outline

Why Java?

• Multi-platform
• OOP to the core
• Compiled AND interpreted
• Client-server
• Elegant
• Extensible

The Basic Tools

• JDK
• javac
• java (JRE)
• text editor
• IDEs

Java contexts

• 'standalone' app
• client - side applet
• server
• servelet
• jsp

The old standard - Hello world

• Code:

• 
  //Hi
  //Hello world on command line
  
  public class Hi{
  
    public static void main(String args[]){
      System.out.println("Hello world!!");
    } // end main
  
  } // end Hi
  

discussion of hello world

• // comment character
• /* this is also a comment */
• use in standard ways - especially when getting started

public static void main

• public - this code will be callable from anywhere
• static - It does not require a previous instance (ignore for now)
• void - this procedure will NOT return a value
• main - this is the function that should automatically be called

(String args[])

• This program is capable of accepting a parameter
• It is expecting an array of Strings
• Capitalization matters.
• We'll see an example shortly

System.out.println("Hello World!!");

• There's no such thing as a command!!
• println is a method
• system is an object
• out is another object
• We are invoking the println method
• ...of the out object
• ...which is a property of the System object

} // end main

• End the main function
• note that in java, the } ends a lot of different things
• comments are very helpful
• indentation is also important

} // end class

• this ends the entire class definition
• Once this is written, we'll save the text file

compiling

• The text is not runnable directly
• We'll compile it into bytecode
• This is a binary code for a virtual machine language
• Not quite as efficient as native machine languages
• use 'javac Hi.java' from command line
• no news is good news

running

• virtual machine is why java is platform independant
• each platform has a java virtual machine
• the vm is written in platform - specific code
• it interprets from java bytecode to local machine language
• use 'java Hi' to run the program

Hello world with command line input:

• //Hi
  //Hello world on command line
  
  public class Hi{
    public static void main(String args[]){
      System.out.println("Hello world!!");
        System.out.println("...and a special hello to " + args[0]);
    } // end main
  } // end Hi
  

how the parameter works...

• args is an array of Strings
• Strings are text values
• an array is a list with a numeric index
• counting starts at zero
• so, args[0] is the first (well, zeroth) element
• it will contain the command line parameters
• (space delimeted, like in C)

String concatenation

• "...and a special hello to " is a String LITERAL
• args[0] is a String VALUE
• the + sign can attach two strings
• note the trailing space

using a condition

• 
  //Hi
  //Hello world on command line
  
  public class Hi{
    public static void main(String args[]){
      System.out.println("Hello world!!");
      if (args.length >0){
        System.out.println("...and a special hello to " + args[0]);
      } // end if
    } // end main
  } // end Hi
  

Why a condition here?

• if there is no parameter, we will get an error
• we should only give a special hello if a paramater was entered
• check the array's length
• length is a property of arrays

if statement

• 
  if (condition){
    ...code...
    ...code...
    ...code...
  } // end if
  

conditions

• statement comparing value to variable
• or variable to variable
• == equal to
• != not equal to
• < less than
• <= less than or equal to
• > greater than
• >= greater than or equal to

more on if statements

• condition must go in parentheses
• true expression will go in { } pair

if - else structure

• 
  //Hi
  

• //Hello world on command line
  
• public class Hi{
  public static void main(String args[]){
  System.out.println("Hello world!!");
  if (args.length >0){
  System.out.println("...and a special hello to " + args[0]);
  } else {
  System.out.println("...Howdy, stranger!");
  } // end if
  } // end main
  } // end Hi
  

Simple GUI

• 
  //Hi with a GUI interface
  
  import java.awt.*;
  
  public class GuiHi1 extends Frame{
    public static void main(String args[]){
      GuiHi1 gh = new GuiHi1();
    } // end main
  
    GuiHi1(){
      super("Hi");
      this.show();
      this.setSize(200,200);
    }// end constructor
  } // end class def
  

Adding components

• //Hi with a GUI interface
  
  import java.awt.*;
  
  public class GuiHi2 extends Frame{
  public static void main(String args[]){
  GuiHi2 gh = new GuiHi2();
  } // end main
  
  GuiHi2(){
  super("Hi");
  this.setLayout(new FlowLayout());
  this.add(new Label("Hello there!"));
  this.add(new Button("OK"));
  this.show();
  this.setSize(200,200);
  }// end constructor
  } // end class def

Event Handling

• //Hi with a GUI interface
  
  import java.awt.*;
  import java.awt.event.*;
  
  public class GuiHi3 extends Frame implements ActionListener{
  public static void main(String args[]){
  GuiHi3 gh = new GuiHi3();
  } // end main
  
  GuiHi3(){
  super("Hi");
  
  Button btnOk = new Button("OK");
  Label lblHi = new Label("Hi there");
  
  btnOk.addActionListener(this);
  this.setLayout(new FlowLayout());
  this.add(lblHi);
  this.add(btnOk);
  this.show();
  this.setSize(200,200);
  }// end constructor
  
  public void actionPerformed(ActionEvent e){
  System.exit(0);
  } // end actionPerformed
  
  } // end class def

GUI input and output

• //Hi with a GUI interface
  
  import java.awt.*;
  import java.awt.event.*;
  
  public class GuiHi4 extends Frame implements ActionListener{
  
  Button btnHi = new Button("Say Hi");
  Button btnExit = new Button("Exit");
  Label lblHi = new Label("Hi there");
  TextField txtIn = new TextField("");
  
  
  public static void main(String args[]){
  GuiHi4 gh = new GuiHi4();
  } // end main
  
  GuiHi4(){
  super("Hi");
  
  
  btnHi.addActionListener(this);
  btnExit.addActionListener(this);
  
  this.setLayout(new GridLayout(0,1));
  this.add(txtIn);
  this.add(btnHi);
  this.add(lblHi);
  this.add(btnExit);
  this.setSize(200,200);
  this.show();
  }// end constructor
  
  public void actionPerformed(ActionEvent e){
  String theCommand = e.getActionCommand();
  if (theCommand.equals("Say Hi")){
  lblHi.setText("Hi, " + txtIn.getText() + "!");
  } else {
  System.exit(0);
  } // end if
  } // end actionPerformed
  
  } // end class def