CPSC 210

B3: Classes & Objects

Learning Goals

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

Constructors

Methods

Fields

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

...

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

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");
    }
}