Classes & OOP

Introduction

In this lesson, you'll learn about classes & oop in Java. Coming from JavaScript, you already have a foundation for understanding this concept. We'll build on that knowledge while highlighting the key differences.

The Java Way

Let's see how Java handles this concept. Here's a typical example:

public abstract class Animal {
  private String name; // private field

  public Animal(String name) {
    this.name = name;
  }

  public String getName() { return name; } // getter

  public abstract String speak(); // must override
}

public class Dog extends Animal {
  public Dog(String name) { super(name); }

  @Override
  public String speak() {
    return getName() + " barks.";
  }
}

Dog d = new Dog("Rex");
System.out.println(d.speak());

Comparing to JavaScript

Here's how you might have written similar code in JavaScript:

class Animal {
  #name; // private field

  constructor(name) {
    this.#name = name;
  }

  get name() { return this.#name; }

  speak() {
    return this.#name + " makes a sound.";
  }
}

class Dog extends Animal {
  constructor(name) { super(name); }

  speak() {
    return this.name + " barks.";
  }
}

const d = new Dog("Rex");
console.log(d.speak());

Step-by-Step Breakdown

#name; // JS private

private String name; // Java private

public abstract String speak(); // subclass MUST implement

speak() { return "barks"; } // no check in JS

@Override
public String speak() { return getName() + " barks."; }

Common Mistakes

When coming from JavaScript, developers often make these mistakes:

• Java uses public/private/protected keywords; JS uses # for private
• Java abstract classes cannot be instantiated; JS has no abstract keyword
• Java uses getter methods by convention; JS has get/set syntax

Key Takeaways

• JS # private → Java private keyword
• Java abstract enforces method implementation
• @Override documents and verifies overrides
• Java getters are methods by convention (getName)