class_object

UML Diagram

Class and Object in Java

Definition

• A class is a blueprint of an object, consisting of properties (data members) and behaviors (member functions).
• An object is a programming representation of a class and its memory is allocated in the Heap section of the memory segment.
• Java is an Object-Oriented Programming Language (OOP), meaning every element is represented as a class and object.
• Java follows four main principles of OOP:
  • Encapsulation
  • Inheritance
  • Polymorphism
  • Abstraction

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Example: Student Class in Java

class Student{
    // Data members or instance variables
    int roll;
    String name;
    double marks;

    // Member functions
    void study(){
        System.out.println(name + " is studying.");
    }
    void info(){
        System.out.println("Roll: " + roll);
        System.out.println("Name: " + name);
        System.out.println("Marks: " + marks);
    }

    // Constructor
    // 0-parameter constructor
    Student(){
        System.out.println("Default constructor called.");
    }

    // Parameterized constructor
    // Student(int roll, String name, double marks){
    //     this.roll = roll;
    //     this.name = name;
    //     this.marks = marks;
    // }

    Student(int r, String n, double m){
        roll = r;
        name = n;
        marks = m;
    }
}

public class Main{
    public static void main(String[] args) {
        Student s1 = new Student(); // Default constructor called
        s1.roll = 101;
        s1.name = "John Doe";
        s1.marks = 85.5;
        s1.study();
        s1.info();

        Student s2 = new Student(102, "Jane Doe", 90.0); // Parameterized constructor called
        s2.study();
        s2.info();
    }
}

Key Points

• Constructors:
  • A default constructor initializes objects with default values.
  • A parameterized constructor initializes objects with specific values.
• Member Functions: study() and info() define behaviors for the Student class.
• Objects Creation:
  • Student s1 = new Student(); → Calls the default constructor.
  • Student s2 = new Student(102, "Jane Doe", 90.0); → Calls the parameterized constructor.