Objects and Classes in Java – The Heart of OOP

Introduction

When you first dive into Java, you quickly hear about objects, classes, and this buzzword: “object-oriented programming” (OOP). But what does that mean in Java’s world, and why should you care? In this post, we’ll peel back the layers on Java’s OOP foundation, objects and classes, and uncover a few quirks you’ll only notice when you start teaching or working with Java in-depth.

What is Object-Oriented Programming in Java?

At its core, OOP is a paradigm where software is organized around “objects”, entities that combine state (data) and behavior (methods). Java’s take on OOP is built on a hierarchy of classes, each defining the blueprint for its objects (oracle.com). A class encapsulates fields and methods, guiding how its objects store data and act on it.

Classes: Blueprints of Behavior

A class in Java is like an architectural plan, it specifies:

  • Fields: Variables that capture the state of each object.
  • Methods: Functions that expose the object’s behavior.

For instance:

public class Car {
    private String color;          // field: an attribute
    public void drive() {          // method: an action
        System.out.println("Vroom!");
    }
}

Here, Car defines its state and behavior, but no actual car exists until you instantiate it.

Objects: Living Instances

An object is the real-world manifestation of a class, like the physical car built from the blueprint. You create one with the new keyword:

Car myCar = new Car();    // instantiate!
myCar.drive();            // outputs "Vroom!"

Under the hood, myCar holds a reference to a spot on the heap where the Car data lives. This reference-based approach is why two variables can point to the same object or have no value (null).

Java’s Reference Semantics: A Subtle Twist

In Java, objects are accessed via references, not by value. That means:

Car a = new Car();
Car b = a;

Both a and b point to the same Car object.

A practical teaching point: changing b.color also changes a.color, since there’s only one object in memory.

Primitive Types vs. Wrapper Classes

One of the quirks you’ll spot as a teacher: Java isn’t “pure” OOP, primitives like int, double, and boolean aren’t objects. Yet Java wraps them in wrapper classes (e.g., Integer, Double, Boolean) when needed (autoboxing) (oracle.com). This dual system can surprise students who assume “everything is an object.”

Why It Matters

Understanding objects, classes, and references lays the groundwork for more advanced topics: inheritance, polymorphism, and more. It also clarifies why Java manages memory via garbage collection, you never delete objects manually; the JVM cleans up unreferenced objects for you.

Key Takeaways

  1. Classes are blueprints; objects are their instances.
  2. Java uses reference semantics for objects.
  3. Primitives differ from reference types, but Java bridges them with wrapper classes.

Stay tuned for the next post, where we’ll explore inheritance in Java and reveal why Java’s single inheritance model has its own set of surprises!

Happy coding!