⭐ 欢迎来到虫虫下载站！ | 📦 资源下载 📁 资源专辑 ℹ️ 关于我们
⭐ 虫虫下载站
📤 上传资源

📄 tij0035.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="tij0034.html">Prev</a> | <a href="tij0036.html">Next</a>
</td>
</tr></table>
<hr>

<H2 ALIGN=LEFT>
Creating
new 
<P>data
types: class
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
everything is an object, what determines how a particular class of object looks
and behaves? Put another way, what establishes the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>type</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of an object? You might expect there to be a keyword called &#8220;type&#8221;
and that certainly would have made sense. Historically, however, most
object-oriented languages have used the keyword 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to mean &#8220;I&#8217;m about to tell you what a new type of object looks
like.&#8221; The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword (which is so common that it will not be emboldened throughout the book)
is followed by the name of the new type. For example:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">class
ATypeName { /* class body goes here */ }
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
introduces a new type, so you can now create an object of this type using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">ATypeName
a = new ATypeName();
</FONT></TT><P></DIV><DIV ALIGN=LEFT><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>ATypeName</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the class body consists only of a comment (the stars and slashes and what is
inside, which will be discussed later in this chapter) so there is not too much
that you can do with it. In fact, you cannot tell it to do much of anything
(that is, you cannot send it any interesting messages) until you define some
methods for it.
</FONT><a name="_Toc375545226"></a><a name="_Toc408018427"></a><P></DIV>
<A NAME="Heading72"></A><H3 ALIGN=LEFT>
Fields
and methods
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
you define a class (and all you do in Java is define classes, make objects of
those classes and send messages to those objects), you can put two types of
elements in your class: data members (sometimes called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>fields</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
and member functions (typically called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>methods</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">).
A data member is an object (that you communicate with via its handle) of any
type. It can also be one of the primitive types (which isn&#8217;t a handle).
If it is a handle to an object, you must initialize that handle to connect it
to an actual 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">,
as seen earlier) in a special function called a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>constructor</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(described fully in Chapter 4). If it is a primitive type you can initialize it
directly at the point of definition in the class. (As you&#8217;ll see later,
handles can also be initialized at the point of definition.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Each
object keeps its own storage for its data members; the data members are not
shared among objects. Here is an example of a class with some data members:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">class</font> DataOnly {
  <font color="#0000ff">int</font> i;
  <font color="#0000ff">float</font> f;
  <font color="#0000ff">boolean</font> b;
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
class doesn&#8217;t 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>do</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
anything, but you can create an object:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">DataOnly
d = new DataOnly();
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can assign values to the data members, but you must first know how to refer to
a member of an object. This is accomplished by stating the name of the object
handle, followed by a period (dot), followed by the name of the member inside
the object (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>objectHandle.member</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">).
For example:
</FONT><P></DIV>

<font color="#990000"><PRE>d.i = 47;
d.f = 1.1f;
d.b = <font color="#0000ff">false</font>; </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It
is also possible that your object might contain other objects that contain data
you&#8217;d like to modify. For this, you just keep &#8220;connecting the
dots.&#8221; For example:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">myPlane.leftTank.capacity
= 100;
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>DataOnly
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">class
cannot do much of anything except hold data, because it has no member functions
(methods). To understand how those work, you must first understand 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>arguments</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>return
values
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which will be described shortly.
</FONT><P></DIV>
<A NAME="Heading73"></A><H4 ALIGN=LEFT>
Default
values for primitive members
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
a primitive data type is a member of a class, it is guaranteed to get a default
value if you do not initialize it:
</FONT><P></DIV>
<DIV ALIGN=LEFT><TABLE BORDER>
<COLGROUP>
      <COL width="81">
      <COL width="84">
</COLGROUP>
<TR VALIGN="TOP">
<TH WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Primitive
type
</FONT><P></DIV>
</TH>
<TH WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Default</FONT><P></DIV>
</TH>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Boolean</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>false</B></FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Char</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8216;\u0000&#8217;
(
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)</FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>byte</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>(byte)0</B></FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>short</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>(short)0</B></FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>0</B></FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>long</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>0L</B></FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>float</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>0.0f</B></FONT><P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD WIDTH=81 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>double</B></FONT><P></DIV>
</TD>
<TD WIDTH=84 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>0.0d</B></FONT><P></DIV>
</TD>
</TR>
</TABLE></DIV>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
carefully that the default values are what Java guarantees when the variable is
used 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>as
a member of a class
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This ensures that member variables of primitive types will always be
initialized (something C++ doesn&#8217;t do), reducing a source of bugs. 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">However,
this guarantee doesn&#8217;t apply to &#8220;local&#8221; variables &#8211;
those that are not fields of a class. Thus, if within a function definition you
have:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">int
x;
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Then
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will get some random value (as in C and C++); it will not automatically be
initialized to zero. You are responsible for assigning an appropriate value
before you use 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If you forget, Java definitely improves on C++: you get a compile-time error
telling you the variable might not have been initialized. (Many C++ compilers
will warn you about uninitialized variables, but in Java these are errors.)
</FONT><a name="_Toc375545227"></a><a name="_Toc408018428"></a><P></DIV>

<div align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0034.html">Prev</a> | <a href="tij0036.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 + -