Java

• Java tutorial / reference
• Live Weather
• Stand Alone Programs
• My final project for Java course at local community college, December 1999
• Tiny InterNet Interface TINI
• SitePlayer
• Servlets, JDBC, Web development
• Palm Pilot
• Cybiko wireless
• Home Automation
• Cruz Family genealogy, click here if you have Internet Explorer
• Cruz Family genealogy, click here if you have Mozilla, Galeon, Netscape
• General Notes about compiling java between Windows and Linux / Tips and Tricks

Java Tutorial / Reference

Parameters / Argument passing to the methods

All paramaters/arguments in Java are passed by value! I had a hard time accepting this in the beginning coming from several programming languages but the sooner you accept these the easier it will get (the more you resist these concept the harder it will get). Don't bother comparring C/C++ pointers to Java's object reference, Java don't have pointers.

Interfaces / Abstracts Concepts
Interface are like contract, pretend you have a blueprint of a house that has 4 bedrooms, 2 baths, garage, frontyard and backyard. The interface part is the blueprint, you will give this to the builder and tell the builder, you must build the house with 4 bedrooms, 2 baths, garage, frontyard and backyard but not tell the builder how to build it. The contract part is that the builder must build the house with 4 bedrooms, 2 baths, garage, frontyard and backyard.

Abstract on the other hand is also like a blueprint of a house that has 4 bedrooms, 2 baths, garage, frontyard and backyard. The abstract part is that you give the builder the blueprint but tell the builder, by the way, I already built the 2 baths and the garage, all you need is to build the 4 bedrooms, frontyard and backyard.

So interface, no implementation of methods, abstract, some implementation of methods. If all your abstract methods has no implementation, then might as well turn this abstract to interface, because that's what interface is pure abstract.

Use interface if you want to force a type like class making sure that the class implementing the interface will implement each and every method defined in the interface. Use abstract if part of your methods are already implemented (although the calling class can override these methods). Note that Java do not have multiple inheritance like C++ but you can implement multiple interfaces, so this is another way to use interface effectively. Interfaces can have variables (member) but they must be public static final. Interfaces, unlike classes can extend more than one iterface. For example: interface Son extends MoM, Dad. Son interface extends both Mom and Dad which means that all methods and constants defined by Mom and Dad are part of the Son contract, and also any method and constant inside the Son interface. Summary: An abstract class can have a partial implementation, protected parts, static methods, whereas interfaces are limited to public constants and methods with no implementation.

Polymorphism / Upcasting I've written an example of trying to show the polymorphism behavior, upcasting example. We have 3 students, named Joe, Tony, and Jeremy, they all went inside the class room, when the instructor did a roll call every student reply with their name. I've created an interface called Person that will enforce a method name getName() whoever implements(behavior) Person. Class Student implements Person so it must implement (behaviour) getName(). I've created an empty class room (Vector) that holds any kind of Objects (Student, Person, etc.) I created three new students named Joe, Tony, Jeremy, and put them inside the class room. When we do the roll call, I call each Person (cast to Student) and each one respond by their own name. Where is polymorphism in these examples.
The line: Person person = (Student)classRoom.elementAt(i); is where the polymorphism happens, when I add each student, I'm adding a student object, however, when I'm calling each student, I'm using Person not Student, but somehow with casting Student to a Person, the person object knows everybody's name. Clear as a mud ? Again, when I add the Student in the class room, they are Student type, but when I call each one, I use a Person object cast to Student. Which brings to the next point, person is cast to Student which is ok because a Student is a Person.
The line: class Student implements Person suggests that any Student is a Person which is true. Upcasting means going up to the hierarchy class, which is ok in this case because any Student is a Person. A Vector is a dynamic (shrinking/expanding) array like as opposed to fix size. A Vector holds only object and not primitive types, once you put an object into the vector it just become any other object, to retrieve, you have to cast it to a known object to make it usable, this is why Person is cast to Student before we can manipulate the Person object, otherwise we don't know what object type is inside the Vector. The file name 'RollCall.java' is here.

Collections Collections (Vector, ArrayList ...) holds reference to objects not the actual objects themselves. Below example proves this. I created an object with initial value, put it inside the vector, then get what's inside the vector and shows the value on the screen. Next, I modify the object outside the vector, then get what's inside the vector and show the value on the screen. This time, the value change eventhough I modify the value outside the vector, and did not put back inside the vector the modified object. This shows that the vector is holding the reference to the object and not the object itself.

     Vector v = new Vector();
     StringBuffer sb = new StringBuffer("John ");    // create a string
     v.add(sb);                                      // add sb in vector
     StringBuffer sbVector = (StringBuffer)v.get(0); // get the string inside the vector
     System.out.println("Before modifying, value is " + sbVector);
     sb.append(" Doe");                              // modify sb outside of the vector
     sbVector = (StringBuffer)v.get(0);              // get the sb inside the vector again
     System.out.println("After modifying, value is " + sbVector);


     Output is:
     Before modifying, value is John
     After modifying, value is John Doe

Accessing variables from current, super, super/super class

How to access same name variables current, super, super/super class
Assumes s is instance of class Son that extends Father.
Assumes Father extends GrandFather.  All classes contains a variable String str.

Son s = new Son();
s.str                // this refers to son's str variable
((Father)s).str      // this refers to father's str variable
((GrandFather)s).str // this refers to grandfather's str variable

What are Wrappers and why have them ? To be continued...

Static To be continued...

Overloading / Overriding Why return types are not part of overloaded signatures ? To be continued...

Servlets To be continued...

JDBC To be continued...

Call backs To be continued...

Live Weather

Java Stand alone program

Final project for Java course (house alarm)

TINI

SitePlayer

The SitePlayer is a 1 x 1.5 inch device which has Ethernet capability, serial, I/O for only $29.95 (not including RJ-45) jacks. You can find more info at www.siteplayer.com. The Java code to toggle the SitePlayer I/O using TCP socket is here, and the Java code to toggle the SitePlayer I/O using UDP is here.. Make sure that if you use the UDP that the UDP receive enable is activate in the SitePlayer. Forum for the siteplayer is at http://groups.yahoo.com/group/siteplayer. Currently the SitePlayer is connected to the network through Ethernet, if any changes in my simulated door/window sensors, a microcontroller instruct the SitePlayer to send a UDP packet to the Linux server sending the data. The Linux server has a UDP server that waits for packet, and if it receives packet it then dumps the data to a MySQL database. This code also shows how to connect to a database using Java's JDBC and assumption that you have MySQL database already running. The code for the Linux server is here. The code for the microcontroller is here.

Servlets, JDBC, Web Developments

Palm Pilot

Cybiko wireless

Home Automation

General notes on compiling Java between Linux and Windows

• Linux directory separator is ' / ', Windows directory separator is ' \ '. (e.g. /home/linux/test.txt vs. E:\tmp\test.txt)
• Linux environment variable is ' $ ', Windows is ' % '. (e.g. javac -classpath $WABACLASSES Test.java vs. javac -classpath %WABACLASSES% Test.java)
• Linux separator for environment var is ' : ', Windows is ' ; '. (e.g. PATH=/home/test:/usr/bin vs. PATH=C:\DOS;D:\temp)
• Linux has to export environment var, Windows can be automated using autoexe.bat. (e.g. export PATH vs. specified on the autoexec.bat file)
• Linux can execute shell script, Windows can execute batch command. (e.g. ./scipt.sh vs. batchcommand)
• Whenever possible use Ant instead of batch or make, it's supposed to be portable between platforms.

Unix/Linux end of line terminator = "\n"

Mac end of line terminator = "\r"

Windows end of line terminator = "\r\n"


If you are having problem with sockets try not to use println() but write explicit "\r\n" to the sockets