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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>
Guaranteed
initialization 
<P>with
the constructor
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can imagine creating a method called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>initialize(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for every class you write. The name is a hint that it should be called before
using the object. Unfortunately, this means the user must remember to call the
method. In Java, the class designer can guarantee initialization of every
object by providing a special method called a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>constructor<A NAME="Index262"></A><A NAME="Index263"></A></I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If a class has a constructor, Java automatically calls that constructor when an
object is created, before users can even get their hands on it. So
initialization is guaranteed.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
next challenge is what to name this method. There are two issues. The first is
that any name you use could clash with a name you might like to use as a member
in the class. The second is that because the compiler is responsible for
calling the constructor, it must always know which method to call. The C++
solution seems the easiest and most logical, so it&#8217;s also used in Java:
The name of the constructor <A NAME="Index264"></A>is
the same as the name of the class. It makes sense that such a method will be
called automatically on initialization.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here&#8217;s
a simple class with a constructor: (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">//: SimpleConstructor.java</font>
<font color="#009900">// Demonstration of a simple constructor</font>
<font color="#0000ff">package</font> c04;

<font color="#0000ff">class</font> Rock {
  Rock() { <font color="#009900">// This is the constructor</font>
    System.out.println("Creating Rock");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> SimpleConstructor {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 10; i++)
      <font color="#0000ff">new</font> Rock();
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Now,
when an <A NAME="Index265"></A>object
is created:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">new
Rock();
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">storage
is allocated and the constructor is called. It is guaranteed that the object
will be properly initialized before you can get your hands on it.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that the coding style of making the first letter of all methods lower case does
not apply to constructors, since the name of the constructor must match the
name of the class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>exactly</I></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">Like
any method, the constructor can have arguments<A NAME="Index266"></A><A NAME="Index267"></A>
to allow you to specify 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>how</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
an object is created. The above example can easily be changed so the
constructor takes an argument:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">class</font> Rock {
  Rock(<font color="#0000ff">int</font> i) {
    System.out.println(
      "Creating Rock number " + i);
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> SimpleConstructor {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 10; i++)
      <font color="#0000ff">new</font> Rock(i);
  }
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Constructor
arguments provide you with a way to provide parameters for the initialization
of an object. For example, if the class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tree</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has a constructor that takes a single integer argument denoting the height of
the tree, you would create a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tree</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object like this:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">Tree
t = new Tree(12);  // 12-foot tree
</FONT></TT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tree(int)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is your only constructor, then the compiler won&#8217;t let you create a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tree</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object any other way. 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Constructors
eliminate a large class of problems and make the code easier to read. In the
preceding code fragment, for example, you don&#8217;t see an explicit call to
some 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>initialize(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method that is conceptually separate from definition. In Java, definition and
initialization are unified concepts &#8211; you can&#8217;t have one without
the other.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
constructor is an unusual type of method because it has no return value<A NAME="Index268"></A><A NAME="Index269"></A>.
This is distinctly different from a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>void</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
return value, in which the method returns nothing but you still have the option
to make it return something else. Constructors return nothing and you
don&#8217;t have an option. If there were a return value, and if you could
select your own, the compiler would somehow need to know what to do with that
return value.
</FONT><a name="_Toc375545276"></a><a name="_Toc408018477"></a><P></DIV>

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