Constructors in

About Constructors

Creating an instance of a class is done by calling its constructor through the new operator. A constructor is a special type of method whose goal is to initialize a newly created instance. Constructors look like regular methods, but without a return type and with a name that matches the classes' name.

class Library
{
    private books;

    public Library()
    {
        this.books = 10;
    }
}

// This will call the constructor
var library = new Library();

Like regular methods, constructors can have parameters. Constructor parameters are usually stored as (private) fields to be accessed later, or else used in some one-off calculation. Arguments can be passed to constructors just like passing arguments to regular methods.

class Building
{
    private int numberOfStories;
    private int totalHeight;

    public Building(int numberOfStories, double storyHeight)
    {
        this.numberOfStories = numberOfStories;
        this.totalHeight = numberOfStories * storyHeight;
    }
}

// Call a constructor with two arguments
var largeBuilding = new Building(55, 6.2)

Specifying a constructor is optional. If no constructor is specified, a parameterless constructor is generated by the compiler:

class Elevator
{
}

// This will call the (empty) generated constructor
var elevator = new Elevator();

If fields have an initial value assigned to them, the compiler will output code in which the assignment is actually done inside the constructor. The following class declarations are thus equivalent (functionality-wise):

class UsingFieldInitialization
{
    private int players = 5;
}

class UsingConstructorInitialization
{
    private int players;

    public UsingConstructorInitialization()
    {
        players = 5;
    }
}