java programming Basic

•  Case Sensitivity – Java is case sensitive meaning an identifier Hello and hello have different meanings in Java.
• Class Names – wit class names the first letter should be in Upper Case. when many words are used to form a name of the class, each inner word’s first letter should be in Upper Case.  for Example class MyFirstJavaClass
•  Method Names – wit method all names should start with a Lower Case letter. When many words are used to form the name of the method, then each inner word’s first letter should be in Upper Case. for Example public void myMethodName()
•  Program File Name – te 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
the names used for classes, variables, and methods are what we refer to as identifiers, with identifiers we have some key points to remember;

•  identifiers should begin with a letter (A to Z or a to z), currency character ($), or an underscore (_).
• identifiers can have any combination of characters after the first characters.
•  keywords are not allowed for use as identifiers.
• identifiers are all case-sensitive.

legal identifiers: age, $salary, _value, __1_value.illegal identifiers: 123abc, -sallary.

There are two types  of modifiers:

1.  Access Modifiers: default, public, protected, private
2. Non-access Modifiers: final, abstract, strictfp

Types of variables in Java

1. Local Variables
2. Class Variables (Static Variables)
3.  Instance Variables (Non-static Variables)

Java Arrays

Arrays are a collection of  multiple variables of the same type

Java Enums
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.

class FreshJuice
{
enum FreshJuiceSize{ SMALL, MEDIUM, LARGE }
FreshJuiceSize size;
}
public class FreshJuiceTest { public static void main(String args[])
{
FreshJuice juice = new FreshJuice();
juice.size = FreshJuice.FreshJuiceSize.MEDIUM ;
System.out.println(“Size: ” + juice.size);
}
}

result:
Size: MEDIUM

Java Keywords
abstract   assert    boolean   break   byte    case   catch    char    class   const   continue     default 
Interfaces    do   double   else  enum extends      final     finally     float    for  goto     if  implements     import instance    of     int 
Comments in Java
supports single-line and multi-line comments very similar to C++.characters available inside any comment are ignored by the 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 white space, possibly with a comment, is known as a blank line, and Java totally ignores it.
Inheritance
classes can be derived from classes. Basically, if you need to create a new class and there 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. 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.