Introduction to Gluster File System Install Configure Gluster HA

GlusterFS is a scalable network file system. Using common off-the-shelf hardware, you can create large, distributed storage solutions for media streaming, data analysis, and other data- and bandwidth-intensive tasks. GlusterFS is free and open source software. In this Article we are going to see Introduction Gluster File System Install Configure Gluster HA.

Why GlusterFS – Introduction to Gluster File System

• GlusterFS is a distributed file system defined to be used in user space that can scale out in building-block fashion to store multiple petabytes of data under a single mount point.
• It is a software based file system which accounts to its own flexibility feature.
• It uses already available disk file systems like ext3, ext4, xfs etc to store data and client will able to access the storage as a local file system.
• GlusterFS cluster aggregates storage blocks over Infiniband
• RDMA (Remote Direct Memory Access) and/or TCP/IP interconnect in a single global namespace. Introduction Gluster File System Install Configure Gluster HA

Advantages of GlusterFS

• Gluster File system uses replication to survive hardware failures and automatically performs self-healing to restore performance. Aggregates on top of existing file systems.
• GlusterFS has no single point of failure. Completely distributed. No centralised meta-data server like Lustre.
• Extensible scheduling interface with modules loaded based on user’s storage I/O access pattern.
• Supports Infiniband RDMA and TCP/IP. Introduction Gluster File System Install Configure Gluster HA
• Entirely implemented in user-space. Easy to port, debug and maintain. Scales on demand. Too easy to deploy and maintain

Targeted Audience

Anyone who is having basic knowledge in Linux/Unix and ability to understand the file system concepts.

How to install and Configure Gluster in HA

Before implementing the Gluster setup, let’s go through the storage concepts in Gluster.

• Brick – Brick is nothing but a directory which can be shared among the trusted storage pool.
• Trusted Storage Pool – Collection of shared files or directories Block Storage – Devices through which the data is being moved across systems in form of blocks
• Cluster – Collaboration of storage servers based on a defined protocol
• Distributed File System – A file system in which data is spread across multiple nodes where users can access the files without knowing the actual location of the server
• FUSE – Loadable kernel module which allows users to create a file system without involving any of the kernel code
• Glusterd – GlusterFS management daemon which is the backbone of the file system which will be running all the time
• Volume – A logical collection of bricks Introduction Gluster File System Install Configure Gluster HA

Ports required

• 24007 TCP for the Gluster Daemon
• 24008 TCP for Infiniband management (optional unless you are using IB (InfiniBand) )
• One TCP port for each brick in a volume. So, for example, if you have 4 bricks in a volume, port 24009 – 24012 would be used in GlusterFS 3.3 & below, 49152 – 49155 from GlusterFS 3.4 & later.

Note: by default Gluster/NFS does not provide services over UDP, it is TCP only. You would need to enable the nfs.mount-udp option if you want to add UDP support for the MOUNT protocol. That’s completely optional and is up to your requirement to use.

Installation and Configuration Gluster HA

Now let us start installing and configuring the GlusterFS. Take two Centos 7 servers and name them as per your choice say Glusterfs1 and Glusterfs2.

[root@ip-172-31-31-246 ~]# cat /etc/os-release 
NAME=”CentOS Linux” 
VERSION=”7 (Core)” ID=”centos” 
ID_LIKE=”rhel fedora” 
VERSION_ID=”7″ 
PRETTY_NAME=”CentOS Linux 7 (Core)” 
ANSI_COLOR=”0;31″ 
CPE_NAME=”cpe:/o:centos:centos:7″ 
HOME_URL=”https://www.centos.org/” 
BUG_REPORT_URL=”https://bugs.centos.org/” 

[root@ip-172-31-31-247 ~]# cat /etc/os-release 
NAME=”CentOS Linux” 
VERSION=”7 (Core)” 
ID=”centos” 
ID_LIKE=”rhel fedora” 
VERSION_ID=”7″ 
PRETTY_NAME=”CentOS Linux 7 (Core)” 
ANSI_COLOR=”0;31″ 
CPE_NAME=”cpe:/o:centos:centos:7″ 
HOME_URL=”https://www.centos.org/” 
BUG_REPORT_URL=”https://bugs.centos.org/”

Note: Make Sure that SELinux and firewall are not blocking the above ports between the two servers.

In /etc/hosts create an entry of like below in both the servers.  Make sure that you are able to reach both the servers in both servers.

Glusterfs1

[root@ip-172-31-31-246 ~]# cat /etc/hosts
 127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
 ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
 172.31.31.246 glusterfs1
 172.31.31.247 glusterfs2

[root@ip-172-31-31-246 ~]# ping -c 2 glusterfs1
 PING glusterfs1 (172.31.31.246) 56(84) bytes of data.
 64 bytes from glusterfs1 (172.31.31.246): icmp_seq=1 ttl=64 time=0.016 ms

[root@ip-172-31-31-246 ~]# ping -c 2 glusterfs2
 PING glusterfs2 (172.31.31.247) 56(84) bytes of data.
 64 bytes from glusterfs2 (172.31.31.247): icmp_seq=1 ttl=64 time=0.992 ms

Glusterfs2

[root@ip-172-31-31-247 ~]# cat /etc/hosts
 127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
 172.31.31.247 glusterfs2
 172.31.31.246 glusterfs1
 [root@ip-172-31-31-247 ~]# ping -c 2 glusterfs2
 PING glusterfs2 (172.31.31.247) 56(84) bytes of data.
 64 bytes from glusterfs2 (172.31.31.247): icmp_seq=1 ttl=64 time=0.016 ms
 64 bytes from glusterfs2 (172.31.31.247): icmp_seq=2 ttl=64 time=0.023 ms

[root@ip-172-31-31-247 ~]# ping -c 2 glusterfs1
 PING glusterfs1 (172.31.31.246) 56(84) bytes of data.
 64 bytes from glusterfs1 (172.31.31.246): icmp_seq=1 ttl=64 time=1.36 ms
 64 bytes from glusterfs1 (172.31.31.246): icmp_seq=2 ttl=64 time=0.619 ms

Enable the Gluster Repo in both the servers. Create a repo file and add the contents as shown below.

[root@ip-172-31-31-247 ~]# cat /etc/yum.repos.d/gluster.repo
 [glusterfs]
 name=GlusterFS – Distributed File System
 baseurl=http://buildlogs.centos.org/centos/7/storage/x86_64/gluster-3.7/
 enabled=1
 skip_if_unavailable=1
 gpgcheck=0
[root@ip-172-31-31-247 ~]# yum clean all
[root@ip-172-31-31-246 ~]# cat /etc/yum.repos.d/gluster.repo
 [glusterfs]
 name=GlusterFS – Distributed File System
 baseurl=http://buildlogs.centos.org/centos/7/storage/x86_64/gluster-3.7/
 enabled=1
 skip_if_unavailable=1
 gpgcheck=0

[root@ip-172-31-31-246 ~]# yum clean all

Install Glusterfs server and start the service. Introduction Gluster File System Install Configure Gluster HA

Install the gluster server on both the servers Introduction to Gluster File System Install Configure Gluster HA

[root@ip-172-31-31-246 ~]# yum install glusterfs-server -y

[root@ip-172-31-31-246 ~]# service glusterd start
 Redirecting to /bin/systemctl start glusterd.service

[root@ip-172-31-31-246 ~]# service glusterd status
 Redirecting to /bin/systemctl status glusterd.service
 ● glusterd.service – GlusterFS, a clustered file-system server
 Loaded: loaded (/usr/lib/systemd/system/glusterd.service;disabled; vendor preset: disabled)
 Active: active (running) since Fri 2016-09-30 07:10:44 UTC; 29min ago
 Process: 11956 ExecStart=/usr/sbin/glusterd -p /var/run/glusterd.pid –log-level $LOG_LEVEL $GLUSTERD_OPTIONS (code=exited, status=0/SUCCESS)
 Main PID: 11957 (glusterd)
 CGroup: /system.slice/glusterd.service
 └─11957 /usr/sbin/glusterd -p /var/run/glusterd.pid –loglevel INFO

 [root@ip-172-31-31-246 ~]# chkconfig glusterd on
 Note: Forwarding request to ‘systemctl enable glusterd.service’.
 Created symlink from /etc/systemd/system/multi-user.target.wants/glusterd.service to /usr/lib/systemd/system/glusterd.service.

[root@ip-172-31-31-247 ~]# yum install glusterfs-server -y

[root@ip-172-31-31-247 ~]# service glusterd start
 Redirecting to /bin/systemctl start glusterd.service

[root@ip-172-31-31-247 ~]# service glusterd status
 Redirecting to /bin/systemctl status glusterd.service
 ● glusterd.service – GlusterFS, a clustered file-system server
 Loaded: loaded (/usr/lib/systemd/system/glusterd.service; disabled; vendor preset: disabled)
 Active: active (running) since Fri 2016-09-30 07:10:44 UTC; 29min ago
 Process: 11956 ExecStart=/usr/sbin/glusterd -p /var/run/glusterd.pid –log-level $LOG_LEVEL $GLUSTERD_OPTIONS (code =exited, status=0/SUCCESS) Main PID: 11957 (glusterd)
 CGroup: /system.slice/glusterd.service
 └─11957 /usr/sbin/glusterd -p /var/run/glusterd.pid –loglevel INFO
 
[root@ip-172-31-31-247 ~]# chkconfig glusterd on
 Note: Forwarding request to ‘systemctl enable glusterd.service’.
 Created symlink from /etc/systemd/system/multi-user.target.wants/glusterd.service to /usr/lib/systemd/system/glusterd.service.

Now that installation has been completed. Let’s create the trusted storage pool.

From glusterfs1 Host

[root@ip-172-31-31-246 ~]# gluster peer probe glusterfs2
 peer probe: success.

From Glusterfs2 Host

[root@ip-172-31-31-247 ~]# gluster peer probe glusterfs1
 peer probe: success. Host glusterfs1 port 24007 already in peer list

Now verify whether both the servers are in the pool or not with the below command, from any of the two servers.

[root@ip-172-31-31-246 ~]# gluster pool list
 UUID Hostname State 2d7c96c0-31d3-48bc-8d61-a9c97d01f07d glusterfs2 Connected
 af8c77f4-f10a-4b28-915e-ae7bea2af4c6 localhost Connected

From the above, it is clear that both the servers are added to Trusted storage Pool.

To add extra servers to the pool list follow the same process as mentioned above.

Attach a new disk to each server and mount on any directory of your choice. We will use this mounted directory as a brick while creating the volumes.

Now we have the GlusterFS server is up and running, let’s create volumes and start accessing the data.

I have two new disks attached to both the servers, which will be used as a brick.

[root@ip-172-31-31-246 ~]# df -h | grep brick
 /dev/xvdf 976M 2.6M 907M 1% /mnt/brick1

[root@ip-172-31-31-247 ~]# df -h | grep brick
 /dev/xvdf 976M 2.6M 907M 1% /mnt/brick2

From any of the Gluster server run the following command to create a Gluster volume.

[root@ip-172-31-31-246 ~]# gluster volume create gvol glusterfs1:/mnt/brcik1 glusterfs2:/mnt/brick2 force
 volume create: gvol: success: please start the volume to access data

Gluster volume has been created successfully. Now let us start the Gluster volume.

[root@ip-172-31-31-246 ~]# gluster volume start gvol
 volume start: gvol: success

Gluster volume has been started successfully and now it is ready to serve the data. Before mounting this in client side, let us first check the gluster volume information.

[root@ip-172-31-31-246 ~]# gluster volume info gvol
 Volume Name: gvol
 Type: Distribute
 Volume ID: 74b01cf1-b1ac-4524-b2ad-ca465c02a888
 Status: Started
 Number of Bricks: 2
 Transport-type: tcp
 Bricks:
 Brick1: glusterfs1:/mnt/brcik1
 Brick2: glusterfs2:/mnt/brick2
 Options Reconfigured: performance.readdir-ahead: on

Mount the Gluster in clients (Windows and Linux)

Linux

Let us mount the gluster volume gvol in client machines to access the data. Add the Gluster servers IP addresses to /etc/hosts in the client machine.

[root@ip-172-31-18-13 ~]# cat /etc/hosts
 127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
 ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
 172.31.31.247 glusterfs2
 172.31.31.246 glusterfs1

Enable the Gluster Repo as we did on the servers. Create a repo file and add the contents as shown below.

[root@ip-172-31-18-13 ~]# cat /etc/yum.repos.d/gluster.repo
 [glusterfs]
 name=GlusterFS – Distributed File System
 baseurl=http://buildlogs.centos.org/centos/7/storage/x86_64/gluster-3.7/
 enabled=1
 skip_if_unavailable=1
 gpgcheck=0

[root@ip-172-31-18-13 ~]# yum clean all

Install the client packages with the following command

[root@ip-172-31-18-13 yum.repos.d]# yum install glusterfs glusterfs-fuse

Create a directory.

[root@ip-172-31-18-13 ~]# mkdir /mnt/glusterclient1

Mount it with the following command.

[root@ip-172-31-18-13 ~]# mount.glusterfs glusterfs1:/gvol /mnt/glusterclient1/

If all goes well, you will be able to see the gluster volume mounted successfully.

[root@ip-172-31-18-13 ~]# df -h
 Filesystem Size Used Avail Use% Mounted on
 /dev/xvda1 8.0G 1.1G 7.0G 14% /
 devtmpfs 478M 0 478M 0% /dev
 tmpfs 496M 0 496M 0% /dev/shm
 tmpfs 496M 13M 484M 3% /run
 tmpfs 496M 0 496M 0% /sys/fs/cgroup
 tmpfs 100M 0 100M 0% /run/user/1000
 /dev/xvdf 976M 2.7M 907M 1% /mnt/brick2
 glusterfs1:/gvol 9.0G 1.1G 7.9G 12% /mnt/glusterclient1

Create files from client and you will be able to see the files in the server

Mount GlusterFS in Windows

For windows machines we need to setup Samba configuration to export the mount point of the Gluster volume. Example, if a Gluster volume is mounted on /mnt/glusterclient1, you must edit smb.conf file to enable exporting this through CIFS.

Open smb.conf file in an editor and add the following lines for a simple configuration:

[gvol]
 comment = Gluster Stuff
 path = /mnt/gluster
 public = yes
 writable = yes
 printable = no
 Restart the samba service.
 service smb restart

Go to the windows machine and follow the steps below.

• Open “Computer” then “Network”
• Then search for the storage network \glusterserverip
• The CIFS exported volume will appear.
• Double click on it.
• Enter username and password specified during smb user creation and then connect and access the share.

 Gluster file system supports different types of volumes based on the requirements. Some volumes are good for scaling storage size, some for improving performance and some for both which are explained below. 

source:-https://arkit.co.in Introduction Gluster File System Install Configure Gluster HA

Hibernate 4.3 Introduction Tutorial

An introduction to Hibernate 4

In this article I will show you how can you use Hibernate 4,  along with a simple example application available to try out what you read about.

What is Hibernate and why should I care?

Hibernate was created to leverage the connection between Java applications and relational databases because it is hard to map back and forth between a database table and a Java object.

And because Hibernate does this, it reduces development time which is consumed by JDBC query-execution and data mapping.

Getting Hibernate

To get the latest version of Hibernate, just visit this site. For this article I will use the version 4.3.10.Final.

If you download and extract the package you can see some sub-folders in the lib folder. Everything under required is required for any project using Hibernate. The other folders contain libraries for special cases. For example under jpa you find the library providing the JPA entity manager support.

Alternatively you can set up a Maven project and add Hibernate as a dependency. In this case you do not need to take care about the other required dependencies of Hibernate, which come along in the required package with the bundle-download. Using Maven is more simple and straightforward so I will use Maven as dependency management.

A simple example

In the simple example I will create a Java application which stores information about books in the database. The database will be an H2 memory database for the sake of simplicity.

Dependencies are managed with Maven and the output is an executable JAR with all dependencies.

The entity

The entity I will store in the database is the following:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

package example; public class Book {    private String isbn;    private String title;    private String author;    Book() {    }    public Book(String isbn, String title, String author) {        this.isbn = isbn;        this.title = title;        this.author = author;    }    public String getIsbn() {        return this.isbn;    }    public String getTitle() {        return this.title;    }    public void setTitle(String title) {        this.title = title;    }    public String getAuthor() {        return this.author;    }    public void setAuthor(String author) {        this.author = author;    }    public void setIsbn(String isbn) {        this.isbn = isbn;    } }

The no-argument constructor is a requirement for all persistent classes because Hibernate creates the object instances per reflection. In this case this constructor is private to prevent creation of books without information.

The dependencies

To get the app running, we need two dependencies in the project: Hibernate and H2. To do this, add the following to the pom.xml:

1 2 3 4 5 6 7 8 9 10 11 12

<dependencies> <dependency>    <groupId>org.hibernate</groupId>    <artifactId>hibernate-core</artifactId>    <version>4.3.10.Final</version> </dependency> <dependency>    <groupId>com.h2database</groupId>    <artifactId>h2</artifactId>    <version>1.4.187</version> </dependency> </dependencies>

Now we are ready to continue on.

Configuring Hibernate

Hibernate needs some configuration to get started. This you need to include in the hibernate.cfg.xml file. It is plain old XML. It contains the database connection properties and the entity mapping files inclusive location.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

<!DOCTYPE hibernate-configuration PUBLIC        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"> <hibernate-configuration>    <session-factory>        <!-- Database connection settings -->        <property name="connection.driver_class">org.h2.Driver</property>        <property name="connection.url">jdbc:h2:mem:db1;DB_CLOSE_DELAY=-1;MVCC=TRUE</property>        <property name="connection.username">sa</property>        <property name="connection.password"/>        <!-- JDBC connection pool (use the built-in) -->        <property name="connection.pool_size">1</property>        <!-- SQL dialect -->        <property name="dialect">org.hibernate.dialect.H2Dialect</property>        <!-- Disable the second-level cache -->        <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>        <!-- Echo all executed SQL to stdout -->        <property name="show_sql">true</property>        <!-- Drop and re-create the database schema on startup -->        <property name="hbm2ddl.auto">create</property>        <mapping resource="hibernate_example/hbm/Book.hbm.xml"/>    </session-factory> </hibernate-configuration>

The entity mapping

To map the right fields to the right column in the database, Hibernate requires a mapping file for the entities. These are located in the .hbm.xml files which start with the entity’s name. In this example Book.hbm.xml.

1 2 3 4 5 6 7 8 9 10 11 12 13

<!DOCTYPE hibernate-mapping PUBLIC      "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="hibernate_example">    <class name="Book" table="BOOKS">        <id name="isbn" type="string"/>        <property name="title"/>        <property name="author"/>    </class> </hibernate-mapping>

The main method

To use the application with Hibernate we still need an entry point — and in Java this is the main method. To start, we need some configuration, like creating a session with a session factory… So let’s look at the code how it goes:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

public class Main {    public static void main(String[] args) {        final Configuration configuration = new Configuration().configure();        final StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties());        final SessionFactory factory = configuration.buildSessionFactory(builder.build());        final Session session = factory.openSession();        final Book book = new Book("93939398948 ", "Java 8", "Author");        session.beginTransaction();        session.save(book);        session.getTransaction().commit();        final List<Book> books = session.createCriteria(Book.class).list();        System.out.println("\n----\n");        System.out.println(MessageFormat.format("Storing {0} books in the database", books.size()));        for (final Book b : books) {            System.out.println(b);        }        System.out.println("\n----\n");        session.close();        factory.close();    } }

After running the application the console should have some log messages and the one book added to the database:
—-

1 2	Storing 1 books in the database Java 8 by JBT (ISBN:9393939894

—–

Conclusion

Hibernate gives a nice feature to leverage mapping between Java objects and relational databases. Naturally this sample application does not show the full power of Hibernate: for better user experience you could add a user interface to create and list books in the application.

In a later article I will show how to get rid of the XML configuration (called by some developers as the “XML-Hell”) and use annotations instead. So stay tuned.

Source:javabeginnerstutorial.co

OOPS Interview Questions Part 2

31.What is Constructor?

• A constructor is a special method whose task is to initialize the object of its class.
• It is special because its name is the same as the class name.
• They do not have return types, not even void and therefore they cannot return values.
• They cannot be inherited, though a derived class can call the base class constructor.
• Constructor is invoked whenever an object of its associated class is created.

32.How does the Java default constructor be provided?

If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

33.Can constructor be inherited?

No, constructor cannot be inherited, though a derived class can call the base class constructor.

34.What are the differences between Contructors and Methods?

	Constructors	Methods
Purpose	Create an instance of a class	Group Java statements
Modifiers	Cannot be abstract, final, native, static, or synchronized	Can be abstract, final, native, static, or synchronized
Return Type	No return type, not even void	void or a valid return type
Name	Same name as the class (first letter is capitalized by convention) -- usually a noun	Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action
this	Refers to another constructor in the same class. If used, it must be the first line of the constructor	Refers to an instance of the owning class. Cannot be used by static methods.
super	Calls the constructor of the parent class. If used, must be the first line of the constructor	Calls an overridden method in the parent class
Inheritance	Constructors are not inherited	Methods are inherited

35.How are this() and super() used with constructors?

• Constructors use this to refer to another constructor in the same class with a different parameter list.
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

36.What are the differences between Class Methods and Instance Methods?

Class Methods	Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class	Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword. Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.	Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class.	Instance methods are not declared as static.

37.How are this() and super() used with constructors?

• Constructors use this to refer to another constructor in the same class with a different parameter list.
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

38.What are Access Specifiers?

One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. Java allows you to control access to classes, methods, and fields via so-called access specifiers..

39.What are Access Specifiers available in Java?

Java offers four access specifiers, listed below in decreasing accessibility:

• Public- public classes, methods, and fields can be accessed from everywhere.
• Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.
• Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.
• Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.

Situation	public	protected	default	private
Accessible to class  from same package?	yes	yes	yes	no
Accessible to class  from different package?	yes	no, unless it is a subclass	no	no

40.What is final modifier?

The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.

• final Classes- A final class cannot have subclasses.
• final Variables- A final variable cannot be changed once it is initialized.
• final Methods- A final method cannot be overridden by subclasses.

41.What are the uses of final method?

There are two reasons for marking a method as final:

• Disallowing subclasses to change the meaning of the method.
• Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.

42.What is static block?

Static block which exactly executed exactly once when the class is first loaded into JVM. Before going to the main method the static block will execute.

43.What are static variables?

Variables that have only one copy per class are known as static variables. They are not attached to a particular instance of a class but rather belong to a class as a whole. They are declared by using the static keyword as a modifier.

   static type  varIdentifier;

where, the name of the variable is varIdentifier and its data type is specified by type.
Note: Static variables that are not explicitly initialized in the code are automatically initialized with a default value. The default value depends on the data type of the variables.

44.What is the difference between static and non-static variables?

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

45.What are static methods?

Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:

• A static method can only call other static methods.
• A static method must only access static data.
• A static method cannot reference to the current object using keywords super or this.

Source:-http://www.developersbook.com/