Lesson 6: Constructors and Constructor Overloading · Object-Oriented Programming with Java

Lesson Overview

This lesson explains constructors, default values, constructors with arguments, multiple constructor arguments, and constructor overloading.

What Is a Constructor?

A constructor is a special method-like block that runs when an object is created. Java calls it automatically when new is used.

When new creates an object, Java normally:

Allocates memory for the object.
Assigns default values to fields.
Calls the matching constructor.

Default field values include:

Field type	Default value
numeric types	`0` or `0.0`
object references	`null`
`boolean`	`false`
`char`	`\u0000`

A constructor must have the same name as the class and cannot have a return type.

No-Argument Constructor

A no-argument constructor accepts no values. It is often used to set initial field values.

Activity: Constructor with Initial Values

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Constructor with One Argument

A constructor can receive a value when an object is created.

EmployeeId record = new EmployeeId(2470);

The value inside the parentheses is passed to the constructor.

Activity: Constructor with One Argument

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Constructor with Multiple Arguments

Constructors can receive several values separated by commas. The order and type of arguments must match the constructor parameters.

Activity: Constructor with Multiple Arguments

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Constructor Overloading

Constructor overloading means writing multiple constructors in the same class with different parameter lists.

If you write a constructor with parameters, Java no longer automatically provides the no-argument constructor. Add your own no-argument constructor if you still need one.

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10Activity: Overloaded Constructors
11Laboratory Exercises

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Try It: Constructors

Add a fourth Rectangle using the square constructor and print its area and perimeter.

class Rectangle {
    double width;
    double height;

    Rectangle() {
        width = 1.0;
        height = 1.0;
    }

    Rectangle(double w, double h) {
        width = w;
        height = h;
    }

    Rectangle(double side) {
        width = side;
        height = side;
    }

    double area() { return width * height; }
    double perimeter() { return 2 * (width + height); }
}

public class Main {
    public static void main(String[] args) {
        Rectangle r1 = new Rectangle();
        Rectangle r2 = new Rectangle(4.0, 3.0);
        Rectangle r3 = new Rectangle(5.0);

        System.out.println("r1 area: " + r1.area());
        System.out.println("r2 area: " + r2.area() + ", perimeter: " + r2.perimeter());
        System.out.println("r3 area: " + r3.area());
    }
}

Field type

Default value

numeric types

0 or 0.0

object references

null

boolean

false

char

\u0000

class Rectangle { double width; double height; Rectangle() { width = 1.0; height = 1.0; } Rectangle(double w, double h) { width = w; height = h; } Rectangle(double side) { width = side; height = side; } double area() { return width * height; } double perimeter() { return 2 * (width + height); } } public class Main { public static void main(String[] args) { Rectangle r1 = new Rectangle(); Rectangle r2 = new Rectangle(4.0, 3.0); Rectangle r3 = new Rectangle(5.0); System.out.println("r1 area: " + r1.area()); System.out.println("r2 area: " + r2.area() + ", perimeter: " + r2.perimeter()); System.out.println("r3 area: " + r3.area()); } }