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学习java的经典书籍

      <font color="#0000ff">throws</font> IOException {
    System.out.println("Blip2.writeExternal");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> readExternal(ObjectInput in)
     <font color="#0000ff">throws</font> IOException, ClassNotFoundException {
    System.out.println("Blip2.readExternal");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Blips {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    System.out.println("Constructing objects:");
    Blip1 b1 = <font color="#0000ff">new</font> Blip1();
    Blip2 b2 = <font color="#0000ff">new</font> Blip2();
    <font color="#0000ff">try</font> {
      ObjectOutputStream o =
        <font color="#0000ff">new</font> ObjectOutputStream(
          <font color="#0000ff">new</font> FileOutputStream("Blips.out"));
      System.out.println("Saving objects:");
      o.writeObject(b1);
      o.writeObject(b2);
      o.close();
      <font color="#009900">// Now get them back:</font>
      ObjectInputStream in =
        <font color="#0000ff">new</font> ObjectInputStream(
          <font color="#0000ff">new</font> FileInputStream("Blips.out"));
      System.out.println("Recovering b1:");
      b1 = (Blip1)in.readObject();
      <font color="#009900">// OOPS! Throws an exception:</font>
<font color="#009900">//!   System.out.println("Recovering b2:");</font>
<font color="#009900">//!   b2 = (Blip2)in.readObject();</font>
    } <font color="#0000ff">catch</font>(Exception e) {
      e.printStackTrace();
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
output for this program is:
</FONT><P></DIV>

<font color="#990000"><PRE>Constructing objects:
Blip1 Constructor
Blip2 Constructor
Saving objects:
Blip1.writeExternal
Blip2.writeExternal
Recovering b1:
Blip1 Constructor
Blip1.readExternal</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
reason that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is not recovered is that trying to do so causes an exception. Can you
see the difference between 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">?
The constructor for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
while the constructor for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is not, and that causes the exception upon recovery. Try making 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
constructor 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">and
removing the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>//!
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">comments
to see the correct results.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>b1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is recovered, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
default constructor is called. This is different from recovering a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Serializable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object, in which the object is constructed entirely from its stored bits, with
no constructor calls. With an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object, all the normal default construction behavior occurs (including the
initializations at the point of field definition), and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>then</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>readExternal(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is called. You need to be aware of this &#8211; in particular the fact that all
the default construction always takes place &#8211; to produce the correct
behavior in your 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here&#8217;s
an example that shows what you must do to fully store and retrieve an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Blip3.java</font>
<font color="#009900">// Reconstructing an externalizable object</font>
<font color="#0000ff">import</font> java.io.*;
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> Blip3 <font color="#0000ff">implements</font> Externalizable {
  <font color="#0000ff">int</font> i;
  String s; <font color="#009900">// No initialization</font>
  <font color="#0000ff">public</font> Blip3() {
    System.out.println("Blip3 Constructor");
    <font color="#009900">// s, i not initialized</font>
  }
  <font color="#0000ff">public</font> Blip3(String x, <font color="#0000ff">int</font> a) {
    System.out.println("Blip3(String x, <font color="#0000ff">int</font> a)");
    s = x;
    i = a;
    <font color="#009900">// s &amp; i initialized only in non-default</font>
    <font color="#009900">// constructor.</font>
  }
  <font color="#0000ff">public</font> String toString() { <font color="#0000ff">return</font> s + i; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> writeExternal(ObjectOutput out)
      <font color="#0000ff">throws</font> IOException {
    System.out.println("Blip3.writeExternal");
    <font color="#009900">// You must do this:</font>
    out.writeObject(s); out.writeInt(i);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> readExternal(ObjectInput in)
     <font color="#0000ff">throws</font> IOException, ClassNotFoundException {
    System.out.println("Blip3.readExternal");
    <font color="#009900">// You must do this:</font>
    s = (String)in.readObject(); 
    i =in.readInt();
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    System.out.println("Constructing objects:");
    Blip3 b3 = <font color="#0000ff">new</font> Blip3("A String ", 47);
    System.out.println(b3.toString());
    <font color="#0000ff">try</font> {
      ObjectOutputStream o =
        <font color="#0000ff">new</font> ObjectOutputStream(
          <font color="#0000ff">new</font> FileOutputStream("Blip3.out"));
      System.out.println("Saving object:");
      o.writeObject(b3);
      o.close();
      <font color="#009900">// Now get it back:</font>
      ObjectInputStream in =
        <font color="#0000ff">new</font> ObjectInputStream(
          <font color="#0000ff">new</font> FileInputStream("Blip3.out"));
      System.out.println("Recovering b3:");
      b3 = (Blip3)in.readObject();
      System.out.println(b3.toString());
    } <font color="#0000ff">catch</font>(Exception e) {
      e.printStackTrace();
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
fields 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">are
initialized only in the second constructor, but not in the default constructor.
This means that if you don&#8217;t initialize 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">in
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>readExternal</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it will be 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(since the storage for the object gets wiped to zero in the first step of
object creation). If you comment out the two lines of code following the
phrases &#8220;You must do this&#8221; and run the program, you&#8217;ll see
that when the object is recovered, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is zero.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you are inheriting from an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object, you&#8217;ll typically call the base-class versions of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>writeExternal(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">and
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>readExternal(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to provide proper storage and retrieval of the base-class components.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">So
to make things work correctly you must not only write the important data from
the object during the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>writeExternal(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">method
(there is no default behavior that writes any of the member objects for an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object), but you must also recover that data in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>readExternal(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. This can be a bit confusing at first because the default construction
behavior for an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object can make it seem like some kind of storage and retrieval takes place
automatically. It does not.
</FONT><P></DIV>
<A NAME="Heading346"></A><H4 ALIGN=LEFT>
The
transient keyword
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
you&#8217;re controlling serialization, there might be a particular subobject
that you don&#8217;t want Java&#8217;s serialization mechanism to automatically
save and restore. This is commonly the case if that subobject represents