I wish creating objects was easier (aka constructors)

Let’s say that the user wants to create a Rectangle whose width is 5 and height is 8. Following code achieves this,

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Rectangle r = new Rectangle();
r.width = 5;
r.height = 8;

However, it would be really nice if one could pass the values for the instance variables in the instantiation statement itself, as,

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Rectangle r = new Rectangle(5, 8);

This is done through constructors.

Constructors

• A constructor is a method defined in the class.

• A constructor must have the same name as the class.

• A constructor has no return type (not even void).

• There may be multiple constructors, each distinguished by its parameter list. Thus, we may have one constructor with no parameters, and another with one int parameter.

• A suitable constructor is automatically called during instantiation based on number of parameters passed. If an appropriate constructor is not found, a compilation error is generated.

Example - Constructors

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public class Rectangle {
    public double width, height;
    
    public Rectangle() { //default constructor
        width = 1;
        height = 1;
    }

    //parameterized constructor for a square
    public Rectangle(double side) { 
        width = side;
        height = side;
    }

    //parameterized constructor - generic
    public Rectangle(double w, double h) { 
        width = w;
        height = h;
    }
    //rest of the code
}

Default constructor

It should be noted that a default constructor (without any parameters) is pre-defined for you by Java and that’s why you can instantiate objects without defining it yourself.

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Rectange r = new Rectangle();

The default constructor assigns the default values for the appropriate data types to the instance variables. However, once you define a parameterized constructor, the built-in default constructor is taken away by Java. Thus, if you want to construct an object with default initial values for the instance variables, you need to re-define that!

Defining the default constructor

Let’s say the default Rectangle instance should be of unit length. We can define the default constructor as,

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public Rectangle() {
    width = 1;
    height = 1;
}

Add a default constructor to the class Circle that assigns the value 1.0 to instance variable radius.

SOLUTION

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 //default constructor
 public Circle() {
     radius = 1;
 }

Add a parameterized constructor to the class Circle that assigns the value of the parameter passed to instance variable radius.

SOLUTION

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 //parameterized constructor
 public Circle(double r) {
 	radius = r;
 }