📄 tij0021.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="tij0020.html">Prev</a> | <a href="tij0022.html">Next</a>
</td>
</tr></table>
<hr>

<H2 ALIGN=LEFT>
Reusing
<P>the
implementation
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Once
a class has been created and tested, it should (ideally) represent a useful
unit of code. It turns out that this reusability is not nearly so easy to
achieve as many would hope; it takes experience and insight to achieve a good
design. But once you have such a design, it begs to be reused. Code reuse is
arguably the greatest leverage that object-oriented programming languages
provide.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
simplest way to reuse a class is to just use an object of that class directly,
but you can also place an object of that class inside a new class. We call this
&#8220;creating a member object.&#8221; Your new class can be made up of any
number and type of other objects, whatever is necessary to achieve the
functionality desired in your new class. This concept is called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>composition</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
since you are composing a new class from existing classes. Sometimes
composition is referred to as a &#8220;has-a&#8221; relationship, as in
&#8220;a car has a trunk.&#8221;
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Composition
comes with a great deal of flexibility. The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>member
objects
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of your new class are usually private, making them inaccessible to client
programmers using the class. This allows you to change those members without
disturbing existing client code. You can also change the member objects 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>at
run time
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which provides great flexibility. Inheritance, which is described next, does
not have this flexibility since the compiler must place restrictions on classes
created with inheritance.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Because
inheritance is so important in object-oriented programming it is often highly
emphasized, and the new programmer can get the idea that inheritance should be
used everywhere. This can result in awkward and overcomplicated designs.
Instead, you should first look to composition when creating new classes, since
it is simpler and more flexible. If you take this approach, your designs will
stay cleaner. It will be reasonably obvious when you need inheritance.
</FONT><a name="_Toc375545192"></a><a name="_Toc408018389"></a><P></DIV>

<div align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0020.html">Prev</a> | <a href="tij0022.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 + -