/
ch14.html
286 lines (173 loc) · 18.2 KB
/
ch14.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
<!doctype html>
<html lang="en">
<head>
<meta http-equiv="x-ua-compatible" content="ie=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="Study guide for the Oracle Certified Professional, Java SE 8 Programmer Exam ">
<title>Java 8 Programmer II Study Guide: Exam 1Z0-809</title>
<link href="css/code.css" rel="stylesheet" type="text/css" />
<link href="css/style.css" rel="stylesheet" type="text/css" />
<link href="https://netdna.bootstrapcdn.com/font-awesome/3.2.1/css/font-awesome.css" rel="stylesheet">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.2/jquery.min.js"></script>
<script src="js/common-sections.js"></script>
</head>
<body>
<div class="nav"></div>
<div class="header">
<div class="title-container">
<div class="chapter-title">
<h1><i class="chapter">Chapter FOURTEEN</i><br />
Optional Class</h1>
<p><br /></p>
<h3 style="text-align: center;"><i>Exam Objectives</i></h3>
<p style="text-align: center;"><i>Develop code that uses the Optional class.</i><br /></p>
</div>
</div>
</div>
<div class="container">
<div class="column">
<h2>The problem with null</h2>
<p>Most programming languages have a data type to represent the absence of a value, and it is known by many names:</p>
<p><code class="hljs">NULL, nil, None, Nothing</code></p>
<p>The <code>null</code> type was introduced in ALGOL W by Tony Hoare in 1965, and it's considered one of the worst mistakes of computer science. In Tony Hoare's own words:</p>
<p><i>I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object-oriented language ((ALGOL W)). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.<br />
– Tony Hoare</i></p>
<p>Still, some may be wondering, what is the problem with <code>null</code>?</p>
<p>Well, if you're a little worried by the problems this code might cause, you know the answer already:</p>
<p><code class="java hljs">String summary = <br />
book.getChapter(<span class="hljs-number">10</span>)<br />
.getSummary().toUpperCase();</code></p>
<p>The problem with that code is that if any of those methods returns a <code>null</code> reference (for example, if the book doesn't have a tenth chapter), a <code>NullPointerException</code> (the most common exception in Java) will be thrown at runtime stopping the program.</p>
<p>What can we do to avoid this exception?</p>
<p>Perhaps, the easiest way is to check for <code>null</code>. Here's one way to it:</p>
<p><code class="java hljs">String summary = <span class="hljs-string">""</span>;<br />
<span class="hljs-keyword">if</span>(book != <span class="hljs-keyword">null</span>) {<br />
Chapter chapter = book.getChapter(<span class="hljs-number">10</span>);<br />
<span class="hljs-keyword"> if</span>(chapter != <span class="hljs-keyword">null</span>) {<br />
<span class="hljs-keyword"> if</span>(chapter.getSummary() != <span class="hljs-keyword">null</span>) {<br />
summary = chapter.getSummary()<br />
.toUpperCase();<br />
}<br />
}<br />
}</code></p>
<p>You don't know if any object in this hierarchy can be <code>null</code>, so you check every object for this value. Obviously, this is not the best solution; it's not very practical and damage readability.</p>
<p>There may be another issue. Is checking for <code>null</code> really desirable? I mean, what if those objects should never be <code>null</code>? By checking for <code>null</code>, we will be hiding the error and not be dealing with it.</p>
<p>Of course, this is also a design issue. For example, if a chapter has no summary yet, what would be better to use as a default value? An empty string or <code>null</code>?</p>
<p>To address this problem, Java 8 introduced the class <code>java.util.Optional<T></code>.</p>
<h2>The Optional class</h2>
<p>The job of this class is to <b>ENCAPSULATE</b> an optional value, an object that can be <code>null</code>.</p>
<p>Using the previous example, if we know that not all chapters have a summary, instead of modeling the class like this:</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Chapter</span></span> {<br />
<span class="hljs-keyword"> private</span> String summary;<br />
<span class="hljs-comment"> // Other attributes and methods</span><br />
}</code></p>
<p>We can use the <code>Optional</code> class:</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Chapter</span></span> {<br />
<span class="hljs-keyword"> private</span> Optional<String> summary;<br />
<span class="hljs-comment"> // Other attributes and methods</span><br />
}</code></p>
<p>So if there's a value, the <code>Optional</code> class just wraps it. Otherwise, an empty value is represented by the method <code>Optional.empty()</code>, that returns a singleton instance of <code>Optional</code>.</p>
<p>By using this class instead of <code>null</code>, first, we explicitly declare that the summary attribute is optional. Then, we can avoid <code>NullPointerExceptions</code> while having at our disposal the useful methods of <code>Optional</code> that we'll review up next.</p>
<p>First, let's see how to create an instance of this class.</p>
<p>To get an empty <code>Optional</code> object, use:</p>
<p><code class="java hljs">Optional<String> summary = Optional.empty();</code></p>
<p>If you are sure that an object is not <code>null</code>, you can wrap it in an <code>Optional</code> object this way:</p>
<p><code class="java hljs">Optional<String> summary = Optional.of(<span class="hljs-string">"A summary"</span>);</code></p>
<p>A <code>NullPointerException</code> will be thrown if the object is <code>null</code>. However, you can use:</p>
<p><code class="java hljs">Optional<String> summary = Optional.ofNullable(<span class="hljs-string">"A summary"</span>);</code></p>
<p>That returns an <code>Optional</code> instance with the specified value if it is non-<code>null</code>. Otherwise, it returns an empty <code>Optional</code>.</p>
<p>If you want to know if an <code>Optional</code> contains a value, you can do it like this:</p>
<p><code class="java hljs"><span class="hljs-keyword">if</span>( summary.isPresent() ) {<br />
<span class="hljs-comment"> // Do something</span><br />
}</code></p>
<p>Or in a more functional style:</p>
<p><code class="java hljs">summary.ifPresent(s -> System.out.println(s));<br />
<span class="hljs-comment">// Or summary.ifPresent(System.out::println);</span></code></p>
<p>The <code>ifPresent()</code> method takes a <code>Consumer<T></code> as an argument that is executed only if the <code>Optional</code> contains a value.</p>
<p>To get the value of an <code>Optional</code> use:</p>
<p><code class="java hljs">String s = summary.get();</code></p>
<p>However, this method will throw a <code>java.util.NoSuchElementException</code> if the <code>Optional</code> doesn't contain a value, so it's better to use the <code>ifPresent(</code>) method.</p>
<p>Alternatively, if we want to return something when the <code>Optional</code> doesn't contain a value, there are three other methods we can use:</p>
<p><code class="java hljs">String summaryOrDefault = summary.orElse(<span class="hljs-string">"Default summary"</span>);</code></p>
<p>The <code>orElse()</code> method returns the argument (that must be of type <code>T</code>, in this case a <code>String</code>) when the <code>Optional</code> is empty. Otherwise, it returns the encapsulated value.</p>
<p><code class="java hljs">String summaryOrDefault =<br />
summary.orElseGet( () -> <span class="hljs-string">"Default summary"</span> );</code></p>
<p>The <code>orElseGet()</code> method takes a <code>Supplier<? extends T></code> as an argument that returns a value when the <code>Optional</code> is empty. Otherwise, it returns the encapsulated value.</p>
<p><code class="java hljs">String summaryOrException =<br />
summary.orElseThrow( () -> <span class="hljs-keyword">new</span> Exception() );</code></p>
<p>The <code>orElseThrow()</code> method takes a <code>Supplier<? extends X></code>, where <code>X</code> is the type of the exception to throw when the <code>Optional</code> is empty. Otherwise, it returns the encapsulated value.</p>
<p>Like streams, there are versions of the <code>Optional</code> class to work with primitives, <code>OptionalInt</code>, <code>OptionalLong</code>, and <code>OptionalDouble</code>, so you can use <code>OptionalInt</code> instead of <code>Optional<Integer></code>:</p>
<p><code class="java hljs">OptionalInt optionalInt = OptionalInt.of(<span class="hljs-number">1</span>);<br />
<span class="hljs-keyword">int</span> i = optionalInt.getAsInt();</code></p>
<p>However, the use of these primitive versions are not encouraged, especially because they lack three useful methods of <code>Optional</code>: <code>filter()</code>, <code>map()</code>, and <code>flatMap()</code>. And since <code>Optional</code> just contains one value, the overhead of boxing/unboxing a primitive is not significant.</p>
<p>The <code>filter()</code> method returns the <code>Optional</code> if a value is present and matches the given predicated. Otherwise, an empty <code>Optional</code> is returned.</p>
<p><code class="java hljs">String summaryStr =<br />
summary.filter(s -> s.length() > <span class="hljs-number">10</span>).orElse(<span class="hljs-string">"Short summary"</span>);</code></p>
<p>The <code>map()</code> method is generally used to transform from one type to another. If the value is present, it applies the provided <code>Function< ? super T, ? extends U ></code> to it. For example:</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> summaryLength = summary.map(s -> s.length()).orElse(<span class="hljs-number">0</span>);</code></p>
<p>The <code>flapMap()</code> method is similar to <code>map()</code>, but it takes an argument of type <code>Function<? super T, Optional<U>></code> and if the value is present, it returns the <code>Optional</code> that results from applying the provided function. Otherwise, it returns an empty <code>Optional</code>.</p>
<p>In Chapter 17, we'll review in more detail the methods <code>map()</code> and <code>flatMap()</code> and how they are used with streams.</p>
<h2>Key Points</h2>
<ul>
<li>The <code>java.util.Optional<T></code> class <b>ENCAPSULATES</b> an optional value, i.e. an object that can be <code>null</code>.</li>
<li>An empty value is represented by the method <code>Optional.empty()</code>.</li>
<li>You can wrap an object in an <code>Optional</code> with the method <code>of()</code>, however, a <code>NullPointerException</code> will be thrown if the object is <code>null</code>.</li>
<li>The method <code>ofNullable()</code> returns an <code>Optional</code> instance with the specified value if it is non-<code>null</code>. Otherwise, it returns an empty <code>Optional</code>.</li>
<li>To get the value of an <code>Optional</code> use the method <code>get()</code>, but it will throw a <code>java.util.NoSuchElementException</code> if the <code>Optional</code> doesn't contain a value, so it's better to use the <code>ifPresent()</code> method that takes a <code>Consumer<T></code> as an argument that is executed only if the <code>Optional</code> contains a value.</li>
<li>The <code>orElse()</code> method returns the argument when the <code>Optional</code> is empty, otherwise, it returns the encapsulated value.</li>
<li>The <code>orElseGet()</code> method takes a <code>Supplier</code> that returns a value when the <code>Optional</code> is empty. Otherwise, it returns the encapsulated value.</li>
<li>The <code>orElseThrow()</code> method takes a <code>Supplier</code> that returns an exception when the <code>Optional</code> is empty. Otherwise, it returns the encapsulated value.</li>
</ul>
<h2>Self Test</h2>
<p>1. Given:</p>
<p><code class="java hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Question_14_1</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
Optional<String> opt = Optional.of(<span class="hljs-string">"1"</span>);<br />
String s = opt.orElseGet(<br />
() -> <span class="hljs-keyword">new</span> RuntimeException()<br />
);<br />
System.out.println(s);<br />
}<br />
}</code></p>
<p>What is the result?<br /> A. <code>1</code><br /> B. Nothing is printed<br /> C. Compilation fails<br /> D. An exception occurs at runtime</p>
<p>2. Which of the following statements is true?<br /> A. The method <code>Optional.isPresent()</code> takes a <code>Consumer<T></code> as an argument that is executed only if the <code>Optional</code> contains a value.<br /> B. The method <code>Optional.of()</code> can create an empty <code>Optional</code>.<br /> C. The method <code>Optional.of()</code> can throw a <code>NullPointerException</code>.<br /> D. The method <code>Optional.ifPresent()</code> takes a <code>Function<T,U></code> as an argument.</p>
<p>3. Given:</p>
<p><code class="java hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Question_14_3</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
System.out.println(ToInt(<span class="hljs-string">"a"</span>).get());<br />
}<br />
<span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-keyword">static</span> Optional<Integer> <span class="hljs-title">ToInt</span><span class="hljs-params">(String s)</span></span> {<br />
<span class="hljs-keyword"> try</span> {<br />
<span class="hljs-keyword"> return</span> Optional.of(Integer.parseInt(s));<br />
} <span class="hljs-keyword">catch</span>(Exception e) {<br />
<span class="hljs-keyword"> return</span> Optional.empty();<br />
}<br />
}<br />
}</code></p>
<p>What is the result?<br /> A. <code>a</code><br /> B. <code>Optional.empty</code><br /> C. Compilation fails<br /> D. An exception occurs at runtime</p>
<p>4. Given:</p>
<p><code class="java hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Question_14_4</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
System.out.println(<br />
Optional.of(<span class="hljs-number">0</span>).orElse(<span class="hljs-number">1</span>)<br />
);<br />
}<br />
}</code></p>
<p>What is the result?<br /> A. <code>0</code><br /> B. <code>1</code><br /> C. Compilation fails<br /> D. An exception occurs at runtime</p>
<div class="answers">
<a href="ch14a.html" target="_blank">Open answers page</a>
</div>
<div class="book-info"></div>
<div class="linkbox">
<div class="previous">
<a href="ch13.html">13. Iterating and Filtering Collections</a>
</div>
<div class="next">
<a href="ch15.html">15. Data Search</a>
</div>
<div style="clear:both;"></div>
</div>
</div>
</div>
</body>
</html>