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content/ta/spring2018/cpsc220/mar27.md

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