Buttons

Appearance of buttons A button has one of three appearances: Disabled by your program Enabled by your program Enabled by your program and pressed by the user

A style rule for buttons Users expect buttons to do things When the user clicks a button, there should be a visible change in the display –If there isn’t, the user wonders “Did the program recognize my button click?” This is an application of the Principle of Least Surprise: A program should surprise the user as little as possible. –In other words: a program should behave the way the user expects it to behave

Calculator example In the Calculator program, every button click changes the display –Almost: Clicking the Clear button repeatedly leaves the display at 0 (duh!) Clicking a base button ( Oct, Dec, Hex ) changes the number in the display –Unless it’s a small number (less or equal to base) –Even then, some buttons are enabled/disabled –This is a subtle change that might not be noticed –It would be better to display Oct / Dec / Hex somewhere

Button constructors import java.awt.*; new Button() –Constructs a button with no label new Button(String label ) –Constructs a button with the given label

Placing buttons with FlowLayout setLayout(new FlowLayout( )); Button button1 = new Button("Button 1"); add(button1);

Placing buttons with GridLayout setLayout(new GridLayout(2, 3)); Button button1 = new Button("Button 1"); add(button1);

Placing buttons with BorderLayout setLayout(new BorderLayout( )); Button button1 = new Button("Button 1"); add(button1, BorderLayout.NORTH);

Using a layout manager Create a container (usually a Panel ) Send it the message setLayout( layout_manager ) to tell it what kind of layout manager to use Send add messages to the container; the kind of add message depends on the layout manager –If BorderLayout, an extra parameter should be used Example: Panel p = new Panel(); p.setLayout(new GridLayout(3, 4)); p.add(button1);

Adding a listener import java.awt.event.*; button1.addActionListener(new MyButtonListener()); class MyButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { code to execute when button1 is pressed } } MyButtonListener is best implemented as a member class, so that it has full access to the fields of the enclosing class

Adding the same listener to several buttons MyListener listener = new MyButtonListener(); button1.addActionListener(listener); button2.addActionListener(listener); button3.addActionListener(listener); class MyButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { code to execute when button1 is pressed } }

Adding an anonymous listener import java.awt.event.*; button1.addActionListener(new ActionListener(){ public void actionPerformed(ActionEvent e) { code to execute when button1 is pressed }}); An anonymous listener is more convenient if: –The actionPerformed method is short, and –The listener is only used for this one button

Enabling the button A Button is a Component A Component can be enabled or disabled Button inherits this method from Component : public void setEnabled(boolean b) To enable a button: button1.setEnabled(true); To disable a button: button1.setEnabled(false); You will never get an Event from a disabled component

Changing the button’s appearance button.setLabel(String label); –Changes the label on a button button.setBackground(Color color); button.setForeground(Color color); –Changes color of background or text of button –These methods are inherited from Component –May not work on all platforms

Changing the button’s font new Font(String name, int style, int size ) –name: The name of a font. It may be: A font on your system (maybe not on my system), or one of the font types "Serif", "Sans-serif", "Monospaced", "Dialog", and "DialogInput" –style: one of Font.PLAIN, Font.BOLD, Font.ITALIC, or Font.BOLD+Font.ITALIC –size: The point size, such as 10, 12, or 18 button.setFont(Font font ); or panel.setFont(Font font ); // default for contents

Writing the listener To listen for a button click: –Write a class that implements ActionListener –Create an instance of that class –Attach the instance to one or more buttons button1.addActionListener(new MyButtonListener()); To implement ActionListener, you must provide this method: public void actionPerformed(ActionEvent e)

Writing actionPerformed actionPerformed must be public void actionPerformed takes an ActionEvent parameter If the listener is attached to only a single button, you can ignore the ActionEvent If the listener is attached to several buttons, you can use the ActionEvent parameter to discover which button was pressed

Examining an ActionEvent public void actionPerformed(ActionEvent e) An ActionEvent is an EventObject –it inherits a method public Object getSource() –getSource() returns the Object that caused the event –So: if (e.getSource() == button1) {...} Alternatively, an ActionEvent has a method public String getActionCommand() that (for a Button ) returns the label on the button –So: if (e.getActionCommand().equals("Button 1")) {...}

Doing something You define what the button does in your button listener If the listener is an instance of a member class or an anonymous class, you have full access to the enclosing class If the listener is an external class, it may be harder to access the information you need Good luck!

The End