Using ObjectsChapter 6

“CarCounter page 89 and 91

Local variables declared in a method only temporary

carCount is a local variable good only while the method exists. It is

an instance variable

Instantiation using random class

• Page 93 “RandomLines”

• Private Random randomPositions = new Random( );– xEnd = randomPositions.nextInt (100);– Using the method to generate a number from

the range 0 to 99

The Swing toolkit

• Imported as a Java library

• Components look the same on every platform (JButtons for example)

Events

• Action – like a button

• Change – adjust a slider

• “extends” current class (child) inherits from another class (parent)

Creating a JButton CarCounter extends JFame page 99

• Private JButton button;

• button = new JButton (“Car Enterng”);– Window.add(button);

• When button clicked actionListener called

General Guides for using objects

1. Determine what is to be imported

2. Select a constructor (with “new”) to be used

3. Add window, if needed

4. Use the instance via it’s methods

Page 102 SumTextFields uses JLabel class to display non-

changing text fields

• private JTextField number1Field, number2Field, sumField;

• JTextField class– A single line area to display text or input text– textField1 = new JTextField(15);– textField2 = new JTextField(“Hello!” ,15);– String s;– s = textField1.getText( );– textField1.setText(s);

JPanel class

• Used for drawing, or to hold objects such as buttons

• panel = new JPanel( );– Panel.setPreferedSize(new

Dimension(200,200));– window.add(panel);

Timer class

• Regularly spaced events

• timers, control speeds

• Not visible as Buttons are

TimerExamplepage 105

• Screenshot shows number of seconds

• Using timer.

JSlider“Raindrops” page 107

• private JSlider slider;

• slider = new JSlider(JSlider.HORIZONTAL, 200, 2000, 1000);

• window.add (slider);

• … gapField = new JTextField(10);

• gapField.setText(Integer.toString

• ( slider.getValue( ) ) );

Summary

• The Java language has a vast number of classes which you can (and ought to) use.

• Do not write your own code without investigating the libraries first.

• You may check http://www.java.sun.com and search tutorials and references about the various classes