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<b>Trail:</b> Learning the Java Language

<br /><b>Lesson:</b> Generics (Updated)

<br /><b>Section:</b> Wildcards

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<div class="linkBHEAD"><a href="capture.html">Wildcard Capture and Helper Methods</a></div>

<div class="nolinkBHEAD">Guidelines for Wildcard Use</div>

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<div id="PageTitle"><h1>Guidelines for Wildcard Use</h1></div>

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<p>

One of the more confusing aspects when learning to program with generics is determining when to use an upper bounded wildcard and when to use a lower bounded wildcard. This page provides some guidelines to follow when designing your code.</p>

<p>

For purposes of this discussion, it is helpful to think of variables as providing one of two functions:</p>

<dl>

<dt><strong>An "In" Variable</strong></dt>

<dd>An "in" variable serves up data to the code. Imagine a copy method with two arguments: <tt>copy(src, dest)</tt>. The <tt>src</tt> argument provides the data to be copied, so it is the "in" parameter.</dd>

<dt><strong>An "Out" Variable</strong></dt>

<dd>An "out" variable holds data for use elsewhere. In the copy example, <tt>copy(src, dest)</tt>, the <tt>dest</tt> argument accepts data, so it is the "out" parameter.</dd>

</dl>

<p>

Of course, some variables are used both for "in" and "out" purposes &mdash; this scenario is also addressed in the guidelines.</p>

<p>

You can use the "in" and "out" principle when deciding whether to use a wildcard and what type of wildcard is appropriate. The following list provides the guidelines to follow:</p>

<div class="note"><hr /><strong>Wildcard Guidelines:</strong>&nbsp;<ul>

<li>An "in" variable is defined with an upper bounded wildcard, using the <tt>extends</tt> keyword.</li>

<li>An "out" variable is defined with a lower bounded wildcard, using the <tt>super</tt> keyword.</li>

<li>In the case where the "in" variable can be accessed using methods defined in the <tt>Object</tt> class, use an unbounded wildcard.</li>

<li>In the case where the code needs to access the variable as both an "in" and an "out" variable, do not use a wildcard.</li>

</ul>

<hr /></div>

<p>

These guidelines do not apply to a method's return type. Using a wildcard as a return type should be avoided because it forces programmers using the code to deal with wildcards.</p>

<p>

A list defined by <tt>List&lt;? extends ...&gt;</tt> can be informally thought of as read-only, but that is not a strict guarantee. Suppose you have the following two classes:</p>

<div class="codeblock"><pre>

class NaturalNumber {

private int i;

public NaturalNumber(int i) { this.i = i; }

// ...

}

class EvenNumber extends NaturalNumber {

public EvenNumber(int i) { super(i); }

// ...

}

</pre></div>

<p>

Consider the following code:</p>

<div class="codeblock"><pre>

List&lt;EvenNumber&gt; le = new ArrayList&lt;&gt;();

List&lt;? extends NaturalNumber&gt; ln = le;

ln.add(new NaturalNumber(35)); // compile-time error

</pre></div>

<p>

Because <tt>List&lt;EvenNumber&gt;</tt> is a subtype of <tt>List&lt;? extends NaturalNumber&gt;</tt>, you can assign <tt>le</tt> to <tt>ln</tt>. But you cannot use <tt>ln</tt> to add a natural number to a list of even numbers. The following operations on the list are possible:</p>

<ul>

<li>You can add <tt>null</tt>.</li>

<li>You can invoke <tt>clear</tt>.</li>

<li>You can get the iterator and invoke <tt>remove</tt>.</li>

<li>You can capture the wildcard and write elements that you've read from the list.</li>

</ul>

<p>

You can see that the list defined by <tt>List&lt;? extends NaturalNumber&gt;</tt> is not read-only in the strictest sense of the word, but you might think of it that way because you cannot store a new element or change an existing element in the list.</p>

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