Java Classes & Objects: Blueprint Definition & Heap Memory Allocation

In Java, classes and objects form the structural foundation of all application logic. A class acts as the blueprint, and an object is the physical living instance produced from that blueprint.

Figure 6: Memory Trace: Class Template in Metaspace, Reference Variable on Call Stack, and Object Instances in Heap.

1. Defining a Class (The Blueprint)

A class definition groups fields (state) and methods (behavior):

public class Car {
    // 1. State / Attributes (Instance Variables)
    String brand;
    String color;
    int currentSpeed;

    // 2. Behavior / Actions (Methods)
    public void accelerate(int speedIncrease) {
        currentSpeed += speedIncrease;
        System.out.println(brand + " accelerated to: " + currentSpeed + " km/h");
    }

    public void applyBrakes() {
        currentSpeed = 0;
        System.out.println(brand + " has come to a complete halt.");
    }
}

2. Instantiating Objects using the new Keyword

To bring a class to life, you instantiate an object using the new operator. Each object possesses its own independent copy of instance variables:

public class CarShowroom {
    public static void main(String[] args) {
        // Step 1: Create first Car object
        Car car1 = new Car();
        car1.brand = "Toyota Fortuner";
        car1.color = "Pearl White";
        car1.currentSpeed = 60;

        // Step 2: Create second Car object
        Car car2 = new Car();
        car2.brand = "Tesla Model 3";
        car2.color = "Midnight Silver";
        car2.currentSpeed = 85;

        // Step 3: Invoke methods via dot operator (.)
        car1.accelerate(20); // Toyota accelerated to 80 km/h
        car2.applyBrakes();   // Tesla has come to a complete halt
    }
}

Memory Trace: Stack vs. Heap Allocation

• Call Stack: When Car car1 is declared inside main(), the JVM allocates 4 or 8 bytes on the Call Stack for the local variable car1. Initially, it holds a memory address pointer (e.g. 0x4A12).
• Heap Memory: The expression new Car() allocates contiguous memory on the Heap for the Car instance. The fields (brand, color, currentSpeed) reside inside this Heap block.

Car c1 = new Car();
c1.brand = "Ford";
Car c2 = c1;
c2.brand = "BMW";
System.out.println(c1.brand);