Java Class 1 Review

  1. Java programs are a psuedo machine code called "Java byte-code" usually executed by a virtual machine (The JVM is an emulator which runs the byte code.) I say usually because SUN has actually created hardware chips which execute the Java byte-code instructions.

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  2. Java program modules are created by writing "class definitions" in "xyz.java" source files and compiling them with a Java compiler to produce "xyz.class" Java byte-code files.

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  3. There are two types of Java virtual machines (JVMs) which run the two types of Java programs.

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    1. Java "applications" are run by starting the JVM (usually java.exe) and giving it the name of a Java class file that has an entry point called "main".

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    2. Java "applets" are run in a special JVM (usually part of a web browser) by pointing it to an HTML document which has the startup info for the Java applet. Applets are not called at entry point "main" but have several special entry points called by the JVM when certain events occur:
      1. init - is called when the applet is first loaded.
      2. start - is called when the applet is made visible in the browser/viewer window.
      3. stop - is called when the applet is no longer visible.
      4. destroy - is called when the applet is removed (eg the browser changes documents).
      5. paint - is called when the browser wants the applet to refresh it's output display.
      6. there are some others (note: you can write a Java pgm that runs as an app or applet.).

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  4. The Java Developer Kit (JDK) has a Java compiler (javac), a Java interpreter (the JVM - java), the basic Java class libraries, an applet testing program (appletviewer), and some other stuff. These are command line utilities.

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  5. Classes are Java files that contain data items (aka variables/properties/values...) and methods (aka code blocks/functions/subroutines...). In order to use the variables and functions, you usually need to create an object from the class.

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  6. Objects are "instances" of "classes" that have been loaded and initialized, sort of like a tear off note from a sticky pad. They are quite useful but hide the fact that code is still executed sequentially in some order. They differ from mainframe started tasks or services/daemons on pc servers/minicomputers because they may not have an independent "thread" or execution process - they may simply reside in memory, waiting to be called by another task.

  7. Note that Java programs do not actually require objects - a Java application starts by running a "main" procedure which exists independently of any object and before any objects which may be created by it. Therefore a better way of looking at applications than the Java designers seem to have is to consider the main Java procedure to be a simple script which may or may not create objects from class files. Consider the following NetRexx program which computes and displays the perfect squares from 1 to 10000 but creates and uses no objects:
        
        loop R=1 to 100
            say R "squared is" R*R
        end

    NetRexx automatically converts this to a class as Java requires, but it is obvious this is a simple code procedure in contrast to the Java equivalent version:

     public class Squares {

    8.              static int R ;
                 public static void main(String[] arg) {
                     for (R=1;R<=100;R++) System.out.println(R + " squared is " + R*R);
                 }   }