Class Definition, Object Instantiation, and Heap vs Stack Allocation

Intermediate13 min read•Updated: 2026-10-22

🏛️ Class Definition, Object Instantiation, and Heap vs Stack Allocation

Introduction

Object-Oriented Programming (OOP) in Java models real-world business domains as interactions between software objects. A class is a blueprint or template, while an object is a concrete instance created from that blueprint.

🔬 Memory Architecture: Stack vs Heap

1STACK MEMORY (Thread-Private, Fast)       HEAP MEMORY (Shared, Managed by GC)
2┌─────────────────────────────────┐       ┌─────────────────────────────────────┐
3│  studentRef (Pointer Address)   │ ────► │  [ Student Object Instance ]        │
4│  primitive int roll = 101       │       │    String name = "Aarav Sharma"     │
5└─────────────────────────────────┘       │    double gpa = 9.2                 │
6                                          └─────────────────────────────────────┘

Stack: Stores method execution frames, primitive local variables, and object reference variables (pointers).
Heap: Stores all object instances and arrays. Cleaned automatically by Garbage Collection (GC).

💻 Class Definition & Instantiation

1public class StudentProfile {
  // Member state (fields)
  private int rollNumber;
  private String fullName;
  private double gpa;
6 
  // Method to initialize
  public void setDetails(int roll, String name, double marks) {
      this.rollNumber = roll;
      this.fullName = name;
      this.gpa = marks;
  }
13 
  public void display() {
      System.out.printf("Student #%d: %s | GPA: %.2f%n", rollNumber, fullName, gpa);
  }
17 
  public static void main(String[] args) {
      // 's1' is a reference on the stack; 'new StudentProfile()' allocates on the heap
      StudentProfile s1 = new StudentProfile();
      s1.setDetails(101, "Aarav Sharma", 9.4);
      s1.display();
23 
      StudentProfile s2 = new StudentProfile();
      s2.setDetails(102, "Sneha Gupta", 8.9);
      s2.display();
  }
28}

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Instance Variables vs Static Variables & Static Methods

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🔬 Memory Architecture: Stack vs Heap

1STACK MEMORY (Thread-Private, Fast)       HEAP MEMORY (Shared, Managed by GC)
2┌─────────────────────────────────┐       ┌─────────────────────────────────────┐
3│  studentRef (Pointer Address)   │ ────► │  [ Student Object Instance ]        │
4│  primitive int roll = 101       │       │    String name = "Aarav Sharma"     │
5└─────────────────────────────────┘       │    double gpa = 9.2                 │
6                                          └─────────────────────────────────────┘

Stack: Stores method execution frames, primitive local variables, and object reference variables (pointers).

Heap: Stores all object instances and arrays. Cleaned automatically by Garbage Collection (GC).

💻 Class Definition & Instantiation

1public class StudentProfile {
  // Member state (fields)
  private int rollNumber;
  private String fullName;
  private double gpa;
6 
  // Method to initialize
  public void setDetails(int roll, String name, double marks) {
      this.rollNumber = roll;
      this.fullName = name;
      this.gpa = marks;
  }
13 
  public void display() {
      System.out.printf("Student #%d: %s | GPA: %.2f%n", rollNumber, fullName, gpa);
  }
17 
  public static void main(String[] args) {
      // 's1' is a reference on the stack; 'new StudentProfile()' allocates on the heap
      StudentProfile s1 = new StudentProfile();
      s1.setDetails(101, "Aarav Sharma", 9.4);
      s1.display();
23 
      StudentProfile s2 = new StudentProfile();
      s2.setDetails(102, "Sneha Gupta", 8.9);
      s2.display();
  }
28}

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Instance Variables vs Static Variables & Static Methods