StatusBar

Un singleton con una JLabel que muestra el estado ("Listo" por defecto), una barra de progreso y un reloj a la derecha.

import javax.swing.JLabel;
import javax.swing.JProgressBar;

import org.jdesktop.swingx.JXStatusBar;

public class StatusBar extends JXStatusBar{

private static StatusBar INSTANCE = new StatusBar();
static final JLabel statusLabel = new JLabel("Listo");
static JProgressBar pbar = new JProgressBar();
static final JLabel hora = new JLabel();

private StatusBar() {
// Exists only to defeat instantiation.
}

public static StatusBar getInstance() {

INSTANCE.setSize(800,20);

JXStatusBar.Constraint c1 = new JXStatusBar.Constraint();
c1.setFixedWidth(200);
INSTANCE.add(statusLabel, c1); // Fixed width of 100 with no inserts

JXStatusBar.Constraint c2 = new JXStatusBar.Constraint(
JXStatusBar.Constraint.ResizeBehavior.FILL); // Fill with no inserts
INSTANCE.add(pbar, c2); // Fill with no inserts - will use remaining space

new Clock(hora);
hora.setSize(100,20);
JXStatusBar.Constraint c3 = new JXStatusBar.Constraint();
c3.setFixedWidth(90);
INSTANCE.add(hora,c3);

return INSTANCE;
}

public void setEstado(String estado){
statusLabel.setText(estado); // Fixed width of 100 with no inserts
INSTANCE.repaint();
}

}

CardLayout

Tres RadioButton que cambian la JLabel.

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BorderFactory;
import javax.swing.BoxLayout;
import javax.swing.ButtonGroup;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JRadioButton;

public class CardLayoutDemo implements ActionListener {
JPanel cards; //a panel that uses CardLayout
final static String[] strings =
{"Component 1",
"Component 2 is so long-winded it makes the container wide",
"Component 3"};

private static JComponent createComponent(String s) {
JLabel l = new JLabel(s);
l.setBorder(BorderFactory.createMatteBorder(5,5,5,5,
Color.DARK_GRAY));
l.setHorizontalAlignment(JLabel.CENTER);
return l;
}

public void addCardsToPane(Container pane) {
JRadioButton[] rb = new JRadioButton[strings.length];
ButtonGroup group = new ButtonGroup();
JPanel buttons = new JPanel();
buttons.setLayout(new BoxLayout(buttons,
BoxLayout.PAGE_AXIS));

for (int i= 0; i < strings.length; i++) {
rb[i] = new JRadioButton("Show component #" + (i+1));
rb[i].setActionCommand(String.valueOf(i));
rb[i].addActionListener(this);
group.add(rb[i]);
buttons.add(rb[i]);
}
rb[0].setSelected(true);

//Create the panel that contains the "cards".
cards = new JPanel(new CardLayout());
for (int i = 0; i < strings.length; i++) {
cards.add(createComponent(strings[i]), String.valueOf(i));
}

pane.add(buttons, BorderLayout.NORTH);
pane.add(cards, BorderLayout.CENTER);
}

public void actionPerformed(ActionEvent evt) {
CardLayout cl = (CardLayout)(cards.getLayout());
cl.show(cards, (String)evt.getActionCommand());
}

public static void main(String[] args) {
JFrame.setDefaultLookAndFeelDecorated(true);

JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

CardLayoutDemo demo = new CardLayoutDemo();
demo.addCardsToPane(frame.getContentPane());

frame.pack();
frame.setVisible(true);
}
}

Singleton

public class ClassicSingleton {
private static ClassicSingleton instance = null;
protected ClassicSingleton() {
// Exists only to defeat instantiation.
}
public static ClassicSingleton getInstance() {
if(instance == null) {
instance = new ClassicSingleton();
}
return instance;
}
}

Notas:

Si dejamos el constructor protected, clases en el mismo paquete podrán crear más de una instancia (sin necesidad de extender al Singleton). Podemos crear un paquete explícito para el Singleton o podemos hacer el constructor private. Si usamos la segunda opción, no se podrán hacer subclases del Singleton; en ese caso es mejor declarar explícitamente que el Singleton es final para temas de optimización en compilación.

Este Singleton no es thread-safe.

¿Opciones para hacerlo thread-safe?

1) Opción conservadora: sincronizar el método getInstance(). Hacer esto es caro.

public synchronized static Singleton getInstance() {
if(singleton == null) {
simulateRandomActivity();
singleton = new Singleton();
}
logger.info("created singleton: " + singleton);
return singleton;
}

2) Opción radical: no permite cambiar en un futuro el número de instancias del Singleton más allá de uno.

public class Singleton {
public final static Singleton INSTANCE = new Singleton();
private Singleton() {
// Exists only to defeat instantiation.
}
}