Class vs Object in Java

Direct Answer

A class declares a type's fields, constructors, and methods. An object is a runtime instance of a class with its own instance-field state. One class can have many objects; static members are associated with the loaded class rather than copied into each object.

Code Example

// The class — a blueprint, no memory allocated here yet
class Student {
    String name;
    int marks;

    void printGrade() {
        System.out.println(name + ": " + (marks >= 50 ? "Pass" : "Fail"));
    }
}

public class Main {
    public static void main(String[] args) {
        // Two objects from the same class — fully independent
        Student s1 = new Student();
        s1.name = "Alice";
        s1.marks = 95;

        Student s2 = new Student();
        s2.name = "Bob";
        s2.marks = 42;

        s1.printGrade();  // Alice: Pass
        s2.printGrade();  // Bob: Fail

        // Proof of independence
        s1.marks = 60;
        System.out.println(s2.marks); // 42 — unaffected
    }
}

Same class, two objects, completely independent data.

Common Mistakes & Pitfalls

• Using "class" and "object" as if they mean the same thing — the class is the definition you write, the object is one live instance built from it; the interviewer hears the slip the moment you make it.
• Assuming all objects of a class share their field values — each object gets its own copy of the instance fields, so changing s1.marks never touches s2.marks unless that field is static.
• Believing Student s; already creates an object — it only creates a null reference; the object exists only after new, and touching s before that throws NullPointerException.
• Thinking the class takes heap memory — only objects live on the heap; the class is loaded once as a template, so "how many objects" and "how many classes" are completely different questions.