Instantiation & constructors

Before an object can be used, its memory must be allocated and its internal attributes set to valid initial values. This initialization process is called instantiation, and it is handled by a special method known as aconstructor.

In languages like Java and C++, constructors adhere to three strict rules:

1. Identical Name: A constructor must have the exact same name as the class (including uppercase letters).
2. No Return Type: Constructors never specify a return type, not even void.
3. Automatic Invocation: You cannot call a constructor like a normal method; it is executed automatically when you use the new keyword.

// Java constructor declaration and invocation
public class Book {
    private String title;
    private double price;

    // Constructor
    public Book(String title, double price) {
        this.title = title;
        this.price = price;
    }
}

// Instantiation:
Book myBook = new Book("Computer Science", 45.00);

In the example above, the constructor parameter is named title and the instance field is also named title. The parameter shadows the field! Using this.title explicitly tells the compiler: “Assign to the title attribute of this specific object instance.” In Python, the equivalent is explicitly passing self as the first parameter (def __init__(self, title): self.title = title).

A class can provide multiple constructors as long as their parameter lists differ in number or types (overloading). For instance, a class can have a default constructor with no parameters setting sensible defaults, and a parameterized constructor accepting custom values.