📄 tij0078.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="tij0077.html">Prev</a> | <a href="tij0079.html">Next</a>
</td>
</tr></table>
<hr>

<H2 ALIGN=LEFT>
Overriding
vs. overloading
<P><A NAME="Index577"></A><A NAME="Index578"></A></H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Let&#8217;s
take a different look at the first example in this chapter. In the following
program, the interface of the method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is changed in the process of overriding it, which means that you haven&#8217;t 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>overridden</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the method, but instead 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>overloaded
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">it.
The compiler allows you to overload methods so it gives no complaint. But the
behavior is probably not what you want. Here&#8217;s the example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: WindError.java </font>
<font color="#009900">// Accidentally changing the interface</font>

<font color="#0000ff">class</font> NoteX {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">final</font> <font color="#0000ff">int</font>
    MIDDLE_C = 0, C_SHARP = 1, C_FLAT = 2;
}

<font color="#0000ff">class</font> InstrumentX {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(<font color="#0000ff">int</font> NoteX) {
    System.out.println("InstrumentX.play()");
  }
}

<font color="#0000ff">class</font> WindX <font color="#0000ff">extends</font> InstrumentX {
  <font color="#009900">// OOPS! Changes the method interface:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(NoteX n) {
    System.out.println("WindX.play(NoteX n)");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> WindError {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> tune(InstrumentX i) {
    <font color="#009900">// ...</font>
    i.play(NoteX.MIDDLE_C);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    WindX flute = <font color="#0000ff">new</font> WindX();
    tune(flute); <font color="#009900">// Not the desired behavior!</font>
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There&#8217;s
another confusing aspect thrown in here. In 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InstrumentX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method takes an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that has the identifier 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>NoteX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
That is, even though 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>NoteX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is a class name, it can also be used as an identifier without complaint. But in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>WindX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
takes a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>NoteX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle that has an identifier 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>n.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(Although you could even say 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(NoteX
NoteX)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
without an error.) Thus it appears that the programmer intended to override 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
but mistyped the method a bit. The compiler, however, assumed that an overload
and not an override was intended. Note that if you follow the standard Java
naming convention, the argument identifier would be 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>noteX,</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
which would distinguish it from the class name.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InstrumentX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is sent the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
message, with one of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>NoteX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
members (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>MIDDLE_C</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
as an argument. Since 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>NoteX</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
contains 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
definitions, this means that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
version of the now-overloaded 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is called, and since that has 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>not</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
been overridden the base-class version is used.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
output is:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">InstrumentX.play()</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
certainly doesn&#8217;t appear to be a polymorphic method call. Once you
understand what&#8217;s happening, you can fix the problem fairly easily, but
imagine how difficult it might be to find the bug if it&#8217;s buried in a
program of significant size.
</FONT><a name="_Toc375545334"></a><a name="_Toc408018537"></a><P></DIV>

<div align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0077.html">Prev</a> | <a href="tij0079.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 + -