Core Concepts of Classes and Objects A class serves as a blueprint for creating objects, defining their structure and behavior. Objects represent specific instances with unique:

Behavior: Actions an object can perform State: Current properties or attributes Idetnity: Unique distinguishnig characteristics

Class-Object Relationship Classes are abstract templates that exist only in code, while objects are runtime instances with actual memory allocation. Key differences:

Classes contain no data until instantiated Each object maintains independent state Object properties change through method interactions

Object Initialization Java requires explicit object construction before use:

Employee developer = new Employee("John", 85000);

Predefined Class Utilization Essential Java API classes:

Math: Mathematical operations String: Text manipulation Scanner: Input processing LocalDate: Date handling

Custom Class Implementation Key components in user-defined classes:

Field Declarations

private String employeeName;  // Instance field
private static int employeeCount;  // Static field

Method Types

Constructor: Initializes new instances Accessor: Retrieves object state Mutator: Modifies object state Static: Class-level operations

Method Overloading Multiple methods sharing a name but differing in parameters:

public Employee(String name) { ... }
public Employee(String name, int id) { ... }

File Input/Output Implementation

import java.io.*;
import java.util.*;

public class FileProcessor {
    public static void main(String[] args) throws IOException {
        // Write to file
        PrintWriter fileWriter = new PrintWriter("data.txt");
        fileWriter.println("Name,Department,Salary");
        fileWriter.println("Alice,Engineering,95000");
        fileWriter.close();
        
        // Read from file
        Scanner fileScanner = new Scanner(new File("data.txt"));
        while(fileScanner.hasNextLine()) {
            String record = fileScanner.nextLine();
            Scanner recordParser = new Scanner(record).useDelimiter(",");
            String name = recordParser.next();
            String department = recordParser.next();
            String salary = recordParser.next();
            System.out.printf("Name: %s | Department: %s | Salary: %s%n", 
                             name, department, salary);
        }
        fileScanner.close();
    }
}

Custom Class Example

public class Employee {
    private String fullName;
    private double annualCompensation;
    private LocalDate startDate;

    public Employee(String name, double salary, int year, int month, int day) {
        fullName = name;
        annualCompensation = salary;
        startDate = LocalDate.of(year, month, day);
    }

    public void adjustCompensation(double percentage) {
        double adjustment = annualCompensation * percentage / 100;
        annualCompensation += adjustment;
    }
}

Static Members Implementation

public class Employee {
    private static int nextIdentifier = 1;
    private int currentIdentifier;

    public void assignIdentifier() {
        currentIdentifier = nextIdentifier;
        nextIdentifier++;
    }

    public static int getNextIdentifier() {
        return nextIdentifier;
    }
}

Geometric Class Implementation

public class Rectangle {
    private double width;
    private double height;

    public Rectangle(double w, double h) {
        width = w;
        height = h;
    }

    public double calculatePerimeter() {
        return 2 * (width + height);
    }

    public double calculateArea() {
        return width * height;
    }
}

public class Circle {
    private double radius;
    private static final double PI = 3.14159;

    public Circle(double r) {
        radius = r;
    }

    public double calculateCircumference() {
        return 2 * PI * radius;
    }

    public double calculateArea() {
        return PI * radius * radius;
    }
}

Package Organization

package com.company.employees;

public class Manager {
    private String name;
    private String department;

    public Manager(String n, String d) {
        name = n;
        department = d;
    }
}