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学习java的经典书籍

s = "abc" + foo + "def" + Integer.toString(47);
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
could imagine how this 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>might</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
work: the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
&#8220;abc&#8221; could have a method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>append(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that creates a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object containing &#8220;abc&#8221; concatenated with the contents of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>foo</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object would then create another new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that added &#8220;def&#8221; and so on.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
would certainly work, but it requires the creation of a lot of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects just to put together this new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and then you have a bunch of the intermediate 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects that need to be garbage-collected. I suspect that the Java designers
tried this approach first (which is a lesson in software design &#8211; you
don&#8217;t really know anything about a system until you try it out in code
and get something working). I also suspect they discovered that it delivered
unacceptable performance.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
solution is a mutable companion class similar to the one shown previously. For 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
this companion class is called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and the compiler automatically creates a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to evaluate certain expressions, in particular when the overloaded operators 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+=</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are used with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects. This example shows what happens:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: ImmutableStrings.java</font>
<font color="#009900">// Demonstrating StringBuffer</font>

<font color="#0000ff">public</font> <font color="#0000ff">class</font> ImmutableStrings {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    String foo = "foo";
    String s = "abc" + foo +
      "def" + Integer.toString(47);
    System.out.println(s);
    <font color="#009900">// The "equivalent" using StringBuffer:</font>
    StringBuffer sb = 
      <font color="#0000ff">new</font> StringBuffer("abc"); <font color="#009900">// Creates String!</font>
    sb.append(foo);
    sb.append("def"); <font color="#009900">// Creates String!</font>
    sb.append(Integer.toString(47));
    System.out.println(sb);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
the creation of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String
s
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the compiler is doing the rough equivalent of the subsequent code that uses 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sb</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is created and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>append(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is used to add new characters directly into the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object (rather than making new copies each time). While this is more efficient,
it&#8217;s worth noting that each time you create a quoted character string
such as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>&#8220;abc&#8221;</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>&#8220;def&#8221;</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the compiler turns those into 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects. So there can be more objects created than you expect, despite the
efficiency afforded through 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><a name="_Toc375545438"></a><a name="_Toc408018673"></a><a name="_Toc375545436"></a><a name="_Toc375545434"></a><P></DIV>
<A NAME="Heading389"></A><H3 ALIGN=LEFT>
The
String and StringBuffer classes
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here
is an overview of the methods available for both <A NAME="Index1503"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
so you can get a feel for the way they interact. These tables don&#8217;t
contain every single method, but rather the ones that are important to this
discussion. Methods that are overloaded are summarized in a single row.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">First,
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class:
</FONT><P></DIV>
<DIV ALIGN=LEFT><TABLE BORDER>
<COLGROUP>
      <COL width="108">
      <COL width="97">
      <COL width="148">
</COLGROUP>
<TR VALIGN="TOP">
<TH WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Method</FONT><P></DIV>
</TH>
<TH WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Arguments,
Overloading
</FONT><P></DIV>
</TH>
<TH WIDTH=148 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Use</FONT><P></DIV>
</TH>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Constructor</FONT><P></DIV>
</TD>
<TD WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Overloaded:
Default, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer,
char
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
arrays, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>byte</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
arrays.
</FONT><P></DIV>
</TD>
<TD WIDTH=148 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Creating
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects.
</FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>length(&#160;)</B></FONT><P></DIV>
</TD>
<TD WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<P></TD>
<TD WIDTH=148 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Number
of characters in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>charAt()</B></FONT><P></DIV>
</TD>
<TD WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int
Index
</B></FONT><P></DIV>
</TD>
<TD WIDTH=148 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>char</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
at a location in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getChars(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getBytes(&#160;)</B></FONT><P></DIV>
</TD>
<TD WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
beginning and end from which to copy, the array to copy into, an index into the
destination array.
</FONT><P></DIV>
</TD>
<TD WIDTH=148 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Copy
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>char</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>bytes</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
into an external array.
</FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toCharArray(&#160;)</B></FONT><P></DIV>
</TD>
<TD WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<P></TD>
<TD WIDTH=148 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Produces
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>char[]</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
containing the characters in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals-IgnoreCase(&#160;)</B></FONT><P></DIV>
</TD>
<TD WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to compare with.
</FONT><P></DIV>
</TD>
<TD WIDTH=148 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">An
equality check on the contents of the two 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s.</FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=108 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>compareTo(&#160;)</B></FONT><P></DIV>
</TD>
<TD WIDTH=97 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">