Design Patterns Java™ Workbook

Appendix B. Solutions

Observer (Chapter 9).................................................................................................................................

SOLUTION 9.1.......................................................................................................................................

One solution is:

public JSlider slider()
{
if (slider == null)
{
slider = new JSlider();
sliderMax = slider.getMaximum();
sliderMin = slider.getMinimum();
slider.addChangeListener(this);
slider.setValue(slider.getMinimum());
}
return slider;
}

public void stateChanged(ChangeEvent e)
{
double val = slider.getValue();
double tp = (val - sliderMin) / (sliderMax - sliderMin);
burnPanel().setTPeak(tp);
thrustPanel().setTPeak(tp);
valueLabel().setText("" + tp);
}

SOLUTION 9.2.......................................................................................................................................

One solution is shown in Figure B.9. To allow a label to register for slider events, the design
in Figure B.9 creates a subclass of JLabel that implements ChangeListener. The new
design lets the components that depend on the slider register their interest and update
themselves. This is arguably an improvement, but we will refactor the design again, to a
model/view/control architecture.