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<!doctype html>
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<title>Java 8 Programmer II Study Guide: Exam 1Z0-809</title>
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<h1><i class="chapter">Appendix A</i><br />
From Java 6/7 to Java 8</h1>
<p><br /></p>
<h3 style="text-align: center;"><i>Exam Objectives</i></h3>
<p style="text-align: center;"><i>Develop code that uses Java SE 8 collection improvements, including Collection.removeIf(), List.replaceAll(), Map.computeIfAbsent(), and Map.computeIfPresent() methods.</i><br /><i>Develop code that uses String objects in the switch statement, binary literals, and numeric literals, including underscores in literals.</i><br /><i>Use Lock, ReadWriteLock, and ReentrantLock classes in the java.util.concurrent.locks.</i><br /><i>Format dates, numbers, and currency values for localization with the NumberFormat and DateFormat classes, including number and date format patterns.</i><br /><i>Recursively access a directory tree by using the DirectoryStream and FileVisitor interfaces</i><br /><i>Find a file by using the PathMatcher interface.</i><br /><i>Observe the changes in a directory by using the WatchService interface.</i></p>
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<h2>New Methods on Collections</h2>
<p>Java 8 brings some new helper methods for <code>Collection</code>s to write less verbose code.</p>
<p>The first one is <code>forEach()</code> (we talked about it earlier):</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">default</span> <span class="hljs-keyword">void</span> <span class="hljs-title">forEach</span><span class="hljs-params">(Consumer<? <span class="hljs-keyword">super</span> T> action)</span></span></code></p>
<p>That calls the <code>accept()</code> method on each element of the collection. So instead of having something like:</p>
<p><code class="java hljs"><span class="hljs-keyword">for</span>(<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i < list.size(); i++) {<br />
System.out.println(list.get(i));<br />
}</code></p>
<p>Or:</p>
<p><code class="java hljs"><span class="hljs-keyword">for</span>(String s : list) {<br />
System.out.println(s);<br />
}</code></p>
<p>Now we can have it as (without creating a stream):</p>
<p><code class="java hljs">list.forEach(System.out::println);</code></p>
<p>Another one is <code>removeif()</code>:</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">default</span> <span class="hljs-keyword">boolean</span> <span class="hljs-title">removeIf</span><span class="hljs-params">(Predicate<? <span class="hljs-keyword">super</span> E> filter)</span></span></code></p>
<p>That iterates over the elements of the collection and removes an element if it matches the given predicate (if the implementation of the <code>Collection</code> doesn't support removal, an <code>UnsupportedOperationException</code> is thrown). So instead of having something like:</p>
<p><code class="java hljs">Iterator<String> it = list.iterator();<br />
<span class="hljs-keyword">while</span> (it.hasNext()) {<br />
String s = it.next();<br />
<span class="hljs-keyword"> if</span>(s != <span class="hljs-keyword">null</span> && s.length() < <span class="hljs-number">3</span>) {<br />
it.remove();<br />
}<br />
}</code></p>
<p>Now we can have just:</p>
<p><code class="java hljs">list.removeIf( s -> s != <span class="hljs-keyword">null</span> && s.length() < <span class="hljs-number">3</span> );</code></p>
<p>We also have <code>replaceAll()</code>:</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">default</span> <span class="hljs-keyword">void</span> <span class="hljs-title">replaceAll</span><span class="hljs-params">(UnaryOperator<E> operator)</span></span></code></p>
<p>That replaces each element of this list with the result of applying the operator to the element. So instead of having something like:</p>
<p><code class="java hljs">Iterator<String> it = list.iterator();<br />
<span class="hljs-keyword">while</span> (it.hasNext()) {<br />
String s = it.next();<br />
it.set(s.toUpperCase());<br />
}</code></p>
<p>Now we just have:</p>
<p><code class="java hljs">list.replaceAll(s -> s.toUpperCase());</code></p>
<h2>New Methods on Maps</h2>
<p>Java 8 also brings new helper methods to <code>Map</code>s. There's two interesting methods in particular.</p>
<p>One is <code>computeIfAbsent()</code>:</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">default</span> V <span class="hljs-title">computeIfAbsent</span><span class="hljs-params">(<br />
K key,<br />
Function<? <span class="hljs-keyword">super</span> K,? extends V> mappingFunction)</span></span></code></p>
<p>This method calculates the value of the given key, and adds it to the map only if:</p>
<ul>
<li>The key is not already in the map</li>
<li>The key has a <code>null</code> value associated</li>
<li>If the calculated value is different than <code>null</code></li>
</ul>
<p>For example:</p>
<p><code class="java hljs">Map<String, String> letters = <span class="hljs-keyword">new</span> HashMap<>();<br />
letters.put(<span class="hljs-string">"a"</span>, <span class="hljs-string">"a"</span>);<br />
letters.put(<span class="hljs-string">"b"</span>, <span class="hljs-keyword">null</span>);<br />
<br />
Function<String, String> func = k -> {<br />
<span class="hljs-keyword"> if</span>(k.startsWith(<span class="hljs-string">"d"</span>)) <span class="hljs-keyword">return</span> <span class="hljs-keyword">null</span>;<br />
<span class="hljs-keyword"> return</span> k.toUpperCase();<br />
};<br />
<span class="hljs-comment"><br />
// Won't update,already in map</span><br />
letters.computeIfAbsent(<span class="hljs-string">"a"</span>, func);<br />
<span class="hljs-comment">// Will update<br /></span> letters.computeIfAbsent(<span class="hljs-string">"b"</span>, func);<br />
<span class="hljs-comment">// Will add</span><br />
letters.computeIfAbsent(<span class="hljs-string">"c"</span>, func);<br />
<span class="hljs-comment">// Won't add</span><br />
letters.computeIfAbsent(<span class="hljs-string">"d"</span>, func);<br />
<span class="hljs-comment">// Yes, there's also a forEach in Maps</span><br />
letters.forEach( (key, value) -><br />
System.out.format(<span class="hljs-string">"%s-%s;"</span>, key, value)<br />
);</code></p>
<p>The output:</p>
<p><code class="java hljs">a-a;b-B;c-C;</code></p>
<p>The other method is <code>computeIfPresent()</code>:</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">default</span> V <span class="hljs-title">computeIfPresent</span><span class="hljs-params">(<br />
K key,<br />
BiFunction<? <span class="hljs-keyword">super</span> K,? <span class="hljs-keyword">super</span> V,? extends V> remappingFunction)</span></span></code></p>
<p>This method updates the value of a given key only if:</p>
<ul>
<li>The key is already in the map</li>
<li>The key has a non-<code>null</code> value associated and the function also returns a non-<code>null</code> value</li>
</ul>
<p>If the key has a non-<code>null</code> associated and the function returns <code>null</code>, the key is removed from the map. For example:</p>
<p><code class="java hljs">Map<String, String> letters = <span class="hljs-keyword">new</span> HashMap<>();<br />
letters.put(<span class="hljs-string">"a"</span>, <span class="hljs-string">"a"</span>);<br />
letters.put(<span class="hljs-string">"b"</span>, <span class="hljs-keyword">null</span>);<br />
letters.put(<span class="hljs-string">"d"</span>, <span class="hljs-string">"d"</span>);<br />
<br />
BiFunction<String, String, String> func = (k, v) -> {<br />
<span class="hljs-keyword"> if</span>(k.startsWith(<span class="hljs-string">"d"</span>)) <span class="hljs-keyword">return</span> <span class="hljs-keyword">null</span>;<br />
<span class="hljs-keyword"> return</span> k.toUpperCase();<br />
};<br />
<br />
<span style="color: rgb(0, 106, 0);">// Will update</span><br />
letters.computeIfPresent(<span class="hljs-string">"a"</span>, func);<br />
<span style="color: rgb(0, 106, 0);">// Won't update</span> <br />
letters.computeIfPresent(<span class="hljs-string">"b"</span>, func);<br />
<span style="color: rgb(0, 106, 0);">// Won't add</span> <br />
letters.computeIfPresent(<span class="hljs-string">"c"</span>, func); <br />
<span style="color: rgb(0, 106, 0);">// Will remove</span><br />
letters.computeIfPresent(<span class="hljs-string">"d"</span>, func); <br />
letters.forEach( (key, value) -><br />
System.out.format(<span class="hljs-string">"%s-%s;"</span>, key, value)<br />
);</code></p>
<p>In this case, the output will be:</p>
<p><code class="java hljs">a-A;b-<span class="hljs-keyword">null</span>;</code></p>
<h2>Switch statement</h2>
<p>A <code>switch</code> statement is another way to represent an <code>if-else</code> statement. This is its syntax:</p>
<p><code class="java hljs">switch(expression) {<br/>
case constant_value_1:<br/>
statements;<br/>
case constant_value_2:<br/>
statements;<br/>
default:<br/>
statements;<br/>
}</code></p>
<p>The switch expression must evaluate to one of the following types:</p>
<ul>
<li><code>byte</code>, <code>short</code>, <code>char</code>, <code>int</code></li>
<li><code>enum</code></li>
<li><code>String</code></li>
</ul>
<p>Anything else, like a <code>float</code>, will generate a compiler error.</p>
<p>Here's an example of a <code>switch</code> statement:</p>
<p><code class="java hljs">String s = <span class="hljs-string">"Jack"</span>;<br/>
<span class="hljs-keyword">switch</span>(s) {<br/>
<span class="hljs-keyword">case</span> <span class="hljs-string">"Mike"</span>:<br/>
System.out.println(<span class="hljs-string">"Good morning Mr. "</span> + s); <br/> <span class="hljs-keyword">break</span>;<br/>
<span class="hljs-keyword">case</span> <span class="hljs-string">"Laura"</span>:<br/>
System.out.println(<span class="hljs-string">"Good morning Mrs. "</span> + s); <br/> <span class="hljs-keyword">break</span>;<br/>
<span class="hljs-keyword">default</span>:<br/>
System.out.println(<span class="hljs-string">"Good morning "</span> + s); <br/>
}</code></p>
<p>There are many things to take into account with a <code>switch</code> statement.</p>
<p>First, each case needs either a constant value or a <code>final</code> variable initialized at declaration. Otherwise, a compile time error will be generated:</p>
<p><code class="java hljs"><span class="hljs-keyword">final</span> <span class="hljs-keyword">int</span> goodFinal = 2;<br/>
<span class="hljs-keyword">final</span> <span class="hljs-keyword">int</span> badFinal;<br/>
badFinal = 3;<br/>
<span class="hljs-keyword">int</span> var = 1;<br/>
<br/>
<span class="hljs-keyword">switch</span>(var) {<br/>
<span class="hljs-keyword">case</span> 1: <span class="hljs-comment">// OK</span><br/>
var *= 2; <br/> <span class="hljs-keyword">break;</span><br/>
<span class="hljs-keyword">case</span> goodFinal: <span class="hljs-comment">// OK</span><br/>
var++; <br/> <span class="hljs-keyword">break;</span><br/>
<span class="hljs-keyword">case</span> badFinal: <span class="hljs-comment">// Compiler error!</span><br/>
var--;<br/>
}</code></p>
<p>It's not valid to have more than <code>case</code> with the same value:</p>
<p><code class="java hljs"><span class="hljs-keyword">final</span> <span class="hljs-keyword">int</span> goodFinal = 2;<br/>
<span class="hljs-keyword">int</span> var = 1;<br/>
<br/>
<span class="hljs-keyword">switch</span>(var) {<br/>
<span class="hljs-keyword">case</span> 1: <span class="hljs-comment">// OK</span><br/>
var *= 2; <br/> <span class="hljs-keyword">break;</span><br/>
<span class="hljs-keyword">case</span> 2: <span class="hljs-comment">// Compiler error!</span><br/>
var++; <br/> <span class="hljs-keyword">break;</span><br/>
<span class="hljs-keyword">case</span> goodFinal: <span class="hljs-comment">// Compiler error!</span><br/>
var--;<br/>
}</code></p>
<p>Besides, <code>case</code> labels are evaluated from top to down beginning from the first <code>case</code> constant that matches the <code>switch</code> expression. This means that all the subsequent statements will be executed until the end of the <code>switch</code> statement, or a <code>break</code> is found. For example:</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> var = 1;<br/>
<br/>
<span class="hljs-keyword">switch</span>(var) {<br/>
<span class="hljs-keyword">case</span> 1:<br/>
System.out.println(<span class="hljs-string">"Shirt"</span>); <br/>
<span class="hljs-keyword">case</span> 2:<br/>
System.out.println(<span class="hljs-string">"Pants"</span>);<br/>
<span class="hljs-keyword">case</span> 3:<br/>
System.out.println(<span class="hljs-string">"Shoes"</span>);<br/>
}</code></p>
<p>Will output:</p>
<p><code class="java hljs">Shirt<br/>
Pants<br/>
Shoes
</code></p>
<p>Because of this, we can have a <span class="hljs-keyword">switch</span> statement like the following:</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> var = 1;<br/>
<br/>
<span class="hljs-keyword">switch</span>(var) {<br/>
<span class="hljs-keyword">case</span> 1:<br/>
<span class="hljs-keyword">case</span> 2:<br/>
System.out.println(<span class="hljs-string">"Pants"</span>);<br/> <span class="hljs-keyword">break;</span><br/>
<span class="hljs-keyword">case</span> 3:<br/>
System.out.println(<span class="hljs-string">"Shoes"</span>);<br/>
}</code></p>
<p>Where, if <code>var</code> is <code>1</code> or <code>2</code>, execution will <i>fall through</i>, and <code>Pants</code> will be printed.</p>
<p>Also, notice that <code>default</code> doesn't need a <code>break</code> because it's the last element of the <code>switch</code> statement. However, <code>default</code> can appear in any position, so watch for it.</p>
Finally, it may be obvious, but <code>switch</code> statement can only test for equality. This is important when using a <code>String</code> expression because we have to take into account the meaning of object equality. In other words, this won't match:
<p><code class="java hljs">String s = <span class="hljs-string">"a"</span>;<br/>
<br/>
<span class="hljs-keyword">switch</span>(s) {<br/>
<span class="hljs-keyword">case</span> <span class="hljs-string">"A"</span>: <span class="hljs-comment">// Strings are case-sensitive, so this case won't match</span><br/>
...<br/>
}</code></p>
<h2>Numeric Literals</h2>
<p>In Java, integers numbers are by default of type <code>int</code> and floating-point numbers are by default of type <code>double</code>.</p>
<p>Integer numbers are the ones with types <code>byte</code>, <code>short</code>, <code>int</code>, and <code>long</code> and generally expressed in decimal base. However, they can also be expressed as hexadecimal or binary numbers.</p>
<p>Hexadecimal numbers consist of numbers <code>0</code> through <code>9</code> and letters <code>A</code> through <code>F</code>. To specify a hexadecimal number, you add the <code>0x</code> or <code>0X</code> prefix.</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> hex = 0x2B; <span class="hljs-comment">// The number 43</span>
</code></p>
<p>Binary numbers consist of the numbers <code>0</code> and <code>1</code>. To specify a binary number, you add the <code>0b</code> or <code>0B</code> prefix.</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> bin = 0b0010110; <span class="hljs-comment">// The number 22</span>
</code></p>
<p>These numbers can be cast to <code>float</code> or <code>double</code> types, but we can't express these types as hexadecimal or binary numbers. For example:</p>
<p><code class="java hljs"><span class="hljs-keyword">float</span> goodFloat = 0xF2; <span class="hljs-comment">// OK, number 242.0</span><br/>
<span class="hljs-keyword">double</span> goodDouble = 0b11110110; ; <span class="hljs-comment">// OK, number 246.0</span><br/>
<span class="hljs-keyword">float</span> badFloat = 0xF2f; <span class="hljs-comment"> // Compilation error!</span><br/>
<span class="hljs-keyword">double</span> badDouble = 0b11110110d; ; <span class="hljs-comment">// Compilation error!</span>
</code></p>
<p>From Java 7, you can place any number of underscores between the digits of numbers to separate groups of digits and improve readability.</p>
<p>You cannot put underscores in the following places:</p>
<ul>
<li>At the beginning or end of a number</li>
<li>Adjacent to a decimal point in a floating point literal</li>
<li>Before an F or L suffix</li>
<li>In positions where a string of digits is expected</li>
</ul>
<p>Here are some examples:</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> i = 34_765; <span class="hljs-comment">// OK</span><br/>
<span class="hljs-keyword">float</span> f = 43.987_876; <span class="hljs-comment">// OK</span><br/>
<span class="hljs-keyword">short</span> s = 0b0000____0101; <span class="hljs-comment">// OK</span><br/>
<span class="hljs-keyword">int</span> i2 = 100_; <span class="hljs-comment">// Compilation error!</span><br/>
<span class="hljs-keyword">int</span> i3 = 0_x1D; <span class="hljs-comment">// Compilation error!</span><br/>
<span class="hljs-keyword">int</span> i4 = 0x_1D; <span class="hljs-comment">// Compilation error!</span><br/>
<span class="hljs-keyword">float</span> f2 = 8945.40_f; <span class="hljs-comment">// Compilation error!</span><br/>
<span class="hljs-keyword">float</span> f3 = 8945_.40f; <span class="hljs-comment">// Compilation error!</span><br/>
</code></p>
<h2>java.util.concurrent.locks</h2>
<h4>Soon...</h4>
<h2>Formatting dates, numbers, and currency values</h2>
<h4>Soon...</h4>
<h2>DirectoryStream and FileVisitor interfaces</h2>
<h4>Soon...</h4>
<h2>PathMatcher</h2>
<h4>Soon...</h4>
<h2>WatchService </h2>
<h4>Soon...</h4>
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