Arrays in Java

Arrays in Java

An array is a data structure that defines an indexed collection of a fixed number of homogeneous data elements. All elements in the array have the same data type. A position in the array is indicated by a non-negative integer value called the index. An element at a given position in the array is accessed using the index. The size of an array is fixed and cannot increase to accommodate more elements.

• In Java, arrays are objects. Arrays can be of primitive data types or reference types.
• In the first case, all elements in the array are of a specific primitive data type. In the second case, all elements are references of a specific reference type. 
• Each array object has a final field called length, which specifies the array size, that is, the number of elements the array can accommodate. The first element is always at index 0 and the last element at index n-1, where n is the value of the length field in the array.
• Simple arrays are one-dimensional arrays.
• An array variable declaration has either the following syntax:<element type>[] <array name>;
  Or
  <element type> <array name>[];
  where <element type> can be a primitive data type or a reference type .

The declaration does not actually create an array. It only declares a reference that can denote an array object.

• Constructing an Array: An array can be constructed for a specific number of elements of the element type, using the new operator. The resulting array reference can be assigned to an array variable of the corresponding type.
  <array name> = new <element type> [<array size>];
• The array declaration and construction can be combined. <element type1>[] <array name> = new <element type2>[<array size>];
• Initializing an Array: Java provides the means of declaring, constructing, and explicitly initializing an array in one declaration statement:
  <element type>[] <array name> = { <array initialize list> };
• Using an Array: The whole array is referenced by the array name, but individual array elements are accessed by specifying an index with the [] operator. The array element access expression has the following syntax:
  <array name> [<index expression>]
• Anonymous Arrays:
  <element type>[] <array name> = new <element type>[] { <array initialize list> };
  can be used to declare an array for example
  int[] myArray = new int[] {1, 4, 6, 8}; 
• Multidimensional Arrays: An array element can be an object reference and arrays are objects, array elements can themselves reference other arrays. In Java, an array of arrays can be defined as follows:
  <element type>[][]...[] <array name>;
  or <element type> <array name>[][]...[];

Java Source File Structure

Java Source File Structure

A Java source file can have the following elements that, if present, must be specified in the following order

• An optional package declaration to specify a package name.
• Zero or more import declarations. Since import declarations introduce class and interface names in the source code, they must be placed before any type declarations.
• Any number of top-level class and interface declarations. Since these declarations belong to the same package, they are said to be defined at the top level, which is the package level.
• The classes and interfaces can be defined in any order. Class and interface declarations are collectively known as type declarations. Technically, a source file need not have any such definitions, but that is hardly useful.

The Java 2 SDK imposes the restriction that at the most one public class definition per source file can be defined. If a public class is defined, the file name must match this public class. If the public class name is MyApp, then the file name must be MyApp.java.

The main() Method 

• The Java interpreter executes a method called main in the class specified on the command line.
• Any class can have a main() method, but only the main() method of the class specified to the Java interpreter is executed to start a Java application.
• The main() method must have public accessibility so that the interpreter can call it.
• It is a static method belonging to the class, so that no object of the class is required to start the execution.
• It does not return a value, that is, it is declared void.
• It always has an array of String objects as its only formal parameter. This array contains any arguments passed to the program on the command line.
• All this adds up to the definition of the main() method:

public static void main(String[] args) { // ... }

Java Language Fundamentals

Java Language Fundamentals

Identifiers

• A name in a program is called an identifier.
• Identifiers can be used to denote classes, methods, variables, and labels.
• In Java an identifier is composed of a sequence of characters, where each character can be either a letter, a digit, a connecting punctuation (such as underscore _), or any currency symbol (such as $, ¢, ¥, or £). However, the first character in an identifier cannot be a digit.
• Identifiers in Java are case sensitive, for example, price and Price are two different identifiers.

Examples of Legal Identifiers: number, Number, sum_$, bingo, $$_100, mål, grüß 

Examples of Illegal Identifiers: 48chevy, all@hands, grand-sum

Keywords

• Keywords are reserved identifiers that are predefined in the language and cannot be used to denote other entities.
• All the keywords are in lowercase, and incorrect usage results in compilation errors.

Literals

• A literal denotes a constant value, that is, the value a literal represents remains unchanged in the program. Identifiers can be used to denote classes, methods, variables, and labels.
• Literals represent numerical (integer or floating-point), character, boolean or string values. In addition, there is the literal null that represents the null reference.

Examples of literals

• Integer 2000, 0, -7 
• Floating-point 3.14, -3.14, .5, 0.5
• Character 'a‘, 'A‘, '0‘, ':‘, '-‘, ')' 
• Boolean true, false 
• String "abba“, "3.14“, "for“, "a piece of the action“

Comments

A program can be documented by inserting comments at relevant places. These comments are for documentation purposes and are ignored by the compiler.

Java provides three types of comments to document a program:

• A single-line comment: // ... to the end of the line
• A multiple-line comment: /* ... */
• A documentation (Javadoc) comment: /** ... */

Primitive data types

Primitive data types in Java can be divided into three main categories:

• Integral types— represent signed integers (byte, short, int, long) and unsigned character values (char)
• Floating-point types (float, double)— represent fractional signed numbers
• Boolean type (boolean)— represent logical values

Primitive data values are not objects. Each primitive data type defines the range of values in the data type, and operations on these values are defined by special operators in the language.

Variable declaration

A variable stores a value of a particular type. A variable has a name, a type, and a value associated with it. In Java, variables can only store values of primitive data types and references to objects. Variables that store references to objects are called reference variables.

Declaring and Initializing Variables

Variable declarations are used to specify the type and the name of variables. This implicitly determines their memory allocation and the values that can be stored in them.

• char a, b, c; // a, b and c are character variables.
• double area; // area is a floating-point variable.
• boolean flag; // flag is a boolean variable.

A declaration can also include initialization code to specify an appropriate initial value for the variable:

• int i = 10; // i is an int variable with initial value 10.
• long big = 2147483648L; // big is a long variable with specified initial value.

Object Reference Variables, An object reference is a value that denotes an object in Java. Such reference values can be stored in variables and used to manipulate the object denoted by the reference value. Before we can use a reference variable to manipulate an object, it must be declared and initialized with the reference value of the object.

• Pizza yummyPizza; // Variable yummyPizza can reference objects of class Pizza.
• Hamburger bigOne; // Variable bigOne can reference objects of class Hamburger
• Pizza yummyPizza = new Pizza("Hot&Spicy"); // Declaration with initializer.

Basic elements of a Java Program

Basic elements of a Java Program

The basic elements of a Java application are introduced in this post.

Basic elements of a Java Program - Class

A class denotes a category of objects, and acts as a blueprint for creating such objects. A class models an abstraction by defining the properties and behaviors for the objects representing the abstraction. An object exhibits the properties and behaviors defined by its class. The properties of an object of a class are also called attributes, and are defined by fields in Java. A field in a class definition is a variable which can store a value that represents a particular property.  The behaviors of an object of a class are also known as operations, and are defined using methods in Java. Fields and methods in a class definition are collectively called members.

Basic elements of a Java Program - Object

The process of creating objects from a class is called instantiation. An object is an instance of a class. The object is constructed using the class as a blueprint and is a concrete instance of the abstraction that the class represents. An object must be created before it can be used in a program. In Java, objects are manipulated through object references. Each object has a unique identity and has its own copy of the fields declared in the class definition.

Instance Member

The fields of an object are called instance variables. The values of the instance variables in an object comprise its state. Two distinct objects can have the same state, if their instance variables have the same values. The methods of an object define its behavior. These methods are called instance methods. It is important to note that these methods pertain to each object of the class. Instance variables and instance methods, which belong to objects, are collectively called instance members.

Method invocation

Objects communicate by message passing. This means that an object can be made to exhibit a particular behavior by invoking the appropriate operation on the object. In Java, this is done by calling a method on the object using the dot '.' operator.  The dot '.' notation also can be used with a reference to access fields of an object.

Static member

In some cases, certain members should only belong to the class, and not be part of any object created from the class. Such a field is called a static variable. It belongs to the class, and not to any object of the class. A static variable is initialized when the class is loaded at runtime. Similarly, a class can have static methods that belong only to the class, and not to any objects of the class.  Static variables and static methods are collectively known as static members, and are distinguished from instance members in a class definition by the keyword static in their declaration.

Java Program

A Java program is a collection of one or more classes, with one of them containing the program's execution starting point. A Java source file can contain more than one class definition. The Java 2 SDK enforces the rule that at the most one class in the source file has public accessibility. The name of the source file is comprised of the name of this public class with .java as extension. Each class definition in a source file is compiled into a separate class file, containing Java byte code. The name of this file is comprised of the name of the class with .class as an extension. All programs must be compiled before they can be run. 

An application is what is normally called a program: source code that is compiled and directly executed. In order to create an application in Java, the program must have a class that defines a method called main. The main() method in the class is the starting point for the execution of any application.

public static void main(String [] args)

The main() method has public accessibility, that is, it is accessible from any class. The keyword static means the method belongs to the class. The keyword void means the method does not return any value. The parameter list, String[] args, is an array of strings used to pass information to the main() method when the application is started.

Introduction to Java

Introduction to Java

Java is a general-purpose computer programming language that is concurrent, class-based and object-oriented. Java is intended to let application developers "write once, run anywhere" (WORA) meaning that compiled Java code can run on all platforms that support Java without the need for recompilation. 

Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of computer architecture. As of 2016, Java is one of the most popular programming languages in use, particularly for client-server web applications. 

Java was originally developed by James Gosling at Sun Microsystems (later acquired by Oracle Corporation) and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++, but it has fewer low-level facilities than either of them.

Principles

There were five primary goals in the creation of the Java language  

1. It must be "simple, object-oriented, and familiar".
2. It must be "robust and secure".
3. It must be "architecture-neutral and portable".
4. It must execute with "high performance".
5. It must be "interpreted, threaded, and dynamic".

Java Virtual Machine (JVM)

Oracle provides a set of programming tools such as javac, java and others in a bundle called Java SDK(JDK) for different platforms (Windows, Linux, etc)

Oracle also provides a runtime bundle with just the JVM (Java Virtual machine) when programming tools are not needed.

JVM is a program that runs on a given platform and takes the bytecode as input and interprets them just as if it were a physical processor executing machine code. javac compiler transforms the Java language source code to bytecode that runs in the JVM.

With most programming languages you either compile or interpret a program so that you can run it on your computer. The Java programming language is unusual in that a program is both compiled and interpreted. With the compiler, first you translate a program into an intermediate language called Java bytecodes – the platform independent codes interpreted by the interpreter on the Java platform. The interpreter parses and runs each Java bytecode instruction on the computer. Compilation happens just once; interpretation occurs each time the program is executed.

You can think of the Java bytecodes as the machine code instructions for the Java Virtual Machine(Java VM). Every JVM interpreter, whether it’s a development tool or a web browser that can run applets, is an implementation of the Java VM.

Java byte codes help make “write once, run anywhere” possible. You can compile your program into bytecodes on any platform that has a Java compiler. The bytecodes can then be put on any implementation of the Java VM. That means that as long as a computer has a Java VM, the same program written in the Java programming language can run on Windows, a Solaris workstation or an iMac.

What is MongoDB?

What is MongoDB?

MongoDB is a document database with the scalability and flexibility that you want with the querying and indexing that you need

MongoDB is a free and open-source cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with schemas. MongoDB is developed by MongoDB Inc. and is free and open-source, published under a combination of the GNU Affero General Public License and the Apache License.

Document Database

A record in MongoDB is a document, which is a data structure composed of field and value pairs. MongoDB documents are similar to JSON objects. The values of fields may include other documents, arrays, and arrays of documents.

{

name :  “Ravi” field: value

age: 26     field: value

}

Key Features

High Performance

MongoDB provides high performance data persistence. In particular, Support for embedded data models reduces I/O activity on database system.

Rich Query Language

MongoDB supports a rich query language to support read and write operations including Create, Retrieve, Update, Delete (CRUD) 

High Availability

MongoDB’s replication facility, called replica set, provides:

automatic failover 

data redundancy.

A replica set is a group of MongoDB servers that maintain the same data set, providing redundancy and increasing data availability.

Horizontal Scalability

MongoDB provides horizontal scalability as part of its core functionality:

Sharding distributes data across a cluster of machines.

MongoDB 3.4 supports creating zones of data based on the shard key. In a balanced cluster, MongoDB directs reads and writes covered by a zone only to those shards inside the zone.

Support for Multiple Storage Engines

MongoDB supports multiple storage engines, such as:

WiredTiger Storage Engine and

MMAPv1 Storage Engine.

Indexing

Fields in a MongoDB document can be indexed with primary and secondary indices.

File storage

MongoDB can be used as a file system with load balancing and data replication features over multiple machines for storing files.

Server-side JavaScript execution

JavaScript can be used in queries, aggregation functions (such as MapReduce), and sent directly to the database to be executed.

Introduction to Node.js

Introduction to Node.js 

Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient. Node.js' package ecosystem, npm, is the largest ecosystem of open source libraries in the world.

Node.js is an open source, cross-platform runtime environment for developing server-side and networking applications. Node.js applications are written in JavaScript, and can be run within the Node.js runtime on OS X, Microsoft Windows, and Linux.

Features of Node.js

Following are some of the important features of Node.js 

• Asynchronous and Event Driven − All APIs of Node.js library are asynchronous, that is, non-blocking. It means a Node.js based server never waits for an API to return data. The server moves to the next API after calling it and a notification mechanism of Events of Node.js helps the server to get a response from the previous API call.
• Fast − Being built on Google Chrome's V8 JavaScript Engine, Node.js library is very fast in code execution.
• Single Threaded and Highly Scalable − Node.js uses a single threaded model with event looping. Event mechanism helps the server to respond in a non-blocking way and makes the server highly scalable. Node.js uses a single threaded program and the same program can provide service to a much larger number of requests than traditional servers like Apache HTTP Server.
• No Buffering − Node.js applications never buffer any data. These applications simply output the data in chunks.
• License − Node.js open source and is released under the MIT license

Where to Use Node.js

Node.js can be used in the following 

• I/O bound Applications
• Data Streaming Applications
• Data Intensive Real-time Applications (DIRT)
• JSON APIs based Applications
• Single Page Applications

3 COMPTIA CERTIFICATION IN DEMAND

3 COMPTIA CERTIFICATION IN DEMAND

In 2016, more than 5.2 million IT jobs contributed to the U.S. economy with an additional 2 million new job postings throughout the year. Peruse those posts, and you’ll find three CompTIA certifications specifically cited as job requirements in thousands of listings: CompTIA Security+, CompTIA A+ and CompTIA Network+.

CompTIA Security+

 CompTIA Security+ is the certification globally trusted to validate foundational, vendor-neutral IT security knowledge and skills. As a benchmark for best practices in IT security, this certification covers the essential principles for network security and risk management – making it an important stepping stone of an IT security career.

In response to CompTIA’s IT Career Insights Survey, 93 percent of IT professionals said they regularly work with security and cyber security technology. Even more importantly, according to CompTIA’s Common IT Employability Skills study, 62 percent of hiring managers in the United States indicated that candidates “must have” these skills to be considered for the job. Hands-on and strategic involvement with security technology makes CompTIA Security+ certification, as well as the forthcoming CompTIA Cybersecurity Analyst (CSA+) certification, a vital part of an IT career, whether you want to enhance your skills for an existing job or apply for a new one.

CompTIA A+

It’s a known fact that IT training helps employees advance in their careers. The CompTIA A+ certification is perfect for individuals new to the field and IT professionals in transition. Because it tests a candidate’s knowledge on all fields of information technology and not on one particular domain, the CompTIA A+ certification is typically given more weight than other vendor certifications.

More than 23,500 job postings in 2016 required applicants to have CompTIA A+ certification. Validating the IT skills required by the most common hardware, software and operating systems while remaining vendor neutral, CompTIA A+ has long been a  foundational certification for IT professionals.

CompTIA Network+

Finally, more than 18,000 jobs posted in 2016 required CompTIA Network+ certification, a vendor-neutral, worldwide networking accreditation that validates an IT professional’s ability to design, configure, manage and troubleshoot both wired and wireless networks. Considering 92 percent of IT professionals in the United States worked with this technology in 2016, CompTIA Network+ is a valuable certification for almost any job applicant.

Although most online job postings do not specifically list IT certification requirements, employers know IT pros with CompTIA certifications are more capable of succeeding in their careers.

When accreditations are preferred, CompTIA Security+ is the fourth-most-requested certification in job postings, behind Certified Information Systems Security Professional, Project Management Professional, and Cisco IT certifications. CompTIA A+ and Network+ ranked 8th and 11th, respectively, according to the Burning Glass Technologies Labor Insights study.

Clearly, CompTIA professional certifications set job candidates apart in a competitive and constantly growing job market—one the U.S. Bureau of Labor Statistics expects to grow 12 percent in the network and computer systems industry over the next seven years.

Domain Model in Hibernate

Domain Model in Hibernate

The application domain model is the central character in an ORM. Domain Model are the classes we would be mapping in our application. These domain model classes needs to follow the Plain Old Java Object (POJO) / JavaBean programming model in order to make Hibernate works best.

Applications using Hibernate uses its proprietary XML mapping file format for this purpose. With JPA, most of this information is now defined in a way that is portable across ORM/JPA providers using annotations and/or standardized XML format.

Mapping types

Hibernate understands both the Java and JDBC representations of application data. The ability to read/write this data from/to the database is the function of a Hibernate type. A type, is an implementation of the org.hibernate.type.Type interface.

To help understand the type categorizations, we take an example of a simple table and domain model that we wish to map.

create table ContactInfo (     id integer not null,     firstName varchar(255),     lastName varchar(255),     middleName varchar(255),     notes varchar(255),     starred boolean not null,     website varchar(255),     primary key (id) )

@Entity(name = "ContactInfo") public static class ContactInfo {     @Id     private Integer id;     private Name name;     private String notes;     private URL website;     private boolean starred;     //Getters and setters }

@Embeddable public class Name {     private String firstName;     private String middleName;     private String lastName;     // getters and setters }

In the broadest sense, Hibernate categorizes types into two groups:

• Value types
• Entity types

Value types

A value type is a piece of data that does not define its own lifecycle. It is, owned by an entity, which defines its lifecycle. All the state of an entity is made up entirely of value types. These state fields or JavaBean properties are termed persistent attributes. The persistent attributes of the ContactInfo class are value types.

Value types are further classified into three sub-categories:

Basic types

In mapping the ContactInfo table, all attributes except for name would be basic types.

Embeddable types

The name attribute is an example of an embeddable type.

Collection types

Collection types are also a distinct category among value types.

Entity types

Entities, exist independently of other objects whereas values do not. Entities are domain model classes which correlate to rows in a database table, using a unique identifier. Entities exist independently and define their own lifecycle. The ContactInfo class is an example of an entity. 

Hibernate Architecture Overview

Hibernate Architecture Overview

Hibernate, is an ORM solution, which sits between the Java application data access layer and the Relational Database. The Java application makes use of the Hibernate APIs to load, store, query, etc its domain data. Hibernate implements the Java Persistence API (JPA) specifications. 

SessionFactory (org.hibernate.SessionFactory)

SessionFactory  is a thread-safe representation of the mapping of the application domain model to a database. SessionFactory acts as a factory for org.hibernate.Session instances. The EntityManagerFactory is the JPA equivalent of a SessionFactory.

A SessionFactory is very expensive to create, so, for any given database, the application should have only one associated SessionFactory. The SessionFactory maintains services that Hibernate uses across all Sessions such as second level caches, connection pools, transaction system integrations, etc.

Session (org.hibernate.Session)

A single-threaded, short-lived object conceptually modeling a "Unit of Work". In JPA nomenclature, the Session is represented by an EntityManager.

The Hibernate Session wraps a JDBC java.sql.Connection and acts as a factory for org.hibernate.Transaction instances. 

Transaction (org.hibernate.Transaction)

A single-threaded, short-lived object used by the application to demarcate individual physical transaction boundaries.