A Class in Java is a template or blueprint used to create objects. An Object is an instance of a class that has state (variables) and behavior (methods).

• Fields/Variables: Data associated with the class.
• Methods: Logic/Behavior of the class.
• Constructors: Special methods called when an object is instantiated.
• this Keyword: Refers to the current instance of the class.

java Standard
public class Person {
    String name;
    int age;

    // Constructor
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void display() {
        System.out.println(name + " is " + age + " years old.");
    }

    public static void main(String[] args) {
        Person p1 = new Person("Alice", 25); // Creating an object
        p1.display();
    }
}

• Default Constructor: Forgetting that Java provides a default constructor ONLY if no other constructors are defined.
• Static vs Instance: Attempting to use this inside a static method (static methods belong to the class, not an instance).
• Null Initialization: Not initializing object references, leading to NullPointerException.