tij319.htm

这也是我们java老师给我们的thinking in java的一些资料

    });
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p><b>Int3</b> is inherited from <b>Int2</b>, and a new primitive member, <b>int j</b>,<b> </b>is added. You might think that you&#146;d need to override <b>clone(&#160;)</b> again to make sure <b>j</b> is copied, but that&#146;s not the case. When <b>Int2</b>&#146;s <b>clone(&#160;)</b> is called as <b>Int3</b>&#146;s <b>clone(&#160;)</b>, it calls <b>Object.clone(&#160;),</b> which determines that it&#146;s working with an <b>Int3</b> and duplicates all the bits in the <b>Int3</b>. As long as you don&#146;t add references that need to be cloned, the one call to <b>Object.clone(&#160;)</b> performs all of the necessary duplication regardless of how far down in the hierarchy <b>clone(&#160;)</b> is defined. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2732" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>You can see what&#146;s necessary in order to do a deep copy of an <b>ArrayList</b>: After the <b>ArrayList</b> is cloned, you have to step through and clone each one of the objects pointed to by the <b>ArrayList</b>. You&#146;d have to do something similar to this to do a deep copy of a <b>HashMap</b>. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2733" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>The remainder of the example shows that the cloning did happen by showing that, once an object is cloned, you can change it, and the original object is left untouched. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2734" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc375545429"></a><a name="_Toc24775999"></a><a name="Heading24970"></a>Deep
copy via serialization</h3>
<p>When you consider Java&#146;s object serialization (introduced in Chapter 12), you might observe that an object that&#146;s serialized and then deserialized is, in effect, cloned. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2735" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>So why not use serialization to perform deep copying? Here&#146;s an example that compares the two approaches by timing them:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: appendixa:Compete.java</font>
<font color=#0000ff>import</font> java.io.*;

<font color=#0000ff>class</font> Thing1 <font color=#0000ff>implements</font> Serializable {}
<font color=#0000ff>class</font> Thing2 <font color=#0000ff>implements</font> Serializable {
  Thing1 o1 = <font color=#0000ff>new</font> Thing1();
}

<font color=#0000ff>class</font> Thing3 <font color=#0000ff>implements</font> Cloneable {
  <font color=#0000ff>public</font> Object clone() {
    Object o = <font color=#0000ff>null</font>;
    <font color=#0000ff>try</font> {
      o = <font color=#0000ff>super</font>.clone();
    } <font color=#0000ff>catch</font>(CloneNotSupportedException e) {
      System.err.println(<font color=#004488>"Thing3 can't clone"</font>);
    }
    <font color=#0000ff>return</font> o;
  }
}

<font color=#0000ff>class</font> Thing4 <font color=#0000ff>implements</font> Cloneable {
  <font color=#0000ff>private</font> Thing3 o3 = <font color=#0000ff>new</font> Thing3();
  <font color=#0000ff>public</font> Object clone() {
    Thing4 o = <font color=#0000ff>null</font>;
    <font color=#0000ff>try</font> {
      o = (Thing4)<font color=#0000ff>super</font>.clone();
    } <font color=#0000ff>catch</font>(CloneNotSupportedException e) {
      System.err.println(<font color=#004488>"Thing4 can't clone"</font>);
    }
    <font color=#009900>// Clone the field, too:</font>
    o.o3 = (Thing3)o3.clone();
    <font color=#0000ff>return</font> o;
  }
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Compete {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>final</font> <font color=#0000ff>int</font> SIZE = 25000;
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) <font color=#0000ff>throws</font> Exception {
    Thing2[] a = <font color=#0000ff>new</font> Thing2[SIZE];
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; a.length; i++)
      a[i] = <font color=#0000ff>new</font> Thing2();
    Thing4[] b = <font color=#0000ff>new</font> Thing4[SIZE];
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; b.length; i++)
      b[i] = <font color=#0000ff>new</font> Thing4();
    <font color=#0000ff>long</font> t1 = System.currentTimeMillis();
    ByteArrayOutputStream buf= <font color=#0000ff>new</font> ByteArrayOutputStream();
    ObjectOutputStream o = <font color=#0000ff>new</font> ObjectOutputStream(buf);
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; a.length; i++)
      o.writeObject(a[i]);
    <font color=#009900>// Now get copies:</font>
    ObjectInputStream in = <font color=#0000ff>new</font> ObjectInputStream(
        <font color=#0000ff>new</font> ByteArrayInputStream(buf.toByteArray()));
    Thing2[] c = <font color=#0000ff>new</font> Thing2[SIZE];
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; c.length; i++)
      c[i] = (Thing2)in.readObject();
    <font color=#0000ff>long</font> t2 = System.currentTimeMillis();
    System.out.println(<font color=#004488>"Duplication via serialization: "</font> +
      (t2 - t1) + <font color=#004488>" Milliseconds"</font>);
    <font color=#009900>// Now try cloning:</font>
    t1 = System.currentTimeMillis();
    Thing4[] d = <font color=#0000ff>new</font> Thing4[SIZE];
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; d.length; i++)
      d[i] = (Thing4)b[i].clone();
    t2 = System.currentTimeMillis();
    System.out.println(<font color=#004488>"Duplication via cloning: "</font> +
      (t2 - t1) + <font color=#004488>" Milliseconds"</font>);
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p><b>Thing2</b> and <b>Thing4</b> contain member objects so that there&#146;s some deep copying going on. It&#146;s interesting to notice that while <b>Serializable</b> classes are easy to set up, there&#146;s much more work going on to duplicate them. Cloning involves a lot of work to set up the class, but the actual duplication of objects is relatively simple. The results are interesting. Here is the output from three different runs:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>Duplication via serialization: 547 Milliseconds
Duplication via cloning: 110 Milliseconds

Duplication via serialization: 547 Milliseconds
Duplication via cloning: 109 Milliseconds

Duplication via serialization: 547 Milliseconds
Duplication via cloning: 125 Milliseconds</PRE></FONT></BLOCKQUOTE><p><br></p>
<p>In earlier versions of the JDK, the time required for serialization was much longer than that of cloning (roughly 15 times slower), and the serialization time tended to vary a lot. More recent versions of the JDK have sped up serialization and apparently made the time more consistent, as well. Here, it&#146;s approximately four times slower, which brings it into the realm of reasonability for use as a cloning alternative. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2736" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc24776000"></a><a name="Heading25053"></a>Adding cloneability
<br>farther down a hierarchy</h3>
<p>If you create a new class, its base class defaults to <b>Object</b>, which defaults to noncloneability (as you&#146;ll see in the next section). As long as you don&#146;t explicitly add cloneability, you won&#146;t get it. But you can add it in at any layer and it will then be cloneable from that layer downward, like this:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: appendixa:HorrorFlick.java</font>
<font color=#009900>// You can insert Cloneability at any level of inheritance.</font>
<font color=#0000ff>package</font> appendixa;
<font color=#0000ff>import</font> java.util.*;

<font color=#0000ff>class</font> Person {}
<font color=#0000ff>class</font> Hero <font color=#0000ff>extends</font> Person {}
<font color=#0000ff>class</font> Scientist <font color=#0000ff>extends</font> Person <font color=#0000ff>implements</font> Cloneable {
  <font color=#0000ff>public</font> Object clone() {
    <font color=#0000ff>try</font> {
      <font color=#0000ff>return</font> <font color=#0000ff>super</font>.clone();
    } <font color=#0000ff>catch</font>(CloneNotSupportedException e) {
      <font color=#009900>// This should never happen: It's Cloneable already!</font>
      <font color=#0000ff>throw</font> <font color=#0000ff>new</font> RuntimeException(e);
    }
  }
}
<font color=#0000ff>class</font> MadScientist <font color=#0000ff>extends</font> Scientist {}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> HorrorFlick {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    Person p = <font color=#0000ff>new</font> Person();
    Hero h = <font color=#0000ff>new</font> Hero();
    Scientist s = <font color=#0000ff>new</font> Scientist();
    MadScientist m = <font color=#0000ff>new</font> MadScientist();
    <font color=#009900>//! p = (Person)p.clone(); // Compile error</font>
    <font color=#009900>//! h = (Hero)h.clone(); // Compile error</font>
    s = (Scientist)s.clone();
    m = (MadScientist)m.clone();
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>Before cloneability was added in the hierarchy, the compiler stopped you from trying to clone things. When cloneability is added in <b>Scientist</b>, then <b>Scientist</b> and all its descendants are cloneable. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2737" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc375545430"></a><a name="_Toc24776001"></a><a name="Heading25088"></a>Why
this strange design?</h3>
<p>If all this seems to be a strange scheme, that&#146;s because it is. You might wonder why it worked out this way. What is the meaning behind this design? <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2738" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>Originally, Java was designed as a language to control hardware boxes, and definitely not with the Internet in mind. In a general-purpose language like this, it makes sense that the programmer be able to clone any object. Thus, <b>clone(&#160;)</b> was placed in the root class <b>Object</b>, <i>but</i> it was a <b>public</b> method so you could always clone any object. This seemed to be the most flexible approach, and after all, what could it hurt? <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]AppendA_2739" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>Well, when Java was seen as the ultimate Internet programming language, things changed. Suddenly, there are security issues, and of course, these issues are dealt with using objects, and you don&#146;t necessarily want anyone to be able to clone your security objects. So what you&#146;re seeing is a lot of patches applied on the original simple and straightforward scheme: <b>clone(&#160;)</b> is now <b>protected</b> in <b>Object</b>. You must override it <i>and</i> <b>implement Cloneable</b> <i>and</i> deal with the exceptions. <font size="-2"><a href="mailto:TIJ3@