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140 lines
3 KiB
Markdown
140 lines
3 KiB
Markdown
# Lecture for March 27
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## In the Real World...
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Objects are known for having characteristics. A car has on average 4 wheels, 2-4 doors, a steering wheel.
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Objects can perform actions. A car can drive, hold cargo, and honk.
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## In the Programming World...
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Object-Oriented Programming
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- Focuses on objects
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- Are not linear
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- Adds organization to a program
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- Fits with human cognition (making abstractions)
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## Class Structure
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```java
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public class Classname {
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// Fields
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// Constructors
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// Methods
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}
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```
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## Fields
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Fields are instance variables, they store values, help define state, and exist in memory for the lifetime of the object.
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```java
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public class Car {
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private double price;
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private double gas;
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}
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```
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## Constructor
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We can build an object through a constructor. It is a special kind of method, this method requires that you not have a return type and that you name it the same as the class itself.
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Constructors help set default field values for the different properties of our class.
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```java
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public class Car {
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// Begin Constructor
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public Car(double cost) {
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this.price = cost;
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this.gas = 0;
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}
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// End Constructor
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private double price;
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private double gas;
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}
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```
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**Note:** The `this` keyword refers to the object's fields. This helps keep it separate from other variables you can create in the method and the input parameters you receive.
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## Accessor Method - "Getter"
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We like to classify methods into two types, accessors and mutators.
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Getter methods return a copy of an instance field. It does not change the state of the object.
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```java
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public double getPrice() {
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return this.price;
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}
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```
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## Mutator Method - "Setter"
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This type of method modifies an instance field. It does not return anything and changes the state of the object.
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```java
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public void setPrice(double cost) {
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this.price = cost;
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}
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```
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## Example of Car Class In All Its Glory
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```java
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public class Car {
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// Instance Variables
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private int mpg;
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private double price;
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// Constructors
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public Car() {
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this.price = 0;
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this.mpg = 0;
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}
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public Car(double cost, int mpg) {
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this.price = cost;
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this.mpg = mpg;
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}
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// Accessors
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public double getPrice() {
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return this.price''
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}
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public int getMpg() {
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return this.mpg;
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}
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// Mutators
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public void setPrice(double cost) {
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this.price = cost;
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}
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public void setMpg(int mpg) {
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this.mpg = mpg;
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}
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}
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```
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## Using Classes
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Just like how we used the `Scanner` class, we can also use our new `Car` class.
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```java
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public class TestCar {
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public static void main(String[] args) {
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// Declare an object reference
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Car c;
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// Initialize the object
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c = new Car();
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// Update the fields of the object
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c.setPrice(3000);
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c.setMpg(22);
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// Print object information
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System.out.println("Price is " + c.getPrice() )
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}
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}
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```
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