website/content/ta/spring2018/cpsc220/mar27.md

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2020-01-15 21:51:49 -05:00
# 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() )
}
}
```