Announcements:

You can get to the course Web site through my home page
The lecture notes and supplementary notes are on the course site
Notes are updated by lecture time
- (subsequent notes are useful but not "official" until modernized for this semester)
You will need the official course textbook as well as some book on Java that will answer your questions
- Note that the Sun Web site has an online reference Web site

Important! There is a Java hello world exercise due next class
- (let's glance at it)

Today's topic: introduction to Java

History of Java

Long, long ago, there was BCPL (created at Bell Labs)

Then, there was B

Then, there was C (approx. 1978)

Unix was written in C

C++ added the important object paradigm to C

C++ is not a particularly nice language

...pointers and other complexities abound

May 1995 - Sun announces Java

Java is "C++ ++"

Java keeps the good things and changes the bad in C++

Example: Java hides pointers

Example: Java has a String data structure

Example: Java defines graphics components

"What's next?"

JavaScript?

Java continues to increase in popularity

I predict that the advantages of Java mean...

Java won't go away any time soon

Java will go away (mostly) eventually

A "Hello World" Java Program

1. Requirements phase:

Print a simple string to the screen in a Java program

2. Specifications phase:

program specs subphase:
- Write a program that prints "Welcome to CPRE 486." to the screen.
project plan specs subphase:

3. Design phase:

It will be small
It will have one "method" ("function")

4. Implementation phase:

Here it is!

import java.lang.*;
public class HelloWorld 
   {
   public static void main

     (String [ ] args)
     {
     System.out.println

        ("Welcome to CPRE 486.");
     }
   }

First, save it in a file HelloWorld.java

File name must correspond to a class name in the file!

Compile, using command
javac HelloWorld.java

javac for "java compiler"
You must type the .java suffix
Reason: javac knows other suffixes so you must say which
Compilation produces HelloWorld.class

To run, type
java HelloWorld, or
jdb HelloWorld to run it in the debugger
- Don't type the suffix this time!
  Because it won't work!
  Reason: You're supposed to give the class name, not the file name
  
  Java knows to add the .class suffix to get the file name
  
  (If you typed HelloWorld.class, Java would then look for HelloWorld.class.class - bad news!)

Analyzing HelloWorld.java

Here it is again:


/*1*/ import java.lang.*;
/*2*/
/*3*/ class HelloWorld 
/*4*/   {
/*5*/   public static void main

                 (String args[])
/*6*/     {
/*7*/     System.out.println

             ("Welcome to CPRE 486");
/*8*/     }
/*9*/   }

Let's start analyzing . . .

/*1*/ import java.lang.*

Many calls are in libraries

To use something in a library, import it

One library is called java.lang

System.out.println() is in java.lang

What happens if you forget to import it?

-Usually: java won't be able to find a call you are using

- for java.lang only:

it will load automagically

/*2*/

(A blank line)

Like C, Java ignores blanks whenever possible

(Which ones could we remove? Where could be add more?)

/*3*/ class HelloWorld

Note: OOPS knowledge is assumed in this course

class bundles data and instructions

(A struct type in C only bundles data)

(A struct type in C++ bundles data and instructions)

(A C++ class bundles data and instructions like a Java class)

A Java class is very like a C++ class

A .java source file is named after the main class in it

Example: the class name HelloWorld is stored in the file HelloWorld.java

What happens if you put it in another file?

Won't work! Error message. Can't find class

Why? To run a java program, say the class name

HelloWorld

Note it is "mnemonic"

(What does "mnemonic" mean?)

Note famous book, Mind of a Mnemonist

Mnemonic variable names are an important software engineering technique!

They typify the concept of self-documenting code

Self-documenting code has at least one major advantage over comments...

what?

C tradition: underbars (e.g. hello_world)

Java "tradition": use caps (e.g. HelloWorld)

Which is better? Why?

/*4*/ {

Opens a block of code

Like in C/C++, Pascal, etc.

Can have variables and methods/functions in this block

Best to indent 3 spaces???

2 & 4 spaces were found best for Pascal

3 was not tested

3 is between 2 and 4

... why indent at all?

/*5*/ public static void main(String args[])

public means visible outside the class

static makes it present in memory during the entire time the program is running, so it is there when the computer wants to run it

void...main...(...String...args[]...)...

/*6*/ {

Opening curly brace starts a block of code
...in this case, the code of the main( ) method/function

/*7*/ System.out.println("Hello World!");

The System module has a locally defined data structure out containing a println( ) routine ("method" in Java, "function" in C)
. . . simply prints to the screen

/*8*/ }

Closing curly brace matches an opening curly brace
. . . in this case, ends the block
. . . also the body of the method/function

/*9*/ }

. . . ends the class definition
. . . this class is very simple
. . . it only has one method/function, and no variables

A HelloWorld.java with a GUI

import java.lang.*;

import javax.swing.JOptionPane;

public class Hello{

public static void main(String [ ] args){

JOptionPane.showMessageDialog(null, "Welcome to CPRE 486");

System.exit(0);

}

Some key differences:

import javax.swing.JOptionPane;

gets an object, JOptionPane, from the javax.swing library
(in contrast, we got everything from the java.lang library)

JOptionPane.showMessageDialog(null, "Welcome to CPRE 486");

call showMessageDialog(...), a method of object JOptionPane
when run, this brings up a dialog box
(you click "ok" to get rid of it)

System.exit(0);

Causes the program to exit
Needed when it brings up a GUI
(Because the GUI has a life of its own, and must be explicitly exited)
...main( ) ends, but the GUI is still running until exited