CS 151 Day 9

Cover page image

Cay S. Horstmann

Day 9 Clicker Question 1

An input stream reads bytes from a file. This is an example of which pattern?

  1. Observer
  2. Iterator
  3. Stream
  4. Blob

Day 9 Clicker Question 2

You attach action listeners to a JButton. In the terminology of the OBSERVER pattern, the JButton is a(n)

  1. Subject
  2. Observer
  3. Concrete observer
  4. Model

Day 9 Clicker Question 3

Consider a program that contains two frames, one with a column of text fields containing numbers, and another that draws a bar graph showing the values of the numbers. When the user edits one of the numbers, the graph should be redrawn.

Which of the following statements are true? Select all that apply.

  1. The graph should be an observer of the text fields.
  2. The model consists of a list of numbers.
  3. Observers should be attached to the text fields.
  4. An observer should be attached to the model.

Lab

lab

Pattern Discussion

Sliders

  1. Put this SliderDemo.java file into your lab9 directory and make a project in that directory. Run the program
  2. A slider is a user interface component that allows a user to specify a continuum of values. To be notified of slider movement, you need to attach a class that implements the ChangeListener interface type. Read the API documentation for JSlider and ChangeListener. Make a table of pattern names and actual names that shows how this is a manifestation of the observer pattern.

Not a Pattern Match

  1. Someone tells you that an Iterator is actually an observer that notifies you whenever the next value is available. Is it? Why or why not?
  2. Someone tells you that a ChangeListener is actually an iterator through a sequence of JSlider values. Show why the Iterator pattern doesn't apply.
  3. Let's make a SliderIterator that actually iterates over the slider values. Make it so that when the slider value changes, the slider value is inserted to a queue. The next method takes the next value (blocking if the queue is empty). 
    public class SliderIterator implements Iterator<Integer>
{
   public SliderIterator(JSlider slider)
   {
      slider.addChangeListener(event -> {
         JSlider source = (JSlider) event.getSource();
         int value = source.getValue();
         ...
      });
   }
   
   public boolean hasNext()
   {
      return true;
   }
   
   public Integer next()
   {      
      try
      {
         return ...
      } 
      catch (InterruptedException ex)
      {
         return null;
      }
   }
   
   public void remove()
   {
      throw new UnsupportedOperationException();
   }
   
   private BlockingQueue<Integer> queue = new LinkedBlockingQueue<>();
}
  4. Then add such an iterator to the end of main: 
    Iterator<Integer> iter = new SliderIterator(frame.getSlider(0));
while (iter.hasNext()) {
   System.out.println(iter.next());
}
    Run the program. What happens when you adjust the first slider?
  5. What is the drawback of using an iterator in this situation?

Discussion

discussion