Java

Basics

Compile Java.
javac <class-name>.java
Compiling with assert
javac -source 1.4 -classpath . *.java
Run java with
java <class-name>
Including the class path so Java can find the source files
javac -classpath . Counter.java Counter2.java CounterTest.java
To see the top error messages
javac <class-name>.java 2>&1 | head

Console reading
Get a string
import java.io.*;
...
    BufferedReader br = new BufferedReader( new InputStreamReader(System.in));
    String s = br.readLine()
   
Menu choice
System.out.println(" 1: add elements to list and print ");
System.out.println(" 2: add some elements in order to print ");
BufferedReader in=new BufferedReader(
  new InputStreamReader(System.in));
int ch;
try
  { ch = in.read(); }
catch (IOException e)
  { System.out.println("error: unable to read selection."); return; }
switch (ch)
{
  case '1': ...
    break;
  default: ...
}
Read an integer
BufferedReader stdin = new BufferedReader(
  new InputStreamReader(System.in) );
String msg = stdin.readLine();
int elem = Integer.parseInt(msg);
Compare strings   String("a").equals("a")

Perform a cast.
void castit(Object x)
{
  // If x is null this code will not be executed.
  if (x instanceof Address)
  {
    Address a = (Address)x;
    System.out.println(a);
  }
}

arrays - create one and then fill it
players = new Player[this.nPlayers];
for (int i=0; i<this.nPlayers; ++i)
  players[i] = new Player();


• debug error messages
  console System.out.println("error: ..."); gui JoptionPane.showMessageDialog(null,"error: message");
• Whenever you program Java you need their API documentation. Downloaded Sun's Java doc contained within their SDK.
• exceptions: convention: use most specific exception. Alternative: use context eg wrap a call in a catchall exception.
• Building a gui app: first always build a console app to separate the engine from the gui.
• Never use nested classes.
• Serialize with strings so object written to and read though strings alone. ie string processing alone, do not use Java's serialization!
• Generally use Java's fixed arrays and not dynamic Array for performance.

Callbacks

These are functions which are called after an event is triggered. In Java there is a tendency to use callbacks in unclean ways. Through inheritance, handling of multiple callbacks with the one handler, a multitude of callback mechanisms ... There are arguments for efficiency but now with todays computers there should be no need for multiple callbacks to be handled by the one handler.

Here is a technique that directly maps the callback to a function. Put this within your class. This furthers OO design by making your program neat and generic.

In this example I avoided inheriting ActionListener by creating an temporary class - I do not know the technical name for this. If I wished to design a gui with many buttons they can all be linked to individual function calls.

Notice that both import statements are necessary, I do not know why only that its another Java bug.

import java.awt.*;
import java.awt.event.*;
...
  JTextField enterFileName;
...
  // The function which is called back and local to the class.
  public void fileRead(String s)
...

    enterFileName.addActionListener
    (
      new ActionListener()
      {
        public void actionPerformed(ActionEvent e)
        {
          String s = e.getActionCommand();
          fileRead(s);
        }
      }
    );

See ThreeButtons.java - Inheritance for callbacks with inheritance and the old style of handling callbacks. What I believe is the better way - less error prone, simpler and the action is exactly matched with the action code. See ThreeButtons2.java - direct. What I like most is that the direct method is more object orientated - the actions in the inheritance are often all implemented in the handler and not as separate function calls as with the direct way.

JList

Vector v3 = new Vector(); ...  // Populate with strings
JList xList = new JList(v3); // Initialize 
wind.add(xList,BorderLayout.CENTER);  // Add to window
int indx = xList.getSelectedIndex();  // Get the users choice

// Add the callback
xList.addListSelectionListener
(
  new ListSelectionListener ()
  {
    public void valueChanged(ListSelectionEvent e)
    { transSelectInstrument(); }
  }
);

GUI

The idea of the gui is its best to allow the user to resize or change everything and still have it looking good. This means avoiding certain classes and using JPane components. For example JTextArea with JScrollPane does not resize properly, so use JTextPane instead.

Application and Applet

An obvious question when programming Java is that if we write an application how can the application be written the same in an applet. To be more specific how can we code so that the application and applet use the same code.

There are going to be some restrictions as the applet has less file access rights, but these can be granted.

Sticking to Swing's JFrame and JApplet the same Gui code can be used. Indeed if everything is in the default constructor then the only changes that need to be made are inheriting a JApplet instead of JFrame. In applets exclude some gui code.

For the applet Java documentation recommends overloading init(). eg placing thread code in here as the object has already been constructed. Alot of people take this recommendation literally and put code that should be in the default constructor and place it here too. Since Java already provides a default init() function that does nothing, you can even get away with the thread calls also being in the constructor - even though an object is not fully constructed starting a runnable thread at the end of the constructor is ok because you have already initialized everything this code is always going to work.

To include Java in a HTML document use can use the applet tag. Class GUIExam2 has been compiled to GUIExam2.class

<applet code="GUIExam2.class" height="300" width="300">
</applet>

JTextPane

JTextPane textpane = new JTextPane();
...
//Append
String msg = ...
textPane.setText( textPane.getText() + mgs );

Just in case you need JTextArea.

JTextArea infoDisp;
JScrollPane infoDispScroll;
...

infoDisp = new JTextArea(10,20);
infoDispScroll = new JScrollPane
( 
  infoDisp,
  JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
  JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS
);

JComboBox

JComboBox(Object[] items) 
JComboBox(Vector items) 

  String combstr[] = { "A","B","C" };
  JComboBox comb = new JComboBox(combstr);

void addActionListener(ActionListener l) 
int getSelectedIndex() 
Object getSelectedItem()

GridLayout

// one row, two columns.
JPanel wind2 = new JPanel(new GridLayout(1,2));
JButton b1 = new JButton("Clear");
wind2.add(b1);
...
setContentPane(wind2);

GridLayout() is one column per component.

JMenuBar

JMenuBar menuBar = new JMenuBar();

JMenu info = new JMenu("Information");
    
// Alt+I to select this menu
info.setMnemonic(KeyEvent.VK_I);      
menuBar.add(info);

info.add(infoCompanyDetails);

JMenuItem infoCompanyDetails = new 
  JMenuItem("Company Details");
infoCompanyDetails.setMnemonic(KeyEvent.VK_D);

// Add the callback
infoCompanyDetails.addActionListener
(
  new ActionListener ()
  {
    public void actionPerformed(ActionEvent e)
      { infoCompanyDetailsAction(); }
  }
);

ButtonGroup

Setting one button turns of all the others.

JPanel buttonPanel = new JPanel();
buttonPanel.setLayout(new BoxLayout(buttonPanel,BoxLayout.X_AXIS));
ButtonGroup buttonGroup = new ButtonGroup();
String[] options = { "a","b","c" };
JRadioButton[] radioButton = new JRadioButton[options.length];
for (int i=0; i<options.length; ++i)
{
  radioButton[i] = new JRadioButton(options[i]);
  buttonPanel.add(radioButton[i]);
  buttonGroup.add(radioButton[i]);

  //shiprb[i].setActionCommand(shippingOptions[i]);
}

//    rb = shiprb[shipChoiceState];
//    rb.setSelected(true);

You would think that turning on a button would be simple with Java but a toggle is anything but simple. Here is code to turn on the button if it is not selected already.

if (paymentButtonInvisible.isSelected()==false)
  paymentButtonInvisible.doClick();

For the radio buttons to have no effect there is no one way to turn them off so Java's doc recommends having an invisible button for the job.

Process Command Line Arguments

  String commandlineGet(String targ, String[] args)
  {
    String s = "";
    for (int i=0; i<args.length; ++i)
    {
      if( args[i].startsWith(targ) )
        s = args[i].substring(targ.length());
    }

    return s;
  }
  

  boolean commandlineProcess( String[] args )
  {
    boolean filenamefound = false;
    
    dataFileName = commandlineGet("data=",args);
    if (dataFileName != "")
      filenamefound = true;

    ...

Gui Get an Integer

class GuiGetInt
{
  public static int get(String s)
  {
    try
    {
      String s2 = 
        JOptionPane.showInputDialog(s);
      
      int k = Integer.parseInt(s2);
      return k;
    }
    catch (Exception e)
    {
      return -1;
    }
  }
}

Code

Gui2.java  

Callback for a Group of Buttons

Not recommended, old style of callbacks suffers from non-uniqueness problem. Instead see Callbacks.

class Keypad extends JPanel implements ActionListener
{
...

  Keypad(...
  
    for(int i = 9; i >= 0; i--)
    {
      p.add(b = new Button("" + (char)('0' + i)));
      b.addActionListener(this);
    }
  ...

  public void actionPerformed(ActionEvent e)  
  {
    String nm = e.getActionCommand();
    //**** READ BELOW
}

The string which the button was given is returned in nm if that button was pressed. Therefore test for that string.

The idea is a group of buttons are treaded as on and all calls or presses of these buttons go through the actionPerformed function.

Runtime Binding

Shipping s = null;
try
{
   Class x = Class.forName(file);
   s = (Shipping) x.newInstance();
}
catch( Exception e)
{
   s = null;
}


file   is the name of the file without the .class extension. You could make the code cleaner.