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
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