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221 lines
4.7 KiB
Markdown
221 lines
4.7 KiB
Markdown
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# CPSC 220 Lecture 4
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## Practice Problem
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1. Create a class called Car
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2. - Create a private variable of int type called year
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- Create a private variable of String type called make
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3. Create accessor methods for all data members.
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4. Create mutator methods for all data methods.
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```java
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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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}
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```
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Local variables are only existent within the curly braces that it is defined in.
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## If Statements and Boolean Expressions
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Boolean expressions return a boolean
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```java
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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
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```
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If statements only occur if the boolean expression is true, otherwise the `else` block is executed.
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```java
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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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}
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```
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You can only have one `else` per `if`. If you have an `if` you don't necessarily need an `else`
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## Local vs Class Variables
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If you have a local variable and the class variable sharing the same name, then the local variable is always used first.
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```java
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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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}
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```
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This is a redundant statement, it makes the argument that is passed in equal to itself.
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To avoid this situation, use the keyword `this` to access the class variable
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```java
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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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}
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```
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The code above runs as expected.
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Rewriting our class with `this`
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```java
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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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}
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```
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## Unreachable Code
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When the code hits a `return` statement, it stops executing the rest of the code in the method. Also throws an Unreachable Code Error.
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```java
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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");
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```
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Here the code above will not compile, though assuming the error doesn't exist then it would only print out "Hello"
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## Constructors
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You cannot have a private or protected constructors.
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Constructors are used to initialize your objects.
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You want to have the class variables to the left of the assignment statement.
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```java
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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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}
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```
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## Testers
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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.
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```java
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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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}
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```
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## More about classes
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```java
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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
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```
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To call public methods outside the class use the variable name to do so.
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```java
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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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}
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```
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## Math library
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The `ceil` method rounds up while the `floor` method runs down.
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```java
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Math.ceil(3.2); // 4
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Math.ceil(4.1); // 4
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```
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