B3.1.4 Constructing Code to Define Classes and Instantiate Objects Notes

Constructing Code to Define Classes and Instantiate Objects

Understanding Classes and Objects

What is a Class?

1. A class is a blueprint or template for creating objects.
2. It defines the attributes (instance variables) and methods (behaviors) that the objects will have.

What is an Object?

1. An object is an instance of a class.
2. It is a concrete entity that exists in memory, with its own set of attribute values.

Defining Classes

Structure of a Class

1. Class Declaration
   1. Specifies the name of the class.
   2. Often begins with the class keyword.
2. Instance Variables (Attributes/Fields)
   1. Represent the state or properties of the class.
   2. Each object created from the class has its own copy of these variables.
3. Methods (Behaviors/Functions)
   1. Define the actions or operations the class can perform.
   2. Can manipulate instance variables or perform other logic.

# Class Declaration
class Bird:
    # Constructor with instance variables
    def __init__(self, species, age):
        self.species = species
        self.age = age

    # Method (Behavior)
    def sing(self):
        print(f"{self.species} is singing!")

    # Accessor
    def get_age(self):
        return self.age

// Class Declaration
public class Bird {
    // Instance Variables
    private String species;
    private int age;

    // Constructor
    public Bird(String species, int age) {
        this.species = species;
        this.age = age;
    }

    // Method (Behavior)
    public void sing() {
        System.out.println(species + " is singing!");
    }

    // Accessor
    public int getAge() {
        return age;
    }
}

Instantiating Objects

What is Instantiation?

1. Instantiation is the process of creating an object from a class.
2. It involves calling the constructor method, which initializes the object’s attributes (state).
3. Each object created from the same class is independent and has its own copy of the instance variables.

# Instantiating objects using the Bird class constructor
sparrow = Bird("Sparrow", 2)
parrot = Bird("Parrot", 5)

# Calling methods on objects
sparrow.sing()                           # Output: Sparrow is singing!
print("Parrot age:", parrot.get_age())   # Output: Parrot age: 5

public class Main {
    public static void main(String[] args) {
        // Instantiating objects using the Bird class constructor
        Bird sparrow = new Bird("Sparrow", 2);
        Bird parrot = new Bird("Parrot", 5);

        // Calling methods on objects
        sparrow.sing();   // Output: Sparrow is singing!
        System.out.println("Parrot age: " + parrot.getAge()); // Output: Parrot age: 5
    }
}

The Role of Constructors

What is a Constructor?

1. A constructor is a special method that initializes an object's state.
2. It sets the initial values for the object's attributes.

Key Features of Constructors

1. Same Name as the Class: In Java, the constructor shares the class name. In Python, it is defined as __init__.
2. No Return Type: Constructors do not have a return type.
3. Automatic Invocation: Called automatically when an object is instantiated.

Key Takeaways

1. Classes are blueprints for objects, defining attributes and methods.
2. Objects are instances of classes, created through instantiation.
3. Constructors initialize an object's state, setting initial values for attributes.