2.8 Constructors

Learning Objectives

•Write a constructor that initializes instance variables from parameters.
•Explain the default (no-argument) constructor Java provides automatically.
•Overload constructors to support multiple initialization patterns.

Introduction

A constructor is a special method that runs automatically when an object is created with `new`, responsible for initializing its instance variables. A constructor shares its name with the class, has no return type (not even `void`), and can be overloaded to support multiple ways of creating an object.

Key Concepts

A constructor's name matches the class exactly and has no return type.

If a class defines no constructors at all, Java supplies a no-argument default constructor automatically.

As soon as you write any constructor yourself, the automatic default constructor is no longer provided.

Constructors can be overloaded (see 2.11) to allow creating objects in different ways.

Java Syntax

Code Examples

The constructor receives `name` and `grade` as arguments and assigns them to the object's fields using `this` to disambiguate from the parameter names.

Common Mistakes

⚠Giving a constructor a return type by mistake, which turns it into a regular method that shares the class's name — not a constructor.
⚠Assuming a no-argument constructor is always available, even after defining a custom constructor (it is not, unless explicitly written).
⚠Forgetting to initialize every important field inside the constructor, leaving objects partially set up.

Practice

Practice 1

Programming Exercisemedium

Write a `Book` class with `title` and `pages` fields and a constructor that initializes both from parameters.

Summary

Constructors initialize new objects, sharing the class's name with no return type. Defining any constructor removes Java's automatic no-argument default.

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