Object Oriented Programming

What is a class?

A class is a user-defined data type which can be used when primitive/basic data types (such as int or char) are not enough to satisfy the needs of the program. It allows the programmer to combine multiple primitive or user-defined data types into one single data type.

What is an object?

An object is an instance of a class. Think of the class as the “blueprint” for the object. The class defines what types of data are going to be stored in the object.

Example: Baby
class Baby:
    String name
    float length
    Date birthdate

To store the baby’s birthdate, we will declare a separate class Date

class Date:
    int year
    int month
    int day

A sample Baby object would look like:

name: "Sally"
length: 20.1
birthdate: Date (year: 2021, month: 12, day: 25)

Syntax for the Baby class in Java or Processing would look like:

class Baby {
    private String name;
    private float length;
    private Date birthdate;

    public Baby(String name0, float length0, Date birthdate0) {
        name = name0;
        length = length0;
        birthdate = birthdate0;
    }
}

The Date class would look like:

class Date {
    private int year;
    private int month;
    private int day;

    public Baby(String year0, float month0, Date day0) {
        year = year0;
        month = month0;
        day = day0;
    }
}

To create a Baby object (an instance of the Baby class):

Date birthdate = new Date(2021, 12, 25);
Baby myBaby = new Baby("Sally", 20.1, birthdate);

To access or modify the fields of an object in Java, you must either create “getter” and “setter” methods or declare the fields as public or protected to allow use of the . operator (not recommended since any class will be able to freely access the fields).

class Baby {
    private String name; // Alternatively, you can declare these as public/protected instead of private
    private float length;
    private Date birthdate;

    public Baby(String name0, float length0, Date birthdate0) {
        name = name0;
        length = length0;
        birthdate = birthdate0;
    }

    // Gets the current name
    public String getName() {
        return name;
    }

    // Sets the name to a new name
    public void setName(String newName) {
        name = newName;
    }

    // Do this for any other fields that need accessing ...
}

public class Main {
    public static void main(String[] args) {
        Baby myBaby = new Baby("Sally", 20.1, new Date(2021, 12, 25));
        System.out.println("My baby's name is " + myBaby.getName());
        myBaby.setName("Samantha"); // Set a new name
        System.out.println("My baby's name is " + myBaby.getName());
    }
}

Documentation for classes and objects in Python can be found here.

Sample implementation of the Baby and Date classes in Python. Remember that all Python instance methods must contain the parameter self.

class Baby:
    # Any instance methods must contain the parameter "self"
    def __init__(self, name, length, birthdate):
        self.name = name
        self.length = length
        self.birthdate = birthdate

class Date:
    def __init__(self, year, month, day):
        self.year = year
        self.month = month
        self.day = day

Getting and setting fields in Python can be done using the . operator.

myBaby = Baby("Sally", 20.1, Date(2021, 12, 25))
print("My baby's name is", myBaby.name)
print("My baby's birth year is", myBaby.birthdate.year)

print("My baby is", myBaby.length, "inches long.")
myBaby.length += 1 # Baby grew by an inch
print("My baby is", myBaby.length, "inches long.")

Python has instance methods for classes as well. Here’s an example:

class Baby:
    # Any instance methods must contain the parameter "self"
    def __init__(self, name, length, birthdate):
        self.name = name
        self.length = length
        self.birthdate = birthdate
    
    # Display info about the baby
    def showOff(self):
        print("My name is", self.name + ". I am", self.length, "inches long, and I was born on", self.birthdate.to_string())

class Date:
    def __init__(self, year, month, day):
        self.year = year
        self.month = month
        self.day = day

    # Convert the date to a readable string and return it
    def to_string(self):
        return str(self.year) + "-" + str(self.month) + "-" + str(self.day)

myBaby = Baby("Sally", 20.1, Date(2021, 12, 25))
myBaby.showOff()

Sample code can be found in the resource package.