Experiment 2.2: Autoboxing, Serialization, and File Handling

int primitive = 20;
Integer wrapped = Integer.valueOf(primitive);
int backAgain = wrapped.intValue();

int a = 20;
Integer i = Integer.valueOf(a);  // explicit
Integer j = a;                   // autoboxing
System.out.println(a + " " + i + " " + j);   // 20 20 20

Integer a = 3;
int i = a.intValue();  // explicit
int j = a;             // unboxing
System.out.println(a + " " + i + " " + j);   // 3 3 3

try (FileInputStream in = new FileInputStream("input.txt");
     FileOutputStream out = new FileOutputStream("output.txt")) {
    int c;
    while ((c = in.read()) != -1) {
        out.write(c);
    }
}

The Eight Wrapper Classes

Problem statement: Write a Java program to calculate the sum of a list of integers using autoboxing and unboxing. Include methods to parse strings into their respective wrapper classes, for example Integer.parseInt().

Algorithm

1. Read the numbers as a single line of text.
2. Split the line into tokens using a whitespace separator.
3. Parse each token with Integer.parseInt() and catch NumberFormatException for invalid text.
4. Add every parsed value to a List<Integer>, which autoboxes the int into an Integer.
5. Traverse the list and add each element to an int accumulator, which unboxes it.
6. Display the valid numbers, their count, the sum, and the average.

Java Program

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class WrapperSumDemo {

    // Parses text into primitives, then stores them as objects
    public static List<Integer> parseNumbers(String[] tokens) {
        List<Integer> numbers = new ArrayList<>();

        for (String token : tokens) {
            try {
                int value = Integer.parseInt(token);
                numbers.add(value);          // autoboxing: int -> Integer
            } catch (NumberFormatException e) {
                System.out.println("Skipping invalid number: " + token);
            }
        }

        return numbers;
    }

    // Adds every element into an int accumulator
    public static int calculateSum(List<Integer> numbers) {
        int sum = 0;

        for (Integer number : numbers) {
            sum += number;                   // unboxing: Integer -> int
        }

        return sum;
    }

    public static double calculateAverage(List<Integer> numbers) {
        if (numbers.isEmpty()) {
            return 0.0;
        }

        return (double) calculateSum(numbers) / numbers.size();
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        try {
            System.out.print("Enter numbers separated by space: ");
            String line = scanner.nextLine().trim();
            String[] tokens = line.split("\\s+");

            List<Integer> numbers = parseNumbers(tokens);

            if (numbers.isEmpty()) {
                System.out.println("No valid numbers were entered.");
                return;
            }

            System.out.println("Valid numbers: " + numbers);
            System.out.println("Count: " + numbers.size());
            System.out.println("Sum of numbers: " + calculateSum(numbers));
            System.out.printf("Average: %.2f%n", calculateAverage(numbers));
        } finally {
            scanner.close();
        }
    }
}

Sample Output

Enter numbers separated by space: 12 8 45 x9 30
Skipping invalid number: x9
Valid numbers: [12, 8, 45, 30]
Count: 4
Sum of numbers: 95
Average: 23.75

Problem statement: Create a Java program to serialize and deserialize a Student object. Serialize a Student object containing id, name, and GPA and save it to a file, deserialize the object from the file, and display the student details. Handle FileNotFoundException, IOException, and ClassNotFoundException using exception handling.

Algorithm

1. Create a Student class with id, name, and GPA that implements Serializable.
2. Declare a serialVersionUID constant inside the class.
3. Read the student details and construct one Student object.
4. Open an ObjectOutputStream over a FileOutputStream and call writeObject().
5. Open an ObjectInputStream over a FileInputStream and call readObject(), casting the result to Student.
6. Catch FileNotFoundException, then IOException, then ClassNotFoundException.
7. Display the details of the restored object if it is not null.

Java Program

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Scanner;

// Only a Serializable class can be written to an object stream
class Student implements Serializable {
    private static final long serialVersionUID = 1L;

    private final int id;
    private final String name;
    private final double gpa;

    public Student(int id, String name, double gpa) {
        this.id = id;
        this.name = name;
        this.gpa = gpa;
    }

    public void displayStudent() {
        System.out.println("Student ID: " + id);
        System.out.println("Name: " + name);
        System.out.println("GPA: " + gpa);
    }
}

public class StudentSerializationDemo {
    private static final String FILE_NAME = "student.ser";

    public static void serializeStudent(Student student) {
        try (ObjectOutputStream oos =
                     new ObjectOutputStream(new FileOutputStream(FILE_NAME))) {
            oos.writeObject(student);
            System.out.println("Student object serialized to " + FILE_NAME);
        } catch (IOException e) {
            System.out.println("Error during serialization: " + e.getMessage());
        }
    }

    public static Student deserializeStudent() {
        try (ObjectInputStream ois =
                     new ObjectInputStream(new FileInputStream(FILE_NAME))) {
            return (Student) ois.readObject();
        } catch (FileNotFoundException e) {
            System.out.println("File not found. Serialize the object first.");
        } catch (IOException e) {
            System.out.println("Error during deserialization: " + e.getMessage());
        } catch (ClassNotFoundException e) {
            System.out.println("Class not found: " + e.getMessage());
        }

        return null;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        try {
            System.out.print("Enter student id: ");
            int id = Integer.parseInt(scanner.nextLine().trim());

            System.out.print("Enter student name: ");
            String name = scanner.nextLine().trim();

            System.out.print("Enter GPA: ");
            double gpa = Double.parseDouble(scanner.nextLine().trim());

            serializeStudent(new Student(id, name, gpa));

            Student restored = deserializeStudent();
            if (restored != null) {
                System.out.println("\nDeserialized Student Details:");
                restored.displayStudent();
            }
        } finally {
            scanner.close();
        }
    }
}

Sample Output

Enter student id: 101
Enter student name: Alice
Enter GPA: 3.8
Student object serialized to student.ser

Deserialized Student Details:
Student ID: 101
Name: Alice
GPA: 3.8

Problem statement: Create a menu-based Java application with the following options: 1. Add an Employee, 2. Display All, 3. Exit. If option 1 is selected, the application should gather details of the employee such as employee name, employee id, designation, and salary, and store it in a file. If option 2 is selected, the application should display all the employee details. If option 3 is selected the application should exit.

Algorithm

1. Create an Employee class with id, name, designation, and salary that implements Serializable.
2. At startup, load the saved list from employees.dat, or create an empty list if the file does not exist.
3. Repeat: display the menu with Add Employee, Display All, and Exit, and read the choice.
4. For choice 1, read the four details, add the record to the list, and write the list to the file.
5. For choice 2, print every record, or a message when the list is empty.
6. For choice 3, save the list once more and end the loop.
7. For any other value, print an error message and show the menu again.

Java Program

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

class Employee implements Serializable {
    private static final long serialVersionUID = 1L;

    private final int id;
    private final String name;
    private final String designation;
    private final double salary;

    public Employee(int id, String name, String designation, double salary) {
        this.id = id;
        this.name = name;
        this.designation = designation;
        this.salary = salary;
    }

    public void display() {
        System.out.printf("%-6d %-15s %-15s %.2f%n", id, name, designation, salary);
    }
}

public class EmployeeFileManager {
    private static final String FILE_NAME = "employees.dat";

    // readObject() returns Object, so the cast to List cannot be checked at compile time
    @SuppressWarnings("unchecked")
    private static List<Employee> loadEmployees() {
        File file = new File(FILE_NAME);

        if (!file.exists()) {
            return new ArrayList<>();   // first run: nothing saved yet
        }

        try (ObjectInputStream ois =
                     new ObjectInputStream(new FileInputStream(FILE_NAME))) {
            return (List<Employee>) ois.readObject();
        } catch (IOException | ClassNotFoundException e) {
            System.out.println("Error loading employees: " + e.getMessage());
            return new ArrayList<>();
        }
    }

    private static void saveEmployees(List<Employee> employees) {
        try (ObjectOutputStream oos =
                     new ObjectOutputStream(new FileOutputStream(FILE_NAME))) {
            oos.writeObject(employees);
        } catch (IOException e) {
            System.out.println("Error saving employees: " + e.getMessage());
        }
    }

    private static void addEmployee(List<Employee> employees, Scanner scanner) {
        System.out.print("Enter Employee ID: ");
        int id = Integer.parseInt(scanner.nextLine().trim());

        System.out.print("Enter Employee Name: ");
        String name = scanner.nextLine().trim();

        System.out.print("Enter Designation: ");
        String designation = scanner.nextLine().trim();

        System.out.print("Enter Salary: ");
        double salary = Double.parseDouble(scanner.nextLine().trim());

        employees.add(new Employee(id, name, designation, salary));
        saveEmployees(employees);           // store the record in the file now
        System.out.println("Employee added successfully!");
    }

    private static void displayEmployees(List<Employee> employees) {
        if (employees.isEmpty()) {
            System.out.println("No employees found.");
            return;
        }

        System.out.println("\nEmployee Details:");
        System.out.println("ID     Name            Designation     Salary");
        for (Employee employee : employees) {
            employee.display();
        }
    }

    public static void main(String[] args) {
        List<Employee> employees = loadEmployees();
        Scanner scanner = new Scanner(System.in);

        try {
            int choice;
            do {
                System.out.println("\n1. Add Employee");
                System.out.println("2. Display All");
                System.out.println("3. Exit");
                System.out.print("Enter your choice: ");
                choice = Integer.parseInt(scanner.nextLine().trim());

                switch (choice) {
                    case 1 -> addEmployee(employees, scanner);
                    case 2 -> displayEmployees(employees);
                    case 3 -> {
                        saveEmployees(employees);
                        System.out.println("Records saved. Exiting...");
                    }
                    default -> System.out.println("Invalid choice! Please enter a valid option.");
                }
            } while (choice != 3);
        } finally {
            scanner.close();
        }
    }
}

Sample Output

1. Add Employee
2. Display All
3. Exit
Enter your choice: 1
Enter Employee ID: 201
Enter Employee Name: Ravi Verma
Enter Designation: Analyst
Enter Salary: 52000
Employee added successfully!

1. Add Employee
2. Display All
3. Exit
Enter your choice: 2

Employee Details:
ID     Name            Designation     Salary
201    Ravi Verma      Analyst         52000.00

1. Add Employee
2. Display All
3. Exit
Enter your choice: 3
Records saved. Exiting...