2020-01-16 04:07:02 +00:00
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2022-02-15 06:14:58 +00:00
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2020-01-16 04:07:02 +00:00
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<article class="article">
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<h1>CPSC 220 Lecture 3</h1>
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<h2>Variables</h2>
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<p>Variable -- Storage of information</p>
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<p>The type cannot change in a variable.</p>
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<p>Examples of types include</p>
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<ul>
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<li>int</li>
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<li>float</li>
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<li>double</li>
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<li>String</li>
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<li>char</li>
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<li>boolean</li>
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</ul>
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<p>Declaration: <code>int num;</code></p>
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<p>Initialization: <code>num = 5;</code></p>
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<p>Declaration + Initialization: <code>int num = 5;</code></p>
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<h3>Possible Errors</h3>
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<p><strong>You cannot declare a variable multiple times.</strong></p>
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<p>Undefined variables are when you do not declare a variable before attempting to use it.</p>
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<h3>Casting</h3>
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<p>You need to cast if you are attempting to lose data or store a larger memory type into a smaller one.</p>
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<p>double -> float -> int <strong>(casting required)</strong></p>
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<pre><code class="language-java">double gpa = 3.2;
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int num1 = 10 * (int)gpa // 30</code></pre>
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<h1>Operations</h1>
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<p>The basic number operations are</p>
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<ul>
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<li>+</li>
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<li>-</li>
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<li>*</li>
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<li>/</li>
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<li>% <em>(the remainder)</em></li>
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</ul>
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<p>Examples</p>
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<pre><code class="language-java">0 % 2 // 0
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1 % 2 // 1
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2 % 2 // 0
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3 % 2 // 1
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4 % 2 // 0
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5 % 2 // 1
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3 % 5 // 3
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7 % 5 // 2</code></pre>
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<p>You can test if something is even using modulus %</p>
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<pre><code class="language-java">// Assuming i was initiliazed to a value earlier
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if (i % 2 == 0) {
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System.out.println("i is even");
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} else {
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System.out.println("i is odd");
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}</code></pre>
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<h1>System input</h1>
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<p>Here is sample code using a Scanner as input</p>
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<pre><code class="language-java">import java.util.Scanner;
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public class ScannerExample {
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public static void main(String[] args) {
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Scanner in;
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in = new Scanner(System.in);
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// Grab numerical values
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int num = in.nextInt();
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float gpa = in.nextFloat();
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double weight = in.nextDouble();
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// Grab a single character
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in.nextLine()
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char initial = in.next().charAt(0);
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// To get the entire line of a string
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in.nextLine();
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String name = in.nextLine();
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}
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}</code></pre>
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<p>You need to use <code>in.nextLine()</code> to grab the carriage return that is left after grabbing a numeric value.</p>
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<h1>Classes and Objects</h1>
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<p>Classes are a new type that you can have multiple things of.</p>
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<p>These classes are blueprints that are made up of primitives or more basic types.</p>
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<p>First create a Pet.java file (Name of the class must match the name of the file)</p>
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<pre><code class="language-java">public class Pet {
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private String name;
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private int years;
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}</code></pre>
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<p>You can then use the Pet class in your main program. The terminology here is that you can create instances or objects of the class.</p>
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<p>In PetTester.java</p>
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<pre><code class="language-java">public class PetTester {
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public static void main(String[] args) {
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Pet myPet;
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myPet = new Pet();
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}
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}</code></pre>
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<p><strong>Both Pet.java and PetTester.java must be in the same directory/folder</strong></p>
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<h3>Mutators/Accessors</h3>
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<p>Since the variables are private we cannot access them in the main program. To work around this, we can write what is called a mutator method.</p>
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<pre><code class="language-java">public class Pet {
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private String name;
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private int years;
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// Mutators
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public void setName(String n) {
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name = n;
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}
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public void setYears(int y) {
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if (y >= 0) {
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years = y;
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} else {
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System.out.println("No one is less than 0 years old.")
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}
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}
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}</code></pre>
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<p>Now let's use these new methods</p>
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<pre><code class="language-java">public class PetTester {
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public static void main(String[] args) {
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Pet mypet;
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myPet = new Pet();
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myPet.setName("Fred");
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myPet.setAge(20);
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}
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}</code></pre>
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<p>We need a method that will allow us to access the data type. Let's add accessors to our pet class.</p>
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<pre><code class="language-java">public class Pet {
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private String name;
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private int years;
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// Mutators
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public void setName(String n) {
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name = n;
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}
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public void setYears(int y) {
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if (y >= 0) {
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years = y;
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} else {
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System.out.println("No one is less than 0 years old.")
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}
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}
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// Accessors
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public String getName() {
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return name;
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}
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public int getYears() {
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return years;
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}
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}</code></pre>
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<p>Now let's get some information from the pet object, such as the age.</p>
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<pre><code class="language-java">public class PetTester {
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public static void main(String[] args) {
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Pet mypet;
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myPet = new Pet();
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myPet.setName("Fred");
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myPet.setYears(20);
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int year = myPet.getYears();
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}
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}</code></pre>
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<h3>Constructors</h3>
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<p>Constructors lets us initialize variables in the class without having to use mutator methods.</p>
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<pre><code class="language-java">public class Pet {
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private String name;
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private int years;
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// Default Constructor
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public Pet() {
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name = "";
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years = 0;
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}
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// Non-Default Constructor
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public Pet(int y, String n) {
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name = n;
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years = y;
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}
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// Mutator Methods
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public void setName(String n) {
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name = n;
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}
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public void setYears(int y) {
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if (y >= 0) {
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years = y;
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} else {
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System.out.println("No one is less than 0 years old.")
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}
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}
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// Accessor Methods
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public String getName() {
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return name;
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}
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public int getYears() {
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return years;
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}
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}</code></pre>
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<p>Now let us see this in action.</p>
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<pre><code class="language-java">public class PetTester {
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public static void main(String[] args) {
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Pet yourPet = new Pet(10, "Fluffy");
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}
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}</code></pre>
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<p>You can have as many constructors as you want, but they must be different.</p>
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<p>Example:</p>
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<pre><code class="language-java">public class Pet {
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...
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pet() {
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name = "";
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years = 0;
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}
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pet(int y, String n) {
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name = n;
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years = y;
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}
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pet(String n) {
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years = 1;
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name = n;
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}
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...
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}</code></pre>
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