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HTML
239 lines
7.6 KiB
HTML
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<h1>CPSC 220 Lecture 4</h1>
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<h2>Practice Problem</h2>
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<ol>
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<li>Create a class called Car</li>
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<li>
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<ul>
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<li>Create a private variable of int type called year</li>
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<li>Create a private variable of String type called make</li>
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</ul>
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</li>
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<li>Create accessor methods for all data members.</li>
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<li>Create mutator methods for all data methods.</li>
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</ol>
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<pre><code class="language-java">public class car { // begin car
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private int year;
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private String make;
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public int getYear(void) {
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return year;
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}
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public String getMake() {
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return make;
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}
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public void setYear(int y) {
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if (y > 1890) {
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year = y;
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} else {
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System.out.println(y + " is not a valid year.");
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}
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}
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public void setMake(String m) {
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make = m;
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}
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}</code></pre>
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<p>Local variables are only existent within the curly braces that it is defined in.</p>
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<h2>If Statements and Boolean Expressions</h2>
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<p>Boolean expressions return a boolean</p>
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<pre><code class="language-java">1 < 4; // 1 is less than 4: TRUE
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3 > 5; // 3 is greater than 5: FALSE
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5 == 5; // 5 is equal to 5: TRUE
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5 != 5; // 5 is not equal to 5: FALSE
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1 >= 1; // 1 is greater than or equal to 1: TRUE
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5 <= 1; // 5 is less than or equal to 1: FALSE</code></pre>
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<p>If statements only occur if the boolean expression is true, otherwise the <code>else</code> block is executed.</p>
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<pre><code class="language-java">if (true) {
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System.out.println("I am always printed");
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} else {
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System.out.println("I am never printed");
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}</code></pre>
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<p>You can only have one <code>else</code> per <code>if</code>. If you have an <code>if</code> you don't necessarily need an <code>else</code></p>
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<h2>Local vs Class Variables</h2>
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<p>If you have a local variable and the class variable sharing the same name, then the local variable is always used first.</p>
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<pre><code class="language-java">public class car { // begin car
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private int year;
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public void setYear(int year) {
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year = year;
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}
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}</code></pre>
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<p>This is a redundant statement, it makes the argument that is passed in equal to itself.</p>
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<p>To avoid this situation, use the keyword <code>this</code> to access the class variable</p>
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<pre><code class="language-java">public class car {
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private int year;
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public void setYear(int year) {
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this.year = year;
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}
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}</code></pre>
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<p>The code above runs as expected.</p>
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<p>Rewriting our class with <code>this</code></p>
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<pre><code class="language-java">public class car { // begin car
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private int year;
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private String make;
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public int getYear(void) {
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return year;
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}
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public String getMake() {
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return make;
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}
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public void setYear(int year) {
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if (y > 1890) {
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this.year = year;
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} else {
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System.out.println(y + " is not a valid year.");
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}
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}
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public void setMake(String make) {
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this.make = make;
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}
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}</code></pre>
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<h2>Unreachable Code</h2>
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<p>When the code hits a <code>return</code> statement, it stops executing the rest of the code in the method. Also throws an Unreachable Code Error.</p>
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<pre><code class="language-java">public int add(int x, int y) {
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return x + y;
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System.out.println("x + y = " + x + y);
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}
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add();
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System.out.println("Hello");</code></pre>
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<p>Here the code above will not compile, though assuming the error doesn't exist then it would only print out "Hello"</p>
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<h2>Constructors</h2>
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<p>You cannot have a private or protected constructors.</p>
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<p>Constructors are used to initialize your objects.</p>
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<p>You want to have the class variables to the left of the assignment statement.</p>
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<pre><code class="language-java">public class car {
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private int year;
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private String make;
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car() {
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year = 1890;
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make = "Ford";
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}
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car(int year, String make) {
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this.year = year;
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this.make = make;
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}
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}</code></pre>
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<h2>Testers</h2>
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<p>Testers are useful to check that the class is implemented correctly. Both the tester and the class have to be in the same folder/directory.</p>
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<pre><code class="language-java">public class carTester {
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public static void main(String[] args) {
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Car myCar; // Declaration
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myCar = new Car(); // Initilization
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Car yourCar = new Car(2009, "Hyundai"); // Declaration + Initialization
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}
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}</code></pre>
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<h2>More about classes</h2>
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<pre><code class="language-java">public class Car {
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private String name;
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private int odometer;
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public void setOdometer(int od) {
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odometer = od;
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}
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public void setName(String n) {
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this.name = n;
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}
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public void changeOilRequest(String name, int od) {
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if (name == this.name) {
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int difference = od - this.odometer;
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if (difference > = 3000) {
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// You can call other methods in the class
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setOdo(od); // Equivalent to "this.setOdo(od);"
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this.odometer = od;
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System.out.println("Ready for oil change.");
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} else {
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System.out.println(name + " not ready for oil change.")
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}
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} // end if
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} // end changeOil request
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} // end class</code></pre>
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<p>To call public methods outside the class use the variable name to do so.</p>
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<pre><code class="language-java">public class CarTester {
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public static void main(String[] args) {
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Car myCar = new Car();
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myCar.setName("Honda")
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myCar.changeOilRequest("Honda", 3400);
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}
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}</code></pre>
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<h2>Math library</h2>
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<p>The <code>ceil</code> method rounds up while the <code>floor</code> method runs down.</p>
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<pre><code class="language-java">Math.ceil(3.2); // 4
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Math.ceil(4.1); // 4</code></pre>
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</article>
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