Understanding Classes and Objects in Java

Classes and objects are the building blocks of every Java program. A class is a blueprint; an object is a concrete instance of that blueprint.

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Class = Blueprint

A class defines:

• What data an object will hold (fields)
• What behavior an object will have (methods)

class Dog {
    // Fields (data)
    String name;
    String breed;
    int age;

    // Methods (behavior)
    void bark() {
        System.out.println(name + " says Woof!");
    }

    void describe() {
        System.out.printf("%s is a %d-year-old %s%n", name, age, breed);
    }
}

A class doesn’t occupy object memory by itself — it’s just the definition.

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Object = Instance

An object is a specific instance created from a class:

Dog dog1 = new Dog();
dog1.name = "Buddy";
dog1.breed = "Golden Retriever";
dog1.age = 3;

Dog dog2 = new Dog();
dog2.name = "Max";
dog2.breed = "German Shepherd";
dog2.age = 5;

dog1.bark();      // Buddy says Woof!
dog2.describe();  // Max is a 5-year-old German Shepherd

Each object has its own copy of the fields. Changing dog1.name doesn’t affect dog2.name.

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Memory Model

Stack                    Heap
┌──────────┐     ┌──────────────────┐
│ dog1 ────────→ │ name: "Buddy"    │
│          │     │ breed: "Golden.."│
│          │     │ age: 3           │
│          │     └──────────────────┘
│ dog2 ────────→ ┌──────────────────┐
│          │     │ name: "Max"      │
│          │     │ breed: "German.."│
│          │     │ age: 5           │
└──────────┘     └──────────────────┘

• Stack: holds the reference variable (dog1, dog2)
• Heap: holds the actual object data

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Multiple Objects, One Class

You can create unlimited objects from one class:

Dog[] kennel = new Dog[100];
for (int i = 0; i < 100; i++) {
    kennel[i] = new Dog();
    kennel[i].name = "Dog #" + i;
}

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Null References

A reference variable can point to nothing:

Dog dog = null;  // no object yet
dog.bark();      // NullPointerException!

Always check for null before using an object:

if (dog != null) {
    dog.bark();
}

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Employee Management Example

Our running example throughout this tutorial:

class Employee {
    String name;
    String department;
    double salary;

    void displayInfo() {
        System.out.printf("%-15s %-12s $%,.2f%n", name, department, salary);
    }
}

// Usage
Employee emp1 = new Employee();
emp1.name = "Alice Johnson";
emp1.department = "Engineering";
emp1.salary = 95000;

Employee emp2 = new Employee();
emp2.name = "Bob Smith";
emp2.department = "Marketing";
emp2.salary = 78000;

System.out.printf("%-15s %-12s %s%n", "Name", "Department", "Salary");
emp1.displayInfo();
emp2.displayInfo();

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Quick Exercises

1. Create a Book class with title, author, pages. Add a method summary() that prints all fields.
2. Create 3 Book objects and store them in an array. Print all summaries in a loop.
3. What happens when you access a field of a null reference?

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Summary

• Class = blueprint (defines fields + methods)
• Object = instance (created with new)
• Each object has its own copy of fields
• References live on the stack; objects live on the heap
• null means “no object” — accessing it throws NullPointerException

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What’s Next?

Let’s learn the syntax for Defining Classes in Java properly — access modifiers, naming, and structure.