncapsulation can be used to validate the data that is to be stored, to do calculations on data that is stored in a field or fields, or for use in introspection (often the case when using javabeans in Struts, for instance).

What is reflection API? How are they implemented.

   Reflection package is used mainlyfor the purpose of getting the class name. by using the getName method we can get name of the class for particular application .

         Reflection is a feature of the Java programming language. It allows an executing Java program to examine or "introspect" upon itself, and manipulate internal properties of the program.

What are the primitive types in Java

According to Java in a Nutshell, 5th ed

boolean, byte, char, short, long float, double, int

Is there a separate stack for each thread in Java

No

What is heap in Java

       JAVA is fully Object oriented language. It has two phases first one is Compilation phase and second one is interpratation phase. The Compilation phase convert the java file to class file (byte code is only readable format of JVM) than Intepratation phase interorate the class file line by line and give the proper result.

In Java, how are objects / values passed around

In Java Object are passed by reference and Primitive data is always pass by value

Do primitive types have a class representation

Primitive data type has a wrapper class to present.

Like for int - Integer  , for byte Byte, for long Long etc ... 

How all can you free memory

With the help of finalize() method.

If a programmer really wants to explicitly request a garbage collection at some point, System.gc() or Runtime.gc() can be invoked, which will fire off a garbage collection at that time. 

Does java do reference counting

    It is more likely that the JVMs you encounter in the real world will use a tracing algorithm in their garbage-collected heaps

What does a static inner class mean? How is it different from any other static member

A static inner class behaves like any ``outer'' class. It may contain methods and fields.

         It is not necessarily the case that an instance of the outer class exists even when we have created an instance of the inner class. Similarly, instantiating the outer class does not create any instances of the inner class. 

         The methods of a static inner class may access all the members (fields or methods) of the inner class but they can access only static members (fields or methods) of the outer class. Thus, f can access the field x, but it cannot access the field y. 

How do you declare constant values in java?

Using Final keyword we can declare the constant values  How all can you instantiate final members  Final member can be instantiate only at the time of declaration. null

How is serialization implemented in Java?

      A particular class has to implement an Interface java.io.Serializable for implementing serialization.  When you have an object passed to a method and when the object is reassigned to a different one, then is the original reference lost  No Reference is not lost. Java always passes the object by reference, now two references is pointing to the same object.

What are the different kinds of exceptions? How do you catch a Runtime exception.

There are 2 types of exceptions.

1. Checked exception

2. Unchecked exception.

   Checked exception is catched at the compile time while unchecked exception is checked at run time.

(1).Checked Exceptions : Environmental error that cannot necessarily be detected by testing; e.g. disk full, broken socket, database unavailable, etc. 

(2). Unchecked exception.

Errors: Virtual machine error: class not found, out of memory, no such method, illegal access to private field, etc. 

Runtime Exceptions :Programming errors that should be detected in testing: index out of bounds, null pointer, illegal argument, etc. 

Checked exceptions must be handled at compile time. Runtime exceptions do not need to be. Errors often cannot be

What are the differences between JIT and HotSpot?

      The Hotspot VM is a collection of techniques, the most significant of which is called "adaptive optimization”.

The original JVMs interpreted bytecodes one at a time. Second-generation JVMs added a JIT compiler, which compiles each method to native code upon first execution, then executes the native code. Thereafter, whenever the method is called, the native code is executed. The adaptive optimization technique used by Hotspot is a hybrid approach, one that combines bytecode interpretation and run-time compilation to native code. 

Hotspot, unlike a regular JIT compiling VM, doesn't do "premature optimization"

What is a memory footprint? How can you specify the lower and upper limits of the RAM used by the JVM? What happens when the JVM needs more memory?

    when JVM needs more memory then it does the garbage collection, and sweeps all the memory which is not being used.

What are the disadvantages of reference counting in garbage collection?

      An advantage of this scheme is that it can run in small chunks of time closely interwoven with the execution of the program. This characteristic makes it particularly suitable for real-time environments where the program can't be interrupted for very long. A disadvantage of reference counting is that it does not detect cycles. A cycle is two or more objects that refer to one another, for example, a parent object that has a reference to its child object, which has a reference back to its parent. These objects will never have a reference count of zero even though they may be unreachable by the roots of the executing program. Another disadvantage is the overhead of incrementing and decrementing the reference count each time. Because of these disadvantages, reference counting currently is out of favor.

Is it advisable to depend on finalize for all cleanups

     The purpose of finalization is to give an opportunity to an unreachable object to perform any clean up before the object is garbage collected, and it is advisable.

can we declare multiple main() methods in multiple classes. ie can we have each main method in its class in our program?

YES                                                         JDBC

How to Interact with DB?

           Generally every DB vendor provides a User Interface through which we can easily execute SQL query’s and get the result (For example Oracle Query Manager for Oracle, and TOAD (www.quest.com) tool common to all the databases). And these tools will help DB developers to create database. But as a programmer we want to interact with the DB dynamically to execute some SQL queries from our application (Any application like C, C++, JAVA etc), and for this requirement DB vendors provide some Native Libraries (Vendor Specific) using this we can interact with the DB i.e. If you want to execute some queries on Oracle DB, oracle vendor provides an OCI (Oracle Call Interface) Libraries to perform the same.

About ODBC

What is ODBC

     ODBC (Open Database Connectivity) is an ISV (Independent software vendor product) composes of native API to connect to different databases through via a single API called ODBC.

Open Database Connectivity (ODBC) is an SQL oriented application programming interface developed by in collaboration with IBM and some other database vendors.

ODBC comes with Microsoft products and with all databases on Windows OS.

ODBC Architecture

“C” function calls

Front End Application

Oracle ODBC SQL server ODBC Sybase ODBC

Oracle

ODBC API

SP API

SQL server

Sybase

SP API

SP API

Oracle DSN My DSN SQL Server DSN Sybase DSN Our DSN

Oracle ODBC SQL server ODBC Sybase ODBC

Oracle

SQL Server

Sybase

Advantages

·         Single API (Protocol) is used to interact with any DB

·         Switching from one DB to another is easy

·         Doesn’t require any modifications in the Application when you want to shift from one DB to other.

What for JDBC?

        As we have studied about ODBC and is advantages and came to know that it provides a common API to interact with any DB which has an ODBC Service Provider’s Implementation written in Native API that can be used in your applications.

   If an application wants to interact with the DB then the options which have been explained up to now in this book are:

1.     Using Native Libraries given by the DB vendor

2.     Using ODBC API

And we have listed there Advantages and Disadvantages.

   But if the application is a JAVA application then the above given options are not recommended to be used due to the following reasons

1.     Native Libraries given by DB vendor

a.    Application becomes vendor dependent and

b.    The application has to use JNI to interact with Native Lib which may cause serious problem for Platform Independency in our applications.

2.     And the second option given was using ODBC API which can solve the 1.a problem but again this ODBC API is also a Native API, so we have to use JNI in our Java applications which lead to the 1.b described problem.

And the answer for these problems is JDBC (Java Data Base Connectivity) which provides a common Java API to interact with any DB.

What is JDBC

       As explained above JDBC standards for Java Data Base Connectivity. It is a specification given by Sun Microsystems and standards followed by X/Open SAG (SQL Access Group) CLI (Call Level Interface) to interact with the DB.

Java programing language methods. The JDBC API provides database-independent connectivity between the JAVA Applications and a wide range of tabular data bases. JDBC technology allows an application component provider to:

·         Perform connection and authentication to a database server

·         Manage transactions

·         Moves SQL statements to a database engine for preprocessing and execution

·         Executes stored procedures

·         Inspects and modifies the results from SELECT statements

JDBC API

JDBC API is divided into two parts

1.     JDBC Core API

2.     JDBC Extension or Optional API

JDBC Core API (java.sql package)

This part of API deals with the following futures

1.    Establish a connection to a DB

2.    Getting DB Details

3.    Getting Driver Details

4.    maintaining Local Transaction

5.    executing query’s

6.    getting result’s (ResultSet)

7.    preparing pre-compiled SQL query’s and executing

8.    executing procedures & functions

JDBC Ext OR Optional API (javax.sql package)

      This part of API deals with the following futures

1.    Resource Objects with Distributed Transaction Management support

2.    Connection Pooling.

These two parts of Specification are the part of J2SE and are inherited into J2EE i.e. this specification API can be used with all the component’s given under J2SE and J2EE.

JDBC Application

JDBC Driver

Oracle DB

MS SQL Server DB

Sybase DB

JDBC API

SPAPI

SPAPI

SP API

JDBC Architecture:

                  In the above show archetecture diagram the JDBC Driver forms an abstraction layer between the JAVA Application and DB, and is implemented by 3^rd party vendors or a DB Vendor. But whoever may be the vendor and what ever may be the DB we need not to worry will just us JDCB API to give instructions to JDBC Driver and then it’s the responsibility of JDBC Driver Provider to convert the JDBC Call to the DB Specific Call.

And this 3^rd party vendor or DB vendor implemented Drivers are classified into 4-Types namely

Types Of Drivers :

1.     Type-1 (JDBC ODBC-Bridge Driver) JDBC-ODBC Bridge Driver

2.     Type-2 (Java-Native API Driver)     Native API Partly JAVA Driver (Thick Driver)

3.     Type-3 (Java Net Protocol Driver)    Intermediate DataBase Access Server

4.     Type-4 (Java Native Protocol driver) Pure JAVA Driver (Thin driver)

Type-1 : JDBC-ODBC Bridge Driver :

                              Since ODBC is written in C-language using pointers, so JAVA does’t support pointers, a java program can’t communate directly with the DataBase.  The JDBC-ODBC bridge drivertransulates JDBC API calls to ODBC API calls.

Architecture

JAVA Application

JDBC ODBC Driver

Native ODBC Client driver Libraries

DBMS Interface client libraries

DBMS Interface Server Libraries

DBMS

This type of Driver is designed to convert the JDBC request call to ODBC call and ODBC response call to JDBC call.

        The JDBC uses this interface in order to communicate with the database, so neither the database nor the middle tier need to be Java compliant. However ODBC binary code must be installed on each client machine that uses this driver. This bridge driver uses a configured data source.

Advantages

• Simple to use because ODBC drivers comes with DB installation/Microsoft front/back office product installation
• JDBC ODBC Drivers comes with JDK software

Disadvantages

·         More number of layers between the application and DB. And more number of API conversions leads to the downfall of the performance.

·         Slower than type-2 driver

Where to use?

   This type of drivers are generaly used at the development time to test your application’s.

Because of the disadvantages listed above it is not used at production time. But if we are not available with any other type of driver implementations for a DB then we are forced to use this type of driver (for example Microsoft Access).

Examples of this type of drivers

JdbcOdbcDriver from sun

Sun’s JdbcOdbcDriver is one of type-1 drivers and comes along with sun j2sdk (JDK).

Setting environment to use this driver

1.     Software
ODBC libraries has to be installed.

2.     classpath 
No additional classpath settings are required apart from the runtime jar (c:\j2sdk1.4\jre\lib\rt.jar) which is defaultly configured.

3.     Path
No additional path configuration is required.

How to use this driver

1.     Driver class name  à   sun.jdbc.odbc.JdbcOdbcDriver

2.     Driver URL   à dbc:odbc:<DSN>
here <DSN> (Data Source Name) is an ODBC datasource name which is used by ODBC driver to locate one of the ODBC Service Provider implementation API which can in-turn connect to DB.

Steps to create <DSN>

1.     run ‘Data Sources (ODBC)’ from Control Panal\Administrative Tools\
(for Windows 2000 server/2000 professional/XP)
run ‘ODBC Data Sources’ from Control Panel\

2. click on Add button available on the above displayed screen. this opens a new window titled ‘Create New Data Source’ which displays all the available DB’s lable DB’s ODBC drivers currently installed on your system.

3. Select the suitable driver and click on Finish

4. Give the required info to the driver (like username, service id etc)