java-optional/index.html at master · tginsberg/java-optional · GitHub

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<!doctype html>
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		<title>java.util.Optional: What and How</title>

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				<!-- Introduction -->
				<section>
					<h1>java.util.Optional</h1>
					<h3>What it is and how to use it</h3>
					<p>
						<small>
						Created by <a href="http://ginsberg.com" target="_blank">Todd Ginsberg</a>
						for <a href="http://www.meetup.com/ChicagoJUG/events/230375291/" target="_blank">CJUG</a>
						</small>
						<br/>
					    <small>2016-06-02</small>
					</p>
				</section>

				<section>
					<section>
						<h2>What is java.util.Optional?</h2>
						<ul>
							<li class="fragment">Introduced in Java 8</li>
							<li class="fragment">Intended to reduce NullPointerExceptions</li>
							<li class="fragment">A <u>signal</u> to other developers that a value may not be present</li>
							<li class="fragment">Container with zero or one elements</li>
						</ul>
					</section>

					<section>
						<h2>What isn't java.util.Optional?</h2>
						<ul>
							<li class="fragment">Not capable of storing null</li>
							<li class="fragment">Not serializable!</li>
							<li class="fragment">Not widely used in the JDK</li>
						</ul>
					</section>

					<section>
					    <blockquote cite="http://stackoverflow.com/questions/26327957/should-java-8-getters-return-optional-type/26328555#26328555">
					    	"Our intention was to provide a limited mechanism for library method return types where there needed to be a
					    	clear way to represent "no result", and using null for such was overwhelmingly likely to cause errors."
					    </blockquote>
					    	<br/>
					    	-- Brian Goetz
					</section>

					<section>
					 	<h3>How did it get here?</h3>
					 	Optional is mainly here to make stream terminal operations possible for empty streams.
						<pre><code data-trim>
new ArrayList&lt;String&gt;().stream().findFirst();
						</code></pre>
					</section>

					<section>
						<h3>Why not also add it to Map?</h3>
						<pre><code data-trim>
public interface Map&lt;K,V&gt; {

    // ...

    default Optional&lt;V&gt; getOptionally(K k) {
        // ...
    }
}
						</code></pre>
						<div class="fragment">
						Maps can contain null values!
						<pre><code data-trim>
Map&lt;String, String&gt; map = new HashMap&lt;&gt;();
map.put("A",null);

// Both look the same - no difference between not present and null.
map.getOptionally("A");
map.getOptionally("B");
						</code></pre>

						</div>

					</section>
				</section>

				<section>
					<h3>States</h3>
					<table>
						<tr>
							<th>value</th>
							<th>isEmpty()</th>
							<th>isPresent()</th>
							<th>.get()</th>
						</tr>
						<tr>
							<td>not&nbsp;null</td>
							<td>false</td>
							<td>true</td>
							<td>value</td>
						</tr>
						<tr>
							<td>null</td>
							<td>true</td>
							<td>false</td>
							<td>NoSuchElementException</td>
						</tr>
					</table>
				</section>

				<section>
					<h3>Creation</h3>
					Use one of the two built-in Factory methods
					<pre><code data-trim>
Optional&lt;String&gt; text = Optional.ofNullable("Hello, world!");
					</code></pre>

					<div class="fragment">
						<h4>Or...</h4>
						<pre><code data-trim>
Optional&lt;String&gt; text = Optional.of(somethingThatIsntNull);
						</code></pre>
					</div>

					<div class="fragment">
						<pre><code data-trim>
Optional&lt;String&gt; text = Optional.of(null);
// -> NullPointerException
						</code></pre>
					</div>
				</section>


				<section>
					<h3>When to use it</h3>
					If you use it at all, only use it for return types where there is no natural way to represent "no result"
					<pre><code data-trim>
// Yes
public Optional&lt;Customer&gt; getCustomerById(long id);
					</code></pre>
					<div class="fragment">
						<pre><code data-trim>
// No
public Optional&lt;List&lt;Customer&gt;&gt; getCustomersForMonth(final YearMonth when);
						</code></pre>
					</div>
					<div class="fragment">
						<pre><code data-trim>
// Prefer:
public List&lt;Customer&gt; getCustomersForMonth(final YearMonth when);
// -> Collections.emptyList();
						</code></pre>
					</div>
				</section>


				<section>
					<section>
						<h3>Don't call .get()!</h3>
						<pre><code data-trim>
Optional&lt;String&gt; text = service.getText();
if(text.isPresent()) {
    System.out.println(text.get());
} else {
    System.out.println("Hello, world!");
}
						</code></pre>
						<div class="fragment">
							Looks just like...
							<pre><code data-trim>
String text = service.getText();
if(text != null) {
    System.out.println(text);
} else {
    System.out.println("Hello, world!");
}
							</code></pre>
						</div>
					</section>

					<section>
						<blockquote cite="http://stackoverflow.com/questions/26327957/should-java-8-getters-return-optional-type/26328555#26328555">
						"In retrospect, we should have called get something like getOrElseThrowNoSuchElementException
						or something that made it far clearer that this was a highly dangerous method that undermined
						the whole purpose of Optional in the first place. Lesson learned."
						</blockquote>
					    	<br/>
					    	-- Brian Goetz
					</section>

					<section>
						<h3>Use one of the safe methods instead!</h3>
						<pre><code data-trim>
Optional&lt;String&gt; text = service.getText();
System.out.println(text.orElse("Hello, world!"));
						</code></pre>
						<div class="fragment">
							<pre><code data-trim>
Optional&lt;String&gt; text = service.getText();
text.ifPresent(this::doSomethingWithText);
							</code></pre>
						</div>
						<div class="fragment">
							<pre><code data-trim>
Optional&lt;String&gt; text = service.getText();
System.out.println(text.orElseThrow(NoSuchElementException::new));
							</code></pre>
						</div>
					</section>

				</section>


				<section>
					<h3>When not to use it</h3>
					<ul>
						<li class="fragment">Method parameters</li>
						<li class="fragment">Fields</li>
						<li class="fragment">In any DTO or Entity, because it is not serializable</li>
						<li class="fragment">If your customers are not using Java 8</li>
						<li class="fragment">Possibly, don't use it at all!</li>
					</ul>

				</section>

				<section>
					<h3>Rules to remember</h3>
					<ul>
						<li class="fragment">When returning them, make <u>absolutely</u> sure they aren't null</li>
						<li class="fragment">Don't call .get()</li>
				</section>

				<section data-transition="none">
					<h1>Thanks!</h1>
					<p>
						<i class="fa fa-twitter"></i>&nbsp;<a href="https://twitter.com/ToddGinsberg" target="_blank">@ToddGinsberg</a>
					</p>
					<p>
						<i class="fa fa-commenting"></i> <a href="http://bit.ly/FeedbackOptional" target="_blank">bit.ly/FeedbackOptional</a>
					</p>
				</section>

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