Intro to Java tan.tplt 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