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<div id="PageTitle"><h1>Initializing Fields</h1></div>
<div id="PageContent">
<!-- Initializing Fields -->
<p>As you have seen, you can often provide an initial value for a field in its declaration:</p>
<div class="codeblock"><pre>
public class BedAndBreakfast {
// initialize to 10
public static int capacity = 10;
// initialize to false
private boolean full = false;
}
</pre></div>
<p>This works well when the initialization value is available and the initialization can be put on one line. However, this form of initialization has limitations because of its simplicity. If initialization requires some logic (for example, error handling or a <code>for</code> loop to fill a complex array), simple assignment is inadequate. Instance variables can be initialized in constructors, where error handling or other logic can be used. To provide the same capability for class variables, the Java programming language includes <i>static initialization blocks</i>.</p>
<div class="note"><hr /><strong>Note:</strong> It is not necessary to declare fields at the beginning of the class definition, although this is the most common practice. It is only necessary that they be declared and initialized before they are used.
<hr /></div>
<h2>Static Initialization Blocks</h2>
<p>A <i>static initialization block</i> is a normal block of code enclosed in braces, <code>{ }</code>, and preceded by the <code>static</code> keyword. Here is an example:</p>
<div class="codeblock"><pre>
static {
// whatever code is needed for initialization goes here
}
</pre></div>
<p>A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.</p>
<p>There is an alternative to static blocks — you can write a private static method:</p>
<div class="codeblock"><pre>
class Whatever {
public static varType myVar = initializeClassVariable();
private static varType initializeClassVariable() {
// initialization code goes here
}
}
</pre></div>
<p>The advantage of private static methods is that they can be reused later if you need to reinitialize the class variable.</p>
<h2>Initializing Instance Members</h2>
<p>Normally, you would put code to initialize an instance variable in a constructor. There are two alternatives to using a constructor to initialize instance variables: initializer blocks and final methods.</p>
<p>Initializer blocks for instance variables look just like static initializer blocks, but without the <code>static</code> keyword:</p>
<div class="codeblock"><pre>
{
// whatever code is needed for initialization goes here
}
</pre></div>
<p>The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of code between multiple constructors.</p>
<p>A <i>final method</i> cannot be overridden in a subclass. This is discussed in
the lesson on
interfaces and inheritance. Here is an example of using a final method for initializing an instance variable:</p>
<div class="codeblock"><pre>
class Whatever {
private varType myVar = initializeInstanceVariable();
protected final varType initializeInstanceVariable() {
// initialization code goes here
}
}
</pre></div>
<p>This is especially useful if subclasses might want to reuse the initialization method. The method is final because calling non-final methods during instance initialization can cause problems.</p>
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