CSC152 2005S, Class 4: An Introduction to Java

Admin:
* Apologies about reading
* Readings can generally be found in the Espresso site
* Readings for Monday available later today or early tomorrow
* Sam absent on Monday and Tuesday.  Cassie teaches.  Apologies.

Overview:
* Theoretical background
* Detour: Packages
* Running Java programs
* Your first program

Non-Quiz:
* What is an object-oriented programming language?
  * A language in which you programs are combinations of objects
  * Objects combine data and capabilities
* What key features do you expect in such a language?
  * Encapsulation - Grouping and Protection
* Why do we use them?
  * We can reuse objects and methods that apply to objects
  * Protection
* Classes vs. objects
  * Class provides the general features
  * Object provides the specifics
* We need to know how to:
  * Tell an object to do something
  * Learn the capabilities of an object
  * Learn how to refer to objects
  * Learn how to make objects
  * Learn how to make objects interact
  * Learn how to define new classes in terms of other classes
  * Define classes
  * Also: Where does the program start?

On to Java
* Java is a popular object-oriented programming language
* Developed at Sun microsystems
  * Designed for large-scale programming
  * Designed for embedded devices
* Ugly syntax, based on the C programming language

Important Java ideas for this course:
* The source code for each class usually gets its own file, named ClassName.java
  * Names follow the "start with capital letter, capitalize each word
* Related classes are shoved into packages
* Package names are usually multi-part with the parts separated by dots
  * Multipart names allow us to distingush different packakges
     greenbe1.hw2
     kellylor.hw2
  * Official standard looks a lot more like backwards domain names
     edu.grinnell.cs.rebelsky.cs152.2005s.hw2
* On our linux systems, the names correspond to directories
  edu.grinnell.cs.rebelsky should be in
     rebelsky
     within cs
     within grinnell
     within edu
     within SOME SPECIFIED STARTING POINT


	package rebelsky.introjava;
LOOK, we're working in package rebelsky.introjava
Form: package name.of.package SEMICOLON

	public class First
WE'RE DEFINING A CLASS, NAMED First, anyone can use it
{ START OF CLASS DEFINITION

  public static void main(String[] args)
    throws Exception
  {

In Java, non-parameter naming is typically a two-step process
* "Declare" the name - Say what kind of things we name
  KindOfThing name;
* "Assign" the name - Say what it names
  name = ...

    java.io.PrintWriter pen;
    pen = new java.io.PrintWriter(System.out, true);
new ClassName creates a new object
    java.lang.String greeting;
    greeting = "Hello";
    pen.println(greeting);

EVERY LINE MUST BE TERMINATED BY A SEMICOLON

  } // main(String[])
} // class First END OF CLASS DEFINITION

To tell an object to do soemthing
	name DOT thingtodo OPENPAREN params CLOSEPAREN
"fred simplify yourself"
	fred.simplify();
"x, add yourself to y"
	x.add(y);

Awful Java convention: Programs start with a method named main which *must* have the form above

  public static void main(String[] args)
    throws Exception

How do we get this damn thing to work, anyway?
* First, translate .java to .class
	/opt/jdk1.5.0/bin/javac ClassName.java
* Next, "execute" the appropriate class
	/opt/jdk1.5.0/bin/java package.name.ClassName