📄 tij0034.html

📁 学习java的经典书籍
💻 HTML
字号:
<html><body>

<table width="100%"><tr>
<td>
<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
</td>
<td align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0033.html">Prev</a> | <a href="tij0035.html">Next</a>
</td>
</tr></table>
<hr>

<H2 ALIGN=LEFT>
You
never need to 
<P>destroy
an object
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
most programming languages, the concept of the lifetime of a variable occupies
a significant portion of the programming effort. How long does the variable
last? If you are supposed to destroy it, when should you? Confusion over
variable lifetimes can lead to a lot of bugs, and this section shows how Java
greatly simplifies the issue by doing all the cleanup work for you.
</FONT><a name="_Toc375545223"></a><a name="_Toc408018424"></a><P></DIV>
<A NAME="Heading69"></A><H3 ALIGN=LEFT>
Scoping</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Most
procedural languages have the concept of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>scope</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This determines both the visibility and lifetime of the names defined within
that scope. In C, C++ and Java, scope is determined by the placement of curly
braces 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>{}</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
So for example:
</FONT><P></DIV>

<font color="#990000"><PRE>{
  <font color="#0000ff">int</font> x = 12;
  <font color="#009900">/* only x available */</font>
  {
    <font color="#0000ff">int</font> q = 96;
    <font color="#009900">/* both x &amp; q available */</font>
  }
  <font color="#009900">/* only x available */</font>
  <font color="#009900">/* q &#8220;out of scope&#8221; */</font>
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
variable defined within a scope is available only to the end of that scope.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Indentation
makes Java code easier to read. Since Java is a free form language, the extra
spaces, tabs and carriage returns do not affect the resulting program.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that you 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>cannot</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
do the following, even though it is legal in C and C++:
</FONT><P></DIV>

<font color="#990000"><PRE>{
  <font color="#0000ff">int</font> x = 12;
  {
    <font color="#0000ff">int</font> x = 96; <font color="#009900">/* illegal */</font>
  }
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
compiler will announce that the variable 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">has
already been defined. Thus the C and C++ ability to &#8220;hide&#8221; a
variable in a larger scope is not allowed because the Java designers thought
that it led to confusing programs.
</FONT><a name="_Toc375545224"></a><a name="_Toc408018425"></a><P></DIV>
<A NAME="Heading70"></A><H3 ALIGN=LEFT>
Scope
of objects
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
objects do not have the same lifetimes as primitives. When you create a Java
object using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it hangs around past the end of the scope. Thus if you use:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">{</FONT></TT><P><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">
 String s = new String("a string");
</FONT></TT><P><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">}
/* end of scope */
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">the
handle 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
vanishes at the end of the scope. However, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
was pointing to is still occupying memory. In this bit of code, there is no way
to access the object because the only handle to it is out of scope. In later
chapters you&#8217;ll see how the handle to the object can be passed around and
duplicated during the course of a program.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It
turns out that because objects created with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
stay around for as long as you want them, a whole slew of programming problems
simply vanish in C++ and Java. The hardest problems seem to occur in C++
because you don&#8217;t get any help from the language in making sure that the
objects are available when they&#8217;re needed. And more importantly, in C++
you must make sure that you destroy the objects when you&#8217;re done with them.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">That
brings up an interesting question. If Java leaves the objects lying around,
what keeps them from filling up memory and halting your program? This is
exactly the kind of problem that would occur in C++. This is where a bit of
magic happens. Java has a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>garbage
collector
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which looks at all the objects that were created with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and figures out which ones are not being referenced anymore. Then it releases
the memory for those objects, so the memory can be used for new objects. This
means that you never need to worry about reclaiming memory yourself. You simply
create objects, and when you no longer need them they will go away by
themselves. This eliminates a certain class of programming problem: the
so-called &#8220;memory leak,&#8221; in which a programmer forgets to release
memory.
</FONT><a name="_Toc375545225"></a><a name="_Toc408018426"></a><P></DIV>

<div align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0033.html">Prev</a> | <a href="tij0035.html">Next</a>
</div>
</body></html>
⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -