Classes and Objects

What is an Object and what is a Class?

An Object is a thing (noun) that has certain characteristics and can perform certain tasks. A Class is the blueprint or definition for that object when it is created.

An example is a Person. A Person is a kind of Class. A person can have certain characteristics that distinguish it from another person. It can have blue eyes, it can be 15 years old, etc. A person can also perform certain tasks. It can walk, it can talk, etc.

A C# Object is an instance of a C# Class. In this instance “Bea” is a “Person”. “Bea” has brown eyes. This is a property (or data field) of “Bea”. “Bea” can speak in English. “Bea” can speak in Spanish. These are methods that “Bea” can perform.

Creating a Class!

C# is an object-oriented programming language, which means everything in C# is associated with an object and a class (the blueprint for the object).

For starters, the line of code that we used to print statements from activity 1 is actually a method call from a predefined class called System!

Console.WriteLine("Hello World");

1. Console is a class that deals with user input and outputs.
2. WriteLine() is a method defined in the class Console.

Another built-in class we have interacted with in the previous exercises is string. The string class defines a set of rules on how a list of characters should behave.

With the following line of code, we created a string object called name using the rules defined in the string class:

string name = "Patrick";

Console, and string are predefined classes in C#. However, we are not limited to these predefined classes, and we can actually create our own data type by writing a class! This is useful for coders to create specific objects to have certain attributes and behaviors. Having access to these user-defined types allows us to build distinct programs.

Let’s learn about the different parts in a class below:

Class

A class is a blueprint or prototype of a new type of object. In general, a class contains three important parts:

Element	Description	Example
data fields/instance variables	variables an object of this class has access to that describe the object	For example, a Person class could have fields eyeColor, age, height.
constructor	method called automatically when an object from this class is created, constructors have the same name as the class	There can be more than one constructor per class
methods	methods for the object of this class to perform certain tasks	Person class could have methods talk and walk.

For example:

public class Person{
    // (1) data fields/instance variables
    private String name; // example
    private int age;
    private int height;

    // (2) constructor - constructors have the same name as the class
    public Person()
    {
        name = "Bea";
        age = 29;
        height = 167;
    }

    // (2) constructor - you can have more than one
    public Person( String nameInput, int ageInput, int heightInput)
    {
        name = nameInput;
        age = ageInput;
        height = heightInput;
    }

    // (3) methods
    public void talk()
    {
        Console.WriteLine($"Hello from {name}");
    }
}

Try it Out 🐥!

Let’s make a Bird class to represent Patrick 🐥 and all of his Bird friends by following the steps below!

First, we start by defining the class name in the format public class name.

public class Bird{}

Second, let’s declare all the fields of the Bird class: species, name, hobby, age, loveMusic.

Each field is declared in the format: access specifier data type name ;.

1. We declare all 5 fields as private in the class Bird. This ensures these fields can only be accessed within this class.

2. Let’s identify the appropriate type for each field:

• species should be a String that stores for exmaple: “duck”, “swan”, “owl”.
• name should be a String that stores for example: “Patrick”.
• hobby should be a String that stores for example: “play basketball”.
• age should be an int that stores for example: 25.
• loveMusic should be a bool that stores either true or false.

For example, to declare species as a private field of class Bird, you would put private String species; .

3. Finish creating the other 4 fields of the Bird class!

Third, let’s create the constructor for the class Bird.

Usually, constructor is the method that initializes value to all the fields in a class. It has the format public class name (parameter). Since we have 5 fields in this class, the constructor will take in 5 parameter/inputs.

public Bird(string speciesInput, string nameInput, string hobbyInput, int ageInput, bool loveMusicInput){
    // constructor body
}

In the constructor body, we need to initialize all the instance variables, by assign each variables to its initial values:

species = speciesInput;
name = nameInput;
hobby = hobbyInput;
age = ageInput;
loveMusic = loveMusicinput;

Lastly, let’s create some methods for the class Bird.

We are gonna create 6 methods for this class!

• getSpecies(); // return species of the bird
• getName(); // return name of the bird
• getHobby(); // return hobby of the bird
• getAge(); // return age of the bird
• getLoveMusic(); // return whether the bird loves music
• ToString(); // return information in a string

Try to write out the first 5 methods with what you learned from the previous exercise about writing methods!

After, let’s write the ToString() method together!

ToString() is a method that returns the string representation of the object.

We will return a String that include all fields of the Bird: name, age, species, hobby, loveMusic by calling the 5 methods you just implemented!

public string ToString(){
    // initialize variable info to have empty String
    string info = "";

    // append information to the String in the format: 
    // Hi, my name is [name]. I am a [age] year old [species] who likes to [hobby].
    info = info + "Hi, my name is " + getName() + 
           ". I am a " + getAge() + " year old " + getSpecies() + 
           " who likes to " + getHobby() + ". ";

    // if the Bird likes music, append the String "I also like music very much!" to info
    if(getLoveMusic()){
        info = info + "I also like music very much!";
    }else{        
        info = info + "However, I do not like music!";
    }

    // return the full string
    return info;
}

Congratulations! You correctly implemented the Bird Class :)

Creating Birds 🐥!

Congratulations! You just wrote you first C# class Bird class. Now let’s learn to write a program that uses Bird objects!

We defined the Bird class to have the following attributes (instance variables) and behaviors (methods):

To create a new object of a particular class, we call the constructor of that class in the format class name variable name = new constructor call.

Recall that the constructor of Bird class is the following:

public Bird(string speciesInput, string nameInput, string hobbyInput, int ageInput, bool loveMusicInput);

Hence, we can create a Bird with these attributes (species - duck; name - Patrick; hobby - hangout with friends; age - 15; loveMusic - true) with this line of code.

Bird patrick = new Bird("duck", "Patrick", "hangout with friends", 15, true);

Try it out yourself and create multiple Birds of different species 🐦🐤🐔🐧!

1. Print the return value of ToString() method. (i.e. Console.WriteLine(patrick.ToString());)
2. Directly print the Bird object, which prompts ToString() to be called in the background. (i.e. Console.WriteLine(patrick);).

Try it out and print out all the information of the bird friends you created 🐦🐤🐔🐧!

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You did it!
Workshop complete

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Methods