CPSC 210

B3: Classes & Objects

Learning Goals

Identify fields, methods and constructors of a class
Distinguish between primitive and reference variables
Use an ArrayList to store data
Identify "active" (or: instantiated) objects at a point of execution in code
Use the debugger 🐞 to step through code and inspect variables

OO Terminology (1)

Class	Cat
Attribute / Field	name, age, weight
Method	meow(3), sleep(), eat()
Constructor (just a special method)	Create a new cat objects (e.g. Luna)
Object / Instance	Luna (created by constructor)

OO Terminology (2)

Class	Dog
Attribute / Field	name, age, weight
Method	bark(2), sleep(), eat()
Constructor (just a special method)	Create new dog objects (e.g. Charlie)
Object / Instance	Charlie (created by constructor)

Class Example: Tank

Methods

From: Space Invaders

Class

"final" Attributes/Fields

Constructor

Attributes/Fields

Tank myTank;

myTank = new Tank(400);

Tank object

direction 1

x 400

myTank

This does not create an object/instance yet, but only a (potential) reference to a tank

We first need to create an object

Objects vs. References (1)

Objects vs. References (2)

myTank.faceLeft();

myTank

Tank object

direction -1

x 400

Objects vs. References (3)

myTank.move();

myTank

Tank object

direction -1

x 399

Pod Activity (Do in Groups of 3-5)

Each student open the SpaceInvadersBase project in VisualStudio code from last class, open Tank.java
- (do not use the downloaded zip file from edX)
Answer the following questions as a group:

What do you think the final keyword does and why is it needed?
What happens when you set x=x on Line 22 ? Why ?
What do you think the this keyword is , and why is it needed?

Types in Java

Java primitive (built-in) types
- byte, short, int, long, float, double, boolean, char
All other types are reference types
- They are used to reference an object

Person
object

Person p = new Person();

int count = 4;

count 4

Pod Activity (Do in Groups of 3-5)

Each student should clone the ArrayListDemo repository
Follow the class demo on the VS Code debugger (this is VERY important!!) -- Add a breakpoint at Line 9
Then, answer the following questions as a group:

The instructor told you about "Step Into", "Step Over", and "Step Out". What do each of these mean?
Slowly step through the Main.java file, Pause at each step and make sure you understand how assignment and objects work!

ArrayList in Java

Used to store data of arbitrary size
In Racket, you treated (listof X) as built-in, where X is a type parameter
Similarly, ArrayList<E> is part of the Java library:
- E is a type parameter that represents the type of data to be stored in the list

...

Lecture Ticket Review

Which of the following are valid Java code segments?

int y = 3;

A a;
a = new A();

B b = make B();

int x = 3;
x.add(1);

public class Person {
    private String name;

    public Person () {
        this.name = "Unnamed Person";
    }

    public void setName(String name) {
        this.name = name;
        System.out.println("Now my name is " + 
        	this.name);
    }

    public String getName() {
        return name;
    }
}

public class Main {
    public static void main(String[] args) {
        Person harry = new Person();
        Person hermione = new Person();
        harry.setName("Harry");
        hermione.setName("Hermione");

        ArrayList<Person> people = 
        	new ArrayList<>();
        people.add(harry);
        people.add(harry);
        people.add(hermione);

        Person someone = people.get(1);
        someone.setName("Ron");
    }
}