Learning Java

Using other people's applets

Some terms with definition

• Applications are normal programs that you use outside the browser.
• Applets are special programs (objects) that you use inside a Web document with the <APPLET> tag.
• The java compiler would compile a source generating bytecodes:these can run on any platform that has a Java interpreter.
• Being Java object oriented,each program is a class of objects.
• In a class we have definitions of methods (functions) that act on the class variables.
• To use a class you must create an instance of it with instance variables that define the attributes of this object.
• JDK indicates the Java Developer's Kit available from Sun with a compiler(javac) a appletviewer(appletviewer) and an interpreter (java).
• The constructor is a special method called when the object is initialized.
• Member data and member functions are other way to call instance variables and instance methods.
• Abstract classes cannot be instantiated and are created only to serve as a basis for making subsclasses.They exist only to express the common properties of all its subclasses. They can have methods defined but left blank (they do nothing) since it is understood that you must define them in the subclasses:these are called an abstract methods.
• An interface is a set of empty methods definitions:these methods must be defined in the class that implements that interface. For example:

         public interface Drawable {
  public void draw();
  }
  

The first step

Object-oriented programming in a nutshell

Learning programming:step 2

Using Netscape to view applications

Object oriented graphics programming;the AWT class library;the actors on stage metaphor

• Component
  • Label
  • Button
  • Checkbox
  • Choice
  • Textfield
  • Scrollbar
  • Container
    • Panel
      • Applet

Paint,repaint,update

Using predefined classes

Learn Java in a hour

Java Packages

• directory bin:basic Java language tools
• directory demo:Demo programs
• directory lib:Class libraries
  • java.lang:basic language classes (String,etc)
  • java.applet:classes for applet manipulation.
  • java.awt:Abstract windowing toolkit (the classes for GUI)
  • java.io:File input/output classes
  • java.net:Networking protocol API
  • java.util:Miscellanea like random-numbers,date,etc

Java basic data types

• boolean:true or false
• byte:8-bit integer
• short:16-bit integer
• int:32 bit integer(this is the normal integer.
• char:used to represent single characters
• float:32 bit IEEE float
• double:64-bit IEEE float (the normal float type).
• String:class used to represent character strings.

Frequently used constants and methods

• Integer: wrapper class used to manipulate integer.
  For example:
  x=Integer.parseInt(s) converts a string to integer
  s=Integer.toString(x) converts a int to string
• System.out.println(s);:used to write a message on the standard output stream.
• main: the method first called from the "system" in a application.It must be declared like this:

  public static void main(String args[]){ }
  

• paint: method called from "netscape" to (re)paint a graphic object.
• Applet: Netscape requires the program to run inside the browser to be a subclass of this class

  public class CiaoATutti extends Applet {
  

• Color.redConstant indicating the red color.Similar constants can be used to indicate all main colors.

Java I/O tutorial

OPEN file filename for I/O
DO while(there are new records)
read/write record
END DO

try {   }
catch (Exception e) { handle exception};