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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
Passing
handles around
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
you pass a <A NAME="Index1441"></A><A NAME="Index1442"></A><A NAME="Index1443"></A>handle
into a method, you&#8217;re still pointing to the same object. A simple
experiment demonstrates this: (See page 
<A HREF=" PAGE#Running_programs">97</A>
if you have trouble executing this program.)
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: PassHandles.java</font>
<font color="#009900">// Passing handles around</font>
<font color="#0000ff">package</font> c12;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> PassHandles {
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> f(PassHandles h) {
    System.out.println("h inside f(): " + h);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    PassHandles p = <font color="#0000ff">new</font> PassHandles();
    System.out.println("p inside main(): " + p);
    f(p);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is automatically invoked in the print statements, and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PassHandles</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
inherits directly from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
with no redefinition of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Thus, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
version of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">is
used, which prints out the class of the object followed by the address where
that object is located (not the handle, but the actual object storage). The
output looks like this:
</FONT><P></DIV>

<font color="#990000"><PRE>p inside main(): PassHandles@1653748
h inside f(): PassHandles@1653748 </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that both 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>p</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>h</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
refer to the same object. This is far more efficient than duplicating a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PassHandles</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object just so that you can send an argument to a method. But it brings up an
important issue.
</FONT><a name="_Toc375545423"></a><a name="_Toc408018655"></a><P></DIV>
<A NAME="Heading366"></A><H3 ALIGN=LEFT>
Aliasing<P><A NAME="Index1444"></A><A NAME="Index1445"></A></H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Aliasing
means that more than one handle is tied to the same object, as in the above
example. The problem with aliasing occurs when someone 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>writes</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to that object. If the owners of the other handles aren&#8217;t expecting that
object to change, they&#8217;ll be surprised. This can be demonstrated with a
simple example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Alias1.java</font>
<font color="#009900">// Aliasing two handles to one object</font>

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Alias1 {
  <font color="#0000ff">int</font> i;
  Alias1(<font color="#0000ff">int</font> ii) { i = ii; }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Alias1 x = <font color="#0000ff">new</font> Alias1(7);
    Alias1 y = x; <font color="#009900">// Assign the handle</font>
    System.out.println("x: " + x.i);
    System.out.println("y: " + y.i);
    System.out.println("Incrementing x");
    x.i++;
    System.out.println("x: " + x.i);
    System.out.println("y: " + y.i);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
the line:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">Alias1
y = x; // Assign the handle
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">a
new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Alias1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle is created, but instead of being assigned to a fresh object 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">,
it&#8217;s assigned to an existing handle. So the contents of handle 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which is the address of the object 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is pointing to, is assigned to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>y</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and thus both 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>y</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are attached to the same object. So when 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is incremented in the statement:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">x.i++;</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>y</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">will
be affected as well. This can be seen in the output:
</FONT><P></DIV>

<font color="#990000"><PRE>x: 7
y: 7
Incrementing x
x: 8
y: 8 </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">One
good solution in this case is to simply not do it: don&#8217;t consciously
alias more than one handle to an object at the same scope. Your code will be
much easier to understand and debug. However, when you&#8217;re passing a
handle in as an argument &#8211; which is the way Java is supposed to work
&#8211; you automatically alias because the local handle that&#8217;s created
can modify the &#8220;outside object&#8221; (the object that was created
outside the scope of the method). Here&#8217;s an example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Alias2.java</font>
<font color="#009900">// Method calls implicitly alias their</font>
<font color="#009900">// arguments.</font>

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Alias2 {
  <font color="#0000ff">int</font> i;
  Alias2(<font color="#0000ff">int</font> ii) { i = ii; }
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> f(Alias2 handle) {
    handle.i++;
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Alias2 x = <font color="#0000ff">new</font> Alias2(7);
    System.out.println("x: " + x.i);
    System.out.println("Calling f(x)");
    f(x);
    System.out.println("x: " + x.i);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
output is:
</FONT><P></DIV>

<font color="#990000"><PRE>x: 7
Calling f(x)
x: 8 </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
method is changing its argument, the outside object. When this kind of
situation arises, you must decide whether it makes sense, whether the user
expects it, and whether it&#8217;s going to cause problems.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
general, you call a method in order to produce a return value and/or a change
of state in the object 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>that
the method is called for
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
(A method is how you &#8220;send a message&#8221; to that object.) It&#8217;s
much less common to call a method in order to manipulate its arguments; this is
referred to as &#8220;calling a method for its <A NAME="Index1446"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>side
effects
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.&#8221;
Thus, when you create a method that modifies its arguments the user must be
clearly instructed and warned about the use of that method and its potential
surprises. Because of the confusion and pitfalls, it&#8217;s much better to
avoid changing the argument.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you need to modify an argument during a method call and you don&#8217;t intend
to modify the outside argument, then you should protect that argument by making
a copy inside your method. That&#8217;s the subject of much of this chapter.
</FONT><a name="_Toc408018656"></a><P></DIV>

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