tij309.htm

这也是我们java老师给我们的thinking in java的一些资料

  <font color=#0000ff>public</font> String what() { <font color=#0000ff>return</font> <font color=#004488>"Wind"</font>; }
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> adjust() {}
}

<font color=#0000ff>class</font> Percussion <font color=#0000ff>extends</font> Instrument {
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> play(Note n) {
    System.out.println(<font color=#004488>"Percussion.play() "</font> + n);
  }
  <font color=#0000ff>public</font> String what() { <font color=#0000ff>return</font> <font color=#004488>"Percussion"</font>; }
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> adjust() {}
}

<font color=#0000ff>class</font> Stringed <font color=#0000ff>extends</font> Instrument {
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> play(Note n) {
    System.out.println(<font color=#004488>"Stringed.play() "</font> + n);
  }
  <font color=#0000ff>public</font> String what() { <font color=#0000ff>return</font> <font color=#004488>"Stringed"</font>; }
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> adjust() {}
}

<font color=#0000ff>class</font> Brass <font color=#0000ff>extends</font> Wind {
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> play(Note n) {
    System.out.println(<font color=#004488>"Brass.play() "</font> + n);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> adjust() {
    System.out.println(<font color=#004488>"Brass.adjust()"</font>);
  }
}

<font color=#0000ff>class</font> Woodwind <font color=#0000ff>extends</font> Wind {
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> play(Note n) {
    System.out.println(<font color=#004488>"Woodwind.play() "</font> + n);
  }
  <font color=#0000ff>public</font> String what() { <font color=#0000ff>return</font> <font color=#004488>"Woodwind"</font>; }
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Music4 {
  <font color=#0000ff>private</font> <font color=#0000ff>static</font> Test monitor = <font color=#0000ff>new</font> Test();
  <font color=#009900>// Doesn't care about type, so new types</font>
  <font color=#009900>// added to the system still work right:</font>
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> tune(Instrument i) {
    <font color=#009900>// ...</font>
    i.play(Note.MIDDLE_C);
  }
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> tuneAll(Instrument[] e) {
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; e.length; i++)
      tune(e[i]);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    <font color=#009900>// Upcasting during addition to the array:</font>
    Instrument[] orchestra = {
      <font color=#0000ff>new</font> Wind(),
      <font color=#0000ff>new</font> Percussion(),
      <font color=#0000ff>new</font> Stringed(),
      <font color=#0000ff>new</font> Brass(),
      <font color=#0000ff>new</font> Woodwind()
    };
    tuneAll(orchestra);
    monitor.expect(<font color=#0000ff>new</font> String[] {
      <font color=#004488>"Wind.play() Middle C"</font>,
      <font color=#004488>"Percussion.play() Middle C"</font>,
      <font color=#004488>"Stringed.play() Middle C"</font>,
      <font color=#004488>"Brass.play() Middle C"</font>,
      <font color=#004488>"Woodwind.play() Middle C"</font>
    });
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>You can see that there&#146;s really no change except in the base class. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1054" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>It&#146;s helpful to create <b>abstract </b>classes and methods because they make the abstractness of a class explicit, and tell both the user and the compiler how it was intended to be used. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1055" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h2>
<a name="_Toc375545338"></a><a name="_Toc24775664"></a><a name="Heading6439"></a>Constructors
and polymorphism</h2>
<p>As usual, constructors are different from other kinds of methods. This is also true when polymorphism is involved. Even though constructors are not polymorphic (they&#146;re actually <a name="Index649"></a><a name="Index650"></a><b>static</b> methods, but the <b>static</b> declaration is implicit), it&#146;s important to understand the way constructors work in complex hierarchies and with polymorphism. This understanding will help you avoid unpleasant entanglements. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1056" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc375545339"></a><a name="_Toc24775665"></a><a name="Heading6441"></a>Order
of constructor calls<br></h3>
<p><a name="Index651"></a><a name="Index652"></a>The order of constructor calls was briefly discussed in Chapter 4 and again in Chapter 6, but that was before polymorphism was introduced. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1057" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>A constructor for the base class is always called during the construction process for a derived class, chaining up the inheritance hierarchy so that a constructor for every base class is called. This makes sense because the constructor has a special job: to see that the object is built properly. A derived class has access to its own members only, and not to those of the base class (whose members are typically <b>private</b>). Only the base-class constructor has the proper knowledge and access to initialize its own elements. Therefore, it&#146;s essential that all constructors get called, otherwise the entire object wouldn&#146;t be constructed. That&#146;s why the compiler enforces a constructor call for every portion of a derived class. It will silently call the default constructor if you don&#146;t explicitly call a base-class constructor in the derived-class constructor body. If there is no default constructor, the compiler will complain. (In the case where a class has no constructors, the compiler will automatically synthesize a default constructor.) <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1058" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>Let&#146;s take a look at an example that shows the effects of composition, inheritance, and polymorphism on the order of construction:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c07:Sandwich.java</font>
<font color=#009900>// Order of constructor calls.</font>
<font color=#0000ff>package</font> c07;
<font color=#0000ff>import</font> com.bruceeckel.simpletest.*;

<font color=#0000ff>class</font> Meal {
  Meal() { System.out.println(<font color=#004488>"Meal()"</font>); }
}

<font color=#0000ff>class</font> Bread {
  Bread() { System.out.println(<font color=#004488>"Bread()"</font>); }
}

<font color=#0000ff>class</font> Cheese {
  Cheese() { System.out.println(<font color=#004488>"Cheese()"</font>); }
}

<font color=#0000ff>class</font> Lettuce {
  Lettuce() { System.out.println(<font color=#004488>"Lettuce()"</font>); }
}

<font color=#0000ff>class</font> Lunch <font color=#0000ff>extends</font> Meal {
  Lunch() { System.out.println(<font color=#004488>"Lunch()"</font>); }
}

<font color=#0000ff>class</font> PortableLunch <font color=#0000ff>extends</font> Lunch {
  PortableLunch() { System.out.println(<font color=#004488>"PortableLunch()"</font>);}
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Sandwich <font color=#0000ff>extends</font> PortableLunch {
  <font color=#0000ff>private</font> <font color=#0000ff>static</font> Test monitor = <font color=#0000ff>new</font> Test();
  <font color=#0000ff>private</font> Bread b = <font color=#0000ff>new</font> Bread();
  <font color=#0000ff>private</font> Cheese c = <font color=#0000ff>new</font> Cheese();
  <font color=#0000ff>private</font> Lettuce l = <font color=#0000ff>new</font> Lettuce();
  <font color=#0000ff>public</font> Sandwich() {
    System.out.println(<font color=#004488>"Sandwich()"</font>);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    <font color=#0000ff>new</font> Sandwich();
    monitor.expect(<font color=#0000ff>new</font> String[] {
      <font color=#004488>"Meal()"</font>,
      <font color=#004488>"Lunch()"</font>,
      <font color=#004488>"PortableLunch()"</font>,
      <font color=#004488>"Bread()"</font>,
      <font color=#004488>"Cheese()"</font>,
      <font color=#004488>"Lettuce()"</font>,
      <font color=#004488>"Sandwich()"</font>
    });
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>This example creates a complex class out of other classes, and each class has a constructor that announces itself. The important class is <b>Sandwich</b>, which reflects three levels of inheritance (four, if you count the implicit inheritance from <b>Object</b>) and three member objects. You can see the output when a <b>Sandwich</b> object is created in <b>main(&#160;)</b>. This means that the order of constructor calls for a complex object is as follows: <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1059" title="Send BackTalk Comment">Feedback</a></font><br></p>
<ol>
<li><a name="Index653"></a><a name="Index654"></a><a name="Index655"></a><a name="Index656"></a>The
base-class constructor is called. This step is repeated recursively such that
the root of the hierarchy is constructed first, followed by the next-derived
class, etc., until the most-derived class is reached. <font size="-2"><a
href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1060" title="Send BackTalk
Comment">Feedback</a></font></li>
<li>Member initializers are called in the order of declaration. <font
size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap07_1061"
title="Send BackTalk Comment">Feedback</a></font></li>
<li>The body of the derived-class constructor is called. <font size="-2"><a