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学习java的经典书籍

method! You’d typically think that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Pie</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>f(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are friendly and therefore not available to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cake</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
They 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>are</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
friendly &#8211; that part is correct. The reason that they are available in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cake.java</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is because they are in the same directory and have no explicit package name.
Java treats files like this as implicitly part of the &#8220;default
package&#8221; for that directory, and therefore friendly to all the other
files in that directory.
</FONT><a name="_Toc375545298"></a><a name="_Toc408018502"></a><P></DIV>
<A NAME="Heading174"></A><H3 ALIGN=LEFT>
private:
you can&#8217;t touch that!
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
<A NAME="Index392"></A>keyword
that means no one can access that member except that particular class, inside
methods of that class. Other classes in the same package cannot access 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">members,
so it&#8217;s as if you&#8217;re even insulating the class against yourself. On
the other hand, it&#8217;s not unlikely that a package might be created by
several people collaborating together, so 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
allows you to freely change that member without concern that it will affect
another class in the same package. The default &#8220;friendly&#8221; package
access is often an adequate amount of hiding; remember, a
&#8220;friendly&#8221; member is inaccessible to the user of the package. This
is nice, since the default access is the one that you normally use. Thus,
you&#8217;ll typically think about access for the members that you explicitly
want to make 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for the client programmer, and as a result, you might not
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">initially
think you&#8217;ll use the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">keyword
often since it&#8217;s tolerable to get away without it. (This is a distinct
contrast with C++.) However, it turns out that the consistent use of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is very important, especially where multithreading is concerned. (As
you&#8217;ll see in Chapter 14.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here&#8217;s
an example of the use of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: IceCream.java</font>
<font color="#009900">// Demonstrates "private" keyword</font>

<font color="#0000ff">class</font> Sundae {
  <font color="#0000ff">private</font> Sundae() {}
  <font color="#0000ff">static</font> Sundae makeASundae() { 
    <font color="#0000ff">return</font> <font color="#0000ff">new</font> Sundae(); 
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> IceCream {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#009900">//! Sundae x = new Sundae();</font>
    Sundae x = Sundae.makeASundae();
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
shows an example in which 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
comes in handy: you might want to control how an object is created and prevent
someone from directly accessing a particular constructor (or all of them). In
the example above, you cannot create a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Sundae</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object via its constructor; instead you must call the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>makeASundae(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method to do it for you.
</FONT><A NAME="fnB25" HREF="#fn25">[25]</A><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Any
method that you&#8217;re certain is only a &#8220;helper&#8221; method for that
class can be made 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to ensure that you don&#8217;t accidentally use it elsewhere in the package and
thus prohibit you from changing or removing the method. Making a method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
guarantees that you retain this option. (However, just because the handle is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn't mean that some other object can't have a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle to the same object. See Chapter 12 for issues about aliasing.)
</FONT><a name="_Toc312373839"></a><a name="_Toc375545299"></a><a name="_Toc408018503"></a><P></DIV>
<A NAME="Heading175"></A><H3 ALIGN=LEFT>
protected:
&#8220;sort of friendly&#8221;
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
access specifier requires a jump ahead to understand<A NAME="Index393"></A>.
First, you should be aware that you don&#8217;t need to understand this section
to continue through the book up through the inheritance chapter. But for
completeness, here is a brief description and example using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
</FONT><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>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword deals with a concept called <A NAME="Index394"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>inheritance</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which takes an existing class and adds new members to that class without
touching the existing class, which we refer to as the <A NAME="Index395"></A><A NAME="Index396"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>base</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>class</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
You can also change the behavior of existing members of the class. To inherit
from an existing class, you say that your new class <A NAME="Index397"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>extends
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">an
existing class, like this:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">class
Foo extends Bar {
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
rest of the class definition looks the same.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you create a new package and you inherit from a class in another package, the
only members you have access to are the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
members of the original package. (Of course, if you perform the inheritance in
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>same</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
package, you have the normal package access to all the &#8220;friendly&#8221;
members.) Sometimes the creator of the base class would like to take a
particular member and grant access to derived classes but not the world in
general. That&#8217;s what 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
does. If you refer back to the file 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cookie.java</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
on page 
<A HREF=" PAGE#_Ref351419800">203</A>,
the following class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>cannot</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
access the &#8220;friendly&#8221; member:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: ChocolateChip.java</font>
<font color="#009900">// Can't access friendly member</font>
<font color="#009900">// in another class</font>
<font color="#0000ff">import</font> c05.dessert.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> ChocolateChip <font color="#0000ff">extends</font> Cookie {
  <font color="#0000ff">public</font> ChocolateChip() {
   System.out.println(
     "ChocolateChip constructor");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    ChocolateChip x = <font color="#0000ff">new</font> ChocolateChip();
    <font color="#009900">//! x.foo(); // Can't access foo</font>
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">One
of the interesting things about inheritance is that if a method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>foo(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
exists in class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cookie</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
then it also exists in any class inherited from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cookie</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
But since 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>foo(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is &#8220;friendly&#8221; in a foreign package, it&#8217;s unavailable to us in
this one. Of course, you could make it 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but then everyone would have access and maybe that&#8217;s not what you want.
If we change the class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cookie</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
as follows:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">public</font> <font color="#0000ff">class</font> Cookie {
  <font color="#0000ff">public</font> Cookie() { 
    System.out.println("Cookie constructor");
  }
  <font color="#0000ff">protected</font> <font color="#0000ff">void</font> foo() {
    System.out.println("foo"); 
  }
}</PRE></font><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>foo(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
still has &#8220;friendly&#8221; access within package 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>dessert</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but it is also accessible to anyone inheriting from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cookie</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
However, it is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>not</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><a name="_Toc375545301"></a><a name="_Toc408018504"></a><P></DIV>
<HR><DIV ALIGN=LEFT><A NAME="fn25" HREF="#fnB25">[25]</A><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
There&#8217;s another effect in this case: Since the default constructor is the
only one defined, and it&#8217;s 
</FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">,
it will prevent inheritance of this class. (A subject that will be introduced
in Chapter 6.)
</FONT><P></DIV>


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