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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>
Initialization
and 
<P>class
loading
<P><A NAME="Index537"></A><A NAME="Index538"></A><A NAME="Index539"></A></H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
many more traditional languages, programs are loaded all at once as part of the
startup process. This is followed by initialization, and then the program
begins. The process of initialization in these languages must be carefully
controlled so that the order of initialization of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
doesn&#8217;t cause trouble. C++, for example, has problems if one 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
expects another 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">to
be valid before the second one has been initialized.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
doesn&#8217;t have this problem because it takes a different approach to
loading. Because everything in Java is an object, many activities become
easier, and this is one of them. As you will learn in the next chapter, the
code for each object exists in a separate file. That file isn&#8217;t loaded
until the code is needed. In general, you can say that until an object of that
class is constructed, the class code doesn&#8217;t get loaded. Since there can
be some subtleties with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods, you can also say, &#8220;Class code is loaded at the point of first
use.&#8221;
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
point of first use is also where the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
initialization takes place. All the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects and the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
code block will be initialized in textual <A NAME="Index540"></A>order
(that is, the order that you write them down in the class definition) at the
point of loading. The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s,
of course, are initialized only once.
</FONT><a name="_Toc375545323"></a><a name="_Toc408018526"></a><P></DIV>
<A NAME="Heading204"></A><H3 ALIGN=LEFT>
Initialization
with inheritance
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It&#8217;s
helpful to look at the whole <A NAME="Index541"></A><A NAME="Index542"></A>initialization
process, including inheritance, to get a full picture of what happens. Consider
the following code:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Beetle.java</font>
<font color="#009900">// The full process of initialization.</font>

<font color="#0000ff">class</font> Insect {
  <font color="#0000ff">int</font> i = 9;
  <font color="#0000ff">int</font> j;
  Insect() {
    prt("i = " + i + ", j = " + j);
    j = 39;
  }
  <font color="#0000ff">static</font> <font color="#0000ff">int</font> x1 = 
    prt("<font color="#0000ff">static</font> Insect.x1 initialized");
  <font color="#0000ff">static</font> <font color="#0000ff">int</font> prt(String s) {
    System.out.println(s);
    <font color="#0000ff">return</font> 47;
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Beetle <font color="#0000ff">extends</font> Insect {
  <font color="#0000ff">int</font> k = prt("Beetle.k initialized");
  Beetle() {
    prt("k = " + k);
    prt("j = " + j);
  }
  <font color="#0000ff">static</font> <font color="#0000ff">int</font> x2 =
    prt("<font color="#0000ff">static</font> Beetle.x2 initialized");
  <font color="#0000ff">static</font> <font color="#0000ff">int</font> prt(String s) {
    System.out.println(s);
    <font color="#0000ff">return</font> 63;
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    prt("Beetle constructor");
    Beetle b = <font color="#0000ff">new</font> Beetle();
  }
} <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 for this program is:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">static</font> Insect.x initialized
<font color="#0000ff">static</font> Beetle.x initialized
Beetle constructor
i = 9, j = 0
Beetle.k initialized
k = 63
j = 39  </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
first thing that happens when you run Java on 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Beetle</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is that the loader goes out and finds that <A NAME="Index543"></A><A NAME="Index544"></A>class.
In the process of loading it, the loader notices that it has a base class
(that&#8217;s what the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>extends
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">keyword
says), which it then loads. This will happen whether or not you&#8217;re going
to make an object of that base class. (Try commenting out the object creation
to prove it to yourself.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
the base class has a base class, that second base class would then be loaded,
and so on. Next, the <A NAME="Index545"></A><A NAME="Index546"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
initialization in the root base class (in this case, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Insect</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
is performed, and then the next derived class, and so on. This is important
because the derived-class static initialization might depend on the base class
member being initialized properly.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">At
this point, the necessary classes have all been loaded so the object can be
created. First, all the primitives in this object are set to their default
values and the object handles are set to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Then
the base-class constructor will be called. In this case the call is automatic,
but you can also specify the constructor call (as the first operation in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Beetle(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
constructor) using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>super</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The base class construction goes through the same process in the same order as
the derived-class constructor. After the base-class constructor completes, the
instance variables are initialized in textual order. Finally, the rest of the
body of the constructor is executed.
</FONT><a name="_Toc375545324"></a><a name="_Toc408018527"></a><P></DIV>

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