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<!doctype html>
<html lang="en">
<head>
<meta http-equiv="x-ua-compatible" content="ie=edge">
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<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>
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<div class="header">
<div class="title-container">
<div class="chapter-title">
<h1><i class="chapter">Chapter SEVEN</i><br />
Collections</h1>
<p><br /></p>
<h3 style="text-align: center;"><i>Exam Objectives</i></h3>
<p style="text-align: center;"><i>Create and use ArrayList, TreeSet, TreeMap, and ArrayDeque objects.</i></p>
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<div class="container">
<div class="column">
<h2>Answers</h2>
<p><b>1. The correct answer is C.</b><br />
<code>push()</code> inserts the element at the front of the deque. After pushing <code>1</code>, <code>2</code>, <code>3</code> the queue looks like <code>[3, 2, 1]</code>.<br />
<code>poll()</code> retrieves and removes the first element of this deque, <code>3</code> in this case.</p>
<p><br /></p>
<p><b>2. The correct answer is A.</b><br />
<code>TreeSet</code> doesn't allow null values because when you add an object, if no <code>Comparator</code> is passed to the constructor of the <code>TreeSet</code> (like in this case), this class assumes that the object implements <code>Comparable</code> and tries to call the <code>compareTo()</code> method.</p>
<p><br /></p>
<p><b>3. The correct answer is B.</b><br /> The <code>remove()</code> method has two versions, one that takes the index of the element to remove, and another that takes the object to remove.</p>
<p>Since we are passing an <code>int</code>, the version that takes an index is chosen and the second element is removed. If we want to remove the first element (with value <code>1</code>), we have to call the <code>remove()</code> method like this: <code>list.remove(new Integer(1))</code>.</p>
<p><br /></p>
<p><b>4. The correct answer is D.</b><br />
<code>HashSet</code> is an implementation of <code>Set</code>. Objects used as keys of a <code>TreeMap</code> are required to implement the <code>hashCode()</code> method. Values are not required to implement anything.</p>
<p><br /></p>
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