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

1

5

4

2

3

Attributes/Fields

Tank myTank;
myTank = new Tank(400);

Tank object

 direction  1

 x  400

 myTank

 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:
  1. What do you think the final keyword does and why is it needed?
     
  2. What happens when you set x=x on Line 22 ? Why ?
     
  3. 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();

 p

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:

 

 

  1. The instructor told you about "Step Into", "Step Over", and "Step Out". What do each of these mean?
     
  2. 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

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0

...

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);

Q1

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");
    }
}

Q2

Lecture Lab: Buggy Pizza

edX > B3 > Lecture Lab

Hypothesis driven debugging: making careful educated guesses, with plans for how to validate or invalidate them.

Extra: Git Branching

Branch: main

Branch: myfavoriteanimal

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B3: Classes & Objects

The End - Thank You!

B3 Classes and Objects

By firas_moosvi

Source: Felix Grund

B3 Classes and Objects

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