Instances of Classes

What are Objects and Classes?: 2.1.1

• Java is object oriented
  • Can be used to model objects in the real world
• Class is used to define a new data type or a blueprint for objects
  • This is why java files start with public class
  • “blueprint/cookie cutter”
  • A type/classification
    • has attributes
      • properties
      • “what the object knows”
      • instance variables
    • has behaviors
      • “what the object does”
      • methods
• Object are variables created from a class definition
  • Instances of a class
  • String is a class
  • “Cookies” (cut from a cookie cutter)
    • Use a class to create as many objects as you want

Video

• class is a blueprint for an object
  • “tells you how to make objects”
  • Can make multiple objects with the same class
    • Called instances
  • Has attributes and methods
• instances can differ
  • has different attributes or instance variables
    • dog1 has blue fur, dog2 has purple fur

Intro to Objects with Turtles: 2.1.2

• Turtle class was written by CS A
  • Defines attributes for graphical turtles
    • Color
    • Position
    • Has methods to move

Video

import java.util.*;
import java.awt.*;

public class TurtleTest
{
  public static void main(String[] args)
  // Main method; runs program
  {
      World habitat = new World(300,300); 
      // Declares variable of type world; calls World constructor to make new world
      // keeps track of new world created
      Turtle yertle = new Turtle(habitat);  
      // Declares variable of type Turtle; calls Turtle constructor with argument habitat
      // Keeps track of a new turtle which was just constructed

      yertle.forward();
      // tells turtle to move forward
      yertle.turnLeft();
      // tells turtle to turn left
      yertle.forward();
      // tells turtle to move forward

      habitat.show(true);
      // Shows steps taken by a turtle object
  }
}

Dot Operator

• dot operator is used to access an object’s attributes and methods
  • executes a method
    • yertle.forward()
      • tells the object yertle to execute the method forward()
      • Give arguments in parenthesis
        • Data used by the method
        • yertle.forward(50)
  • accesses a member variable
    • They don’t talk about being able to do this in this lesson, but if I went along with saying that the dot operator only runs methods I would kind of be lying
    • depends on variable scope
      • this is it’s own can of worms which is why they probably didn’t mention it
      • if you don’t understand scope, don’t worry about this tangent; they’ll cover it later :)

Creating Turtle Objects: 2.1.3

• Class instances can be named anything
  • Are all unique objects

A class diagram of Turtle

Turtle: SimpleTurtle

Attributes

• name: String
• bodyColor: Color
• width: int = 15
• height: int = 18
• xpos: int = 0
• ypos: int = 0

Methods

• forward()
• backward()
• turnLeft()
• turnRight()
• penUp()
• penDown()
• forward(pixels: int)
• backward(pixels: int)
• turn(degrees: double)
• moveTo(x: int, y: int)
• setColor(theColor: Color)
• setWidth(theWidth: int)
• setHeight(theHeight: int)

Programming Challenge - Turtle Drawing: 2.1.4

Nothing to note; just complete the task

Summary: 2.1.5

• class defines a new data type
  • formal implementation or blueprint of the attributes and behaviors of objects of that class
• object is a specific instance of a class
  • declared as variables of a class type
• attribute/instance variable
  • data the object “knows” about itself
• method is something an object can do
  • turtle can forward()

AP Practice: 2.1.6

• A class is a blueprint for an object
  • Useful for modeling real world objects
  • Each instance has different attributes
• Belt
  • has a length
  • has a color
  • has a isReversable boolean

Closing Thoughts

My main complaint with this section was the assertion that the dot operator could only be used to access methods. I understand that accessing attributes wasn’t explained to avoid the complexity which comes along with scope, but I feel like the explanation of the dot operator could have been phrased better.