JAVA COURSE 1 Computer Engineering Association

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(); }

Command to compile the first program  In java, we use the jdk, which can be downloaded from the website  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]->[setting]- >[system]->[advance] 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 Path=[jdkdirectory]/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

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

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\cla sses  Then we type the following program as shown

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)  {}

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.

Syntax about method  Access modifier: [public|private|protected|default]  [abstract|final|synchronized|native|static](opti onal)  The order of the above two position is not important.  Return type [type|void]  Name of method  Argument  Exception throw(optional)  ()(abstract or interface method don't need ())

Meaning of these class syntax  First key word

 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 C|C++ code  Void means no return type  Throw, we explain later

Variable  They are divided into following types:  Static variable  Instance variable  Local variable  Final variable  Transient variable  Volatile variable

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

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

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.

Local variable  Life Only within the same  Dead after the method

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

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!.

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?

 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

 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)