Java Basic Syntax : Java Basic

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 words 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.

When saving the file you should save it using the class name (Remember java is case sensitive) and append '.java' to the end of the name. (if the file name and the class name do not match your program will not compile).

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 : defualt, public , protected, private
  • Non-access Modifiers : final, abstract, strictfp
We will be looking into more details about modifiers in the next section.

Java Variables:
We would see following type 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 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.
abstractassertbooleanbreak
bytecasecatchchar
classconstcontinuedefault
dodoubleelseenum
extendsfinalfinallyfloat
forgotoifimplements
importinstanceofintinterface
longnativenewpackage
privateprotectedpublicreturn
shortstaticstrictfpsuper
switchsynchronizedthisthrow
throwstransienttryvoid
volatilewhile


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.
public class MyFirstJavaProgram{

   /* This is my first java program.
    * This will print 'Hello World' as the output
    * This is an example of multi-line comments.
    */

    public static void main(String []args){
       // This is an example of single line comment
       /* This is also an example of single line comment. */
       System.out.println("Hello World"); 
    }
} 

Using Blank Lines:

A line containing only whitespace, possibly with a comment, is known as a blank line, and Java totally ignores it.
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 with out having to rewrite the code in a new class. In this scenario the existing class is called the super class 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. Interfaces play a vital role when it comes to the concept of inheritance.

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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