Java Encapsulation

public class BankAccount {
    // private fields — hidden from outside
    private String owner;
    private double balance;

    public BankAccount(String owner, double initialBalance) {
        this.owner   = owner;
        this.balance = (initialBalance >= 0) ? initialBalance : 0;
    }

    // Getter
    public String getOwner() { return owner; }

    // Getter with read-only access
    public double getBalance() { return balance; }

    // Controlled mutation — validation inside setter
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            System.out.printf("Deposited $%.2f. New balance: $%.2f%n", amount, balance);
        } else {
            System.out.println("Deposit amount must be positive.");
        }
    }

    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            System.out.printf("Withdrew $%.2f. New balance: $%.2f%n", amount, balance);
        } else {
            System.out.println("Invalid withdrawal amount.");
        }
    }

    public static void main(String[] args) {
        BankAccount acc = new BankAccount("Alice", 1000.0);
        System.out.println("Owner: " + acc.getOwner());
        acc.deposit(500.0);
        acc.withdraw(200.0);
        acc.withdraw(2000.0);  // Invalid
        // acc.balance = 999999;  // ERROR: balance is private
    }
}

import java.io.Serializable;

public class Student implements Serializable {
    private String name;
    private int    age;
    private double gpa;
    private boolean enrolled;

    // No-arg constructor (required for JavaBean)
    public Student() {}

    // Getters
    public String  getName()     { return name; }
    public int     getAge()      { return age; }
    public double  getGpa()      { return gpa; }
    public boolean isEnrolled()  { return enrolled; }  // boolean uses 'is'

    // Setters with validation
    public void setName(String name)       { this.name = name; }
    public void setAge(int age)            { if (age > 0) this.age = age; }
    public void setGpa(double gpa)         { if (gpa >= 0 && gpa <= 4.0) this.gpa = gpa; }
    public void setEnrolled(boolean e)     { this.enrolled = e; }

    @Override
    public String toString() {
        return String.format("Student{name='%s', age=%d, gpa=%.2f, enrolled=%b}",
                             name, age, gpa, enrolled);
    }

    public static void main(String[] args) {
        Student s = new Student();
        s.setName("Bob");
        s.setAge(20);
        s.setGpa(3.75);
        s.setEnrolled(true);
        System.out.println(s);
    }
}