Java Introduction -
Why to Learn java Programming?
Java is a MUST for students and working professionals to become a great Software Engineer specially when they are working in Software Development Domain. I will list down some of the key advantages of learning Java Programming:
Object Oriented − In Java, everything is an Object. Java can be easily extended since it is based on the Object model.
Platform Independent − Unlike many other programming languages including C and C++, when Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by the Virtual Machine (JVM) on whichever platform it is being run on.
Simple − Java is designed to be easy to learn. If you understand the basic concept of OOP Java, it would be easy to master.
Secure − With Java's secure feature it enables to develop virus-free, tamper-free systems. Authentication techniques are based on public-key encryption.
Architecture-neutral − Java compiler generates an architecture-neutral object file format, which makes the compiled code executable on many processors, with the presence of Java runtime system.
Portable − Being architecture-neutral and having no implementation dependent aspects of the specification makes Java portable. Compiler in Java is written in ANSI C with a clean portability boundary, which is a POSIX subset.
Robust − Java makes an effort to eliminate error prone situations by emphasizing mainly on compile time error checking and runtime checking.
Basic Syntax
About Java programs, it is very important to keep in mind the following points.
Case Sensitivity − Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
Class Names − For all class names the first letter should be in Upper Case. If several words are used to form a name of the class, each inner word's first letter should be in Upper Case.
Example: class MyFirstJavaClass
Method Names − All method names should start with a Lower Case letter. If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case.
Example: public void myMethodName()
Program File Name − Name of the program file should exactly match the class name.
Example: Assume 'MyFirstJavaProgram' is the class name. Then the file should be saved as 'MyFirstJavaProgram.java'
public static void main(String args[]) − Java program processing starts from the main() method which is a mandatory part of every Java program.
Java Identifiers
All Java components require names. Names used for classes, variables, and methods are called identifiers.
In Java, there are several points to remember about identifiers. They are as follows −
All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).
After the first character, identifiers can have any combination of characters.
A key word cannot be used as an identifier.
Most importantly, identifiers are case sensitive.
Examples of legal identifiers: age, $salary, _value, __1_value.
Examples of illegal identifiers: 123abc, -salary.
Java Modifiers
Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers −
Access Modifiers − default, public , protected, private
Non-access Modifiers − final, abstract
We will be looking into more details about modifiers in the next section.
Java Variables
Following are the types of variables in Java −
Local Variables
Class Variables (Static Variables)
Instance Variables (Non-static Variables)
Java Arrays
Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap. We will look into how to declare, construct, and initialize in the upcoming chapters.
Java Enums
Enums were introduced in Java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.
With the use of enums it is possible to reduce the number of bugs in your code.
For example, if we consider an application for a fresh juice shop, it would be possible to restrict the glass size to small, medium, and large. This would make sure that it would not allow anyone to order any size other than small, medium, or large.
Java Keywords
The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.
| abstract | assert | boolean | break |
| byte | case | catch | char |
| class | const | continue | default |
| do | double | else | enum |
| extends | final | finally | float |
| for | goto | if | implements |
| import | instanceof | int | interface |
| long | native | new | package |
| private | protected | public | return |
| short | static | strictfp | super |
| switch | synchronized | this | throw |
| throws | transient | try | void |
| volatile | while |
Comments in Java
Java supports single-line and multi-line comments very similar to C and C++. All characters available inside any comment are ignored by Java compiler.
Example
public static void main(String []args){
// This is single line comments
/* This is multiple line comments */
}
Inheritance
In Java, classes can be derived from classes. Basically, if you need to create a new class and here is already a class that has some of the code you require, then it is possible to derive your new class from the already existing code.
This concept allows you to reuse the fields and methods of the existing class without having to rewrite the code in a new class. In this scenario, the existing class is called the superclass and the derived class is called the subclass.
Interfaces
In Java language, an interface can be defined as a contract between objects on how to communicate with each other. Java sport multiple Interfaces inheritance class. Interface is a collection of abstract class. Interface class is a by default a abstract class. Interface class extends only interface class.
An interface defines the methods, a deriving class (subclass) should use. But the implementation of the methods is totally up to the subclass.
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