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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
The
observer pattern
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
<A NAME="Index2935"></A><A NAME="Index2936"></A>observer
pattern solves a fairly common problem: What if a group of objects needs to
update themselves when some object changes state? This can be seen in the
&#8220;model-view&#8221; aspect of Smalltalk&#8217;s MVC
(model-view-controller), or the almost-equivalent &#8220;Document-View
Architecture.&#8221; Suppose that you have some data (the
&#8220;document&#8221;) and more than one view, say a plot and a textual view.
When you change the data, the two views must know to update themselves, and
that&#8217;s what the observer facilitates. It&#8217;s a common enough problem
that its solution has been made a part of the standard 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>java.util</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
library.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There
are two types of objects used to implement the observer pattern in Java. The <A NAME="Index2937"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class keeps track of everybody who wants to be informed when a change happens,
whether the &#8220;state&#8221; has changed or not. When someone says
&#8220;OK, everybody should check and potentially update themselves,&#8221; the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class performs this task by calling the <A NAME="Index2938"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyObservers(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method for each one on the list. The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyObservers(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is part of the base class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There
are actually two &#8220;things that change&#8221; in the observer pattern: the
quantity of observing objects and the way an update occurs. That is, the
observer pattern allows you to modify both of these without affecting the
surrounding code.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
following example is similar to the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ColorBoxes</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
example from Chapter 14. Boxes are placed in a grid on the screen and each one
is initialized to a random color. In addition, each box 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>implements</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the <A NAME="Index2939"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface and is registered with an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. When you click on a box, all of the other boxes are notified that a
change has been made because the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object automatically calls each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observer
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">object&#8217;s
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>update(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. Inside this method, the box checks to see if it&#8217;s adjacent to the
one that was clicked, and if so it changes its color to match the clicked box.
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: BoxObserver.java</font>
<font color="#009900">// Demonstration of Observer pattern using</font>
<font color="#009900">// Java's built-in observer classes.</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.awt.event.*;
<font color="#0000ff">import</font> java.util.*;

<font color="#009900">// You must inherit a new type of Observable:</font>
<font color="#0000ff">class</font> BoxObservable <font color="#0000ff">extends</font> Observable {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> notifyObservers(Object b) {
    <font color="#009900">// Otherwise it won't propagate changes:</font>
    setChanged();
    <font color="#0000ff">super</font>.notifyObservers(b);
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> BoxObserver <font color="#0000ff">extends</font> Frame {
  Observable notifier = <font color="#0000ff">new</font> BoxObservable();
  <font color="#0000ff">public</font> BoxObserver(<font color="#0000ff">int</font> grid) {
    setTitle("Demonstrates Observer pattern");
    setLayout(<font color="#0000ff">new</font> GridLayout(grid, grid));
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> x = 0; x &lt; grid; x++)
      <font color="#0000ff">for</font>(<font color="#0000ff">int</font> y = 0; y &lt; grid; y++)
        add(<font color="#0000ff">new</font> OCBox(x, y, notifier));
  }   
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#0000ff">int</font> grid = 8;
    <font color="#0000ff">if</font>(args.length &gt; 0)
      grid = Integer.parseInt(args[0]);
    Frame f = <font color="#0000ff">new</font> BoxObserver(grid);
    f.setSize(500, 400);
    f.setVisible(<font color="#0000ff">true</font>);
    f.addWindowListener(
      <font color="#0000ff">new</font> WindowAdapter() {
        <font color="#0000ff">public</font> <font color="#0000ff">void</font> windowClosing(WindowEvent e) {
          System.exit(0);
        }
      });
  }
}

<font color="#0000ff">class</font> OCBox <font color="#0000ff">extends</font> Canvas <font color="#0000ff">implements</font> Observer {
  Observable notifier;
  <font color="#0000ff">int</font> x, y; <font color="#009900">// Locations in grid</font>
  Color cColor = newColor();
  <font color="#0000ff">static</font> <font color="#0000ff">final</font> Color[] colors = { 
    Color.black, Color.blue, Color.cyan, 
    Color.darkGray, Color.gray, Color.green,
    Color.lightGray, Color.magenta, 
    Color.orange, Color.pink, Color.red, 
    Color.white, Color.yellow 
  };
  <font color="#0000ff">static</font> <font color="#0000ff">final</font> Color newColor() {
    <font color="#0000ff">return</font> colors[
      (<font color="#0000ff">int</font>)(Math.random() * colors.length)
    ];
  }
  OCBox(<font color="#0000ff">int</font> x, <font color="#0000ff">int</font> y, Observable notifier) {
    <font color="#0000ff">this</font>.x = x;
    <font color="#0000ff">this</font>.y = y;
    notifier.addObserver(<font color="#0000ff">this</font>);
    <font color="#0000ff">this</font>.notifier = notifier;
    addMouseListener(<font color="#0000ff">new</font> ML());
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> paint(Graphics  g) {
    g.setColor(cColor);
    Dimension s = getSize();
    g.fillRect(0, 0, s.width, s.height);
  }
  <font color="#0000ff">class</font> ML <font color="#0000ff">extends</font> MouseAdapter {
    <font color="#0000ff">public</font> <font color="#0000ff">void</font> mousePressed(MouseEvent e) {
      notifier.notifyObservers(OCBox.<font color="#0000ff">this</font>);
    }
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> update(Observable o, Object arg) {
    OCBox clicked = (OCBox)arg;
    <font color="#0000ff">if</font>(nextTo(clicked)) {
      cColor = clicked.cColor;
      repaint();
    }
  }
  <font color="#0000ff">private</font> <font color="#0000ff">final</font> <font color="#0000ff">boolean</font> nextTo(OCBox b) {
    <font color="#0000ff">return</font> Math.abs(x - b.x) &lt;= 1 &amp;&amp; 
           Math.abs(y - b.y) &lt;= 1;
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
you first look at the online documentation for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it&#8217;s a bit confusing because it appears that you can use an ordinary 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object to manage the updates. But this doesn&#8217;t work; try it &#8211; inside 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BoxObserver</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
create an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object instead of a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BoxObservable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object and see what happens: nothing. To get an effect, you 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>must</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
inherit from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and somewhere in your derived-class code call <A NAME="Index2940"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>setChanged(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is the method that sets the &#8220;changed&#8221; flag, which means that
when you call <A NAME="Index2941"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyObservers(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
all of the observers will, in fact, get notified. In the example above 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>setChanged(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is simply called within 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyObservers(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but you could use any criterion you want to decide when to call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>setChanged(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BoxObserver</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
contains a single 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">object
called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifier</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and every time an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OCBox</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is created, it is tied to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifier</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
In 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OCBox</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
whenever you click the mouse the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyObservers(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is called, passing the clicked object in as an argument so that all the
boxes receiving the message (in their 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>update(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">method)
know who was clicked and can decide whether to change themselves or not. Using
a combination of code in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyObservers(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>update(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
you can work out some fairly complex schemes.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It
might appear that the way the observers are notified must be frozen at compile
time in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyObservers(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. However, if you look more closely at the code above you&#8217;ll see
that the only place in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BoxObserver</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OCBox</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
where you're aware that you&#8217;re working with a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BoxObservable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is at the point of creation of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">object
&#8211; from then on everything uses the basic 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface. This means that you could inherit other 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Observable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
classes and swap them at run-time if you want to change notification behavior
then.
</FONT><a name="_Toc408018798"></a><P></DIV>

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