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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>
Composition
syntax
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Until
now, composition has been used quite frequently. You simply place object
handles inside new classes. For example, suppose you&#8217;d like an object
that holds several 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects, a couple of primitives and an object of another class. For the
non-primitive objects, just put handles inside your new class, and for the
primitives just define them inside your class: (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">//: SprinklerSystem.java</font>
<font color="#009900">// Composition for code reuse</font>
<font color="#0000ff">package</font> c06;

<font color="#0000ff">class</font> WaterSource {
  <font color="#0000ff">private</font> String s;
  WaterSource() {
    System.out.println("WaterSource()");
    s = <font color="#0000ff">new</font> String("Constructed");
  }
  <font color="#0000ff">public</font> String toString() { <font color="#0000ff">return</font> s; }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> SprinklerSystem {
  <font color="#0000ff">private</font> String valve1, valve2, valve3, valve4;
  WaterSource source;
  <font color="#0000ff">int</font> i;
  <font color="#0000ff">float</font> f;
  <font color="#0000ff">void</font> print() {
    System.out.println("valve1 = " + valve1);
    System.out.println("valve2 = " + valve2);
    System.out.println("valve3 = " + valve3);
    System.out.println("valve4 = " + valve4);
    System.out.println("i = " + i);
    System.out.println("f = " + f);
    System.out.println("source = " + source);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    SprinklerSystem x = <font color="#0000ff">new</font> SprinklerSystem();
    x.print();
  }
} <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 methods defined in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>WaterSource
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">is
special: 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
You will learn later that every non-primitive object has a <A NAME="Index427"></A><A NAME="Index428"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method, and it&#8217;s called in special situations when the compiler wants a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
but it&#8217;s got one of these objects. So in the expression:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">System.out.println("source
= " + source);
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">the
compiler sees you trying to add a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>&#8220;source
= &#8220;
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>WaterSource</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This doesn&#8217;t make sense to it, because you can only &#8220;add&#8221; a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to another 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so it says &#8220;I&#8217;ll turn 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>source</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
into a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
by calling 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">!&#8221;
After doing this it can combine the two 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
and pass the resulting 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>System.out.println(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Any time you want to allow this behavior with a class you create you need only
write a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. <A NAME="Index429"></A><A NAME="Index430"></A></FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">At
first glance, you might assume &#8211; Java being as safe and careful as it is
&#8211; that the compiler would automatically construct objects for each of the
handles in the above code, for example calling the default constructor for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>WaterSource</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to initialize 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>source</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The output of the print statement is in fact:
</FONT><P></DIV>

<font color="#990000"><PRE>valve1 = <font color="#0000ff">null</font>
valve2 = <font color="#0000ff">null</font>
valve3 = <font color="#0000ff">null</font>
valve4 = <font color="#0000ff">null</font>
i = 0
f = 0.0
source = <font color="#0000ff">null</font> </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Primitives
that are fields in a class are automatically <A NAME="Index431"></A><A NAME="Index432"></A>initialized
to zero, as noted in Chapter 2. But the object handles are initialized to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and if you try to call methods for any of them you&#8217;ll get an exception.
It&#8217;s actually pretty good (and useful) that you can still print them out
without throwing an exception.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It
makes sense that the compiler doesn&#8217;t just create a default object for
every handle because that would incur unnecessary overhead in many cases. If
you want the handles initialized, you can do it:
</FONT><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	At
the point the objects are defined. This means that they&#8217;ll always be
initialized before the constructor is called.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	In
the constructor for that class
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Right
before you actually need to use the object. This can reduce overhead, if there
are situations where the object doesn&#8217;t need to be created.
</FONT></OL><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">All
three approaches are shown here:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Bath.java</font>
<font color="#009900">// Constructor initialization with composition</font>

<font color="#0000ff">class</font> Soap {
  <font color="#0000ff">private</font> String s;
  Soap() {
    System.out.println("Soap()");
    s = <font color="#0000ff">new</font> String("Constructed");
  }
  <font color="#0000ff">public</font> String toString() { <font color="#0000ff">return</font> s; }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Bath {
  <font color="#0000ff">private</font> String 
    <font color="#009900">// Initializing at point of definition:</font>
    s1 = <font color="#0000ff">new</font> String("Happy"), 
    s2 = "Happy", 
    s3, s4;
  Soap castille;
  <font color="#0000ff">int</font> i;
  <font color="#0000ff">float</font> toy;
  Bath() {
    System.out.println("Inside Bath()");
    s3 = <font color="#0000ff">new</font> String("Joy");
    i = 47;
    toy = 3.14f;
    castille = <font color="#0000ff">new</font> Soap();
  }
  <font color="#0000ff">void</font> print() {
    <font color="#009900">// Delayed initialization:</font>
    <font color="#0000ff">if</font>(s4 == <font color="#0000ff">null</font>)
      s4 = <font color="#0000ff">new</font> String("Joy");
    System.out.println("s1 = " + s1);
    System.out.println("s2 = " + s2);
    System.out.println("s3 = " + s3);
    System.out.println("s4 = " + s4);
    System.out.println("i = " + i);
    System.out.println("toy = " + toy);
    System.out.println("castille = " + castille);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Bath b = <font color="#0000ff">new</font> Bath();
    b.print();
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Bath
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">constructor
a statement is executed before any of the initializations take place. When you
don&#8217;t initialize at the point of definition, there&#8217;s still no
guarantee that you&#8217;ll perform any initialization before you send a
message to an object handle &#8211; except for the inevitable run-time exception.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here&#8217;s
the output for the program:
</FONT><P></DIV>

<font color="#990000"><PRE>Inside Bath()
Soap()
s1 = Happy
s2 = Happy
s3 = Joy
s4 = Joy
i = 47
toy = 3.14
castille = Constructed </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>print(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is called it fills in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s4</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
so that all the fields are properly initialized by the time they are used.
</FONT><a name="_Toc305593252"></a><a name="_Toc305628724"></a><a name="_Toc312374015"></a><a name="_Toc375545307"></a><a name="_Toc408018510"></a><P></DIV>

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