JAVA COURSE 1

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JAVA COURSE 1

• Computer Engineering Association

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Compile your first program
Public class Hello public class
Hello() System.out.println(Hello)
puclic static void main(STring args) System.
out.println(In the main) Hello temp new
Hello()
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Command to compile the first program

• In java, we use the jdk, which can be downloaded
  from the website www.java.sun.com
• After installation, if we don't want to give the
  absolute path everytime to compile the program,
  we need to set the path
• In windows 2000, go to control
  panel-gtsetting-gtsystem-gtadvance and change
  the system enviroment variable, PATH , adding the
  path of jdk directory\bin
• In linux, you need to edit the .bash_profile file
• Add Pathjdkdirectory/bin
• Export Path
• Exec bash --login to make it effective
• For windows 95/95/me, you need to edit the
  autoexec.bat file and add the path as Path ...
  same as above

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Package

• Java use the package mechanism to resolve all the
  class name
• e.g., there are two classes as Hello1, Hello1
• But they are different, as they are put in
  diofferent directory.
• How about we import them, as if include mechanism
  in C, C
• So we simple type import directory.Hello1
• And thus we can use it.
• The java put all their classes in
  jdkdirectory\clases

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Simple program to demonstrate package

• Before we need to set the classpath
• Same as before setting path, but we set CLASSPATH
  now
• CLASSPATH\root\demo\root\j2sdk1.4.0\classes
• Then we type the following program as shown

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Syntax Class definition

• Class definition is defined by the following
  words
• public(optional)
• final(optional)
• abstract(optional)
• If we declare final, then we cannot declare
  abstract, vice versa. And the poition of the
  above words is not rigid.
• Class
• Class name(same as file name)
• Extends (optional)
• implements(optional)

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Meaning of the class keyword

• Public means all classes can access it, whetherit
  is in the same package or not
• If not public, then the class is accessed only by
  the class in the same package.
• Final means the class cannot be inherited
• Abstract means the class must be inherited.
• Extends means to inherit a class
• Implements means to inherit from the interface
• The is needed and should not be omitted.

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Syntax about method

• Access modifier publicprivateprotecteddefault
  
• abstractfinalsynchronizednativestatic(option
  al)
• The order of the above two position is not
  important.
• Return type typevoid
• Name of method
• Argument
• Exception throw(optional)
• ()(abstract or interface method don't need ())

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Meaning of these class syntax

• First key word

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• Final is the method declares a method that cannot
  be overridden in the subclass
• Abstract means the method that should be
  implement in the subclass
• Static means the method can be access though no
  object is created, but they can access instance
  variable or static variable
• Synchronized is used in mutil-threading
• Native means it is in CC code
• Void means no return type
• Throw, we explain later

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Variable

• They are divided into following types
• Static variable
• Instance variable
• Local variable
• Final variable
• Transient variable
• Volatile variable

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• CLASS SCOPE SYNTAX
• Public, private, default, protected
• Transient, final, static, volatile
• type
• Name
• LOCAL VARIABLE (DECLARE WITHIN METHOD)
• Final(optional)
• Type
• Name

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Meaning of these words

• The modifier are the same as method modifier.
• Final means, const if C, C
• Static means the variable can be accessed even
  the object is not there.
• Transient, volatile, talk later

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Class variable characteristics

• Access through object
• Like Hello temp new Hello()
• temp.you, (assumed we have you variable)
• Static variable can be accessed though no object
  is there. Hello.temp no object is created.

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Local variable

• Life Only within the same
• Dead after the method

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Data type

• In java, everything is class, except the
  following data type, which is not class
• Int
• Char
• Boolean
• Float
• Double
• Byte(8 bit)
• Long (64 bit)
• Short(16 bit)
• String is object. We will talk about it later

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About concept of referenial variable

• e.g Hello temp new Hello()
• The, there is only one object allocated in heap
  area.
• What is heap and stack?
• Stack is used for the store of local variable
• And heap is for the dynamic allocation of memory,
  when we use new keyword, we allocate memory for
  the object.
• How about we type Hello temp2 temp
• They refer to the same object!.

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Concept of object -oriented programming

• Encapsulation
• We package the variable within a class, and so we
  ensure some variable are not modified from
  outside
• Inheritance
• We need to know the concept of ISA and HASA
• Like there is a employer class
• And we create a hour_employeer class to inherit
  from it.
• The main point is we ask ourselves, is hour_work
  a type of employee?

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• Casting
• As we know hour_employee is a type of employee
  and inherit from it and thus we can say
  hour_employee is employee
• e.g.
• Employee temp new hour_employee
• It works
• But now it is employee object
• The above is called static casting which cast
  within compile time
• And (hour_employee) temp, is called dynamic
  casting, that a superclass type want to cast
  itself to be the subclass. It may not work

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• Polymorphism
• We write a method a() in the super class
  superclass
• And a() in the subclass sub, which extends
  superclass
• We declare
• Superclass temp new subclass()
• temp.a() is called not based on the type, but the
  actual object allocated in the heap memory
• How about no a() declare in the sub?
• A() is called based on the type
• And this fexibility allow easy programming
• For example we write a method called
• Public void outName(Superclass a)
• We can write outName(subclass)