Constructors and Constructor Overloading

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

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:

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

Default field values include:

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

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

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.

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

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.

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