Class v.s. Object

I. Analogy: Animals in the Zoo

Let’s say we visit a zoo. We see a cat, a dog, and a rabbit. These are actural animals we can touch - they are objects. But when we describe what we saw, we might just say: “I saw some animals.” The word “animal” is abstract - it refers to the concept or type, not a specific instance that we can point to. In Java, that abstract idea is represented by a class. A class is a blueprint to create an object.

II. Viewing Classes from the Perspective of Data Types

From the perspective of data types, a class in Java can be seen as a user-defined composite data type. It achieves this by encapsulating other data types (both primitive and class types) as its own fields, thereby defining a new, more complex data structure.

Defining a Class

A Class defines:

• State —— Fields
  • the properties an object holds (e.g., name, age, species)
• Behaviour —— Methods
  • the actions the object can perform (e.g., eat, sleep, hunt)
• Construction —— Constructor
  • how an object is created (via Constructors)

public class Animal {
    private String species;
    private String name;
    private int age;

    // Default constructor
    public Animal() {}

    private void eat() {}
    
    private void sleep() {}
    
    private void hunt() {}
    
}

• Fields
  • species, name and age represent the state of an animal. These are also called member variables or instance variables. They do NOT occupy memory at compile time. Only when we instantiate the class (i.e., create an actual animal object), memory is allocated.
• Methods
  • eat(), sleep(), and hunt() define the behavious of the animal.
• Constructor
  • Animal() is a default constructor, automatically provided by the Java compiler if not explicitly written.

Instantiating Objects

Standard way: via a parameterized constructor

e.g.

public class Animal {
    private String species;
    private String name;
    private int age;

    public Animal(String species, String name, int age) {
        this.species = species;
        this.name = name;
        this.age = age;
    }
}

Animal animal = new Animal("Cat","Oreo", 4);