Java toString(), Encapsulation, Getters/Setters & Object Best Practices

class Student {
    // Encapsulated fields
    private String name;
    private int age;
    private double gpa;
    private boolean isEnrolled;

    // Parameterized constructor
    public Student(String name, int age, double gpa) {
        setName(name);  // Reuse setter validation in constructor!
        setAge(age);
        setGpa(gpa);
        this.isEnrolled = true;
    }

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

    // Setters with validation
    public void setName(String name) {
        if (name == null || name.isBlank())
            throw new IllegalArgumentException("Name cannot be null or blank.");
        this.name = name.trim();
    }

    public void setAge(int age) {
        if (age < 16 || age > 80)
            throw new IllegalArgumentException("Age out of valid range: " + age);
        this.age = age;
    }

    public void setGpa(double gpa) {
        if (gpa < 0.0 || gpa > 4.0)
            throw new IllegalArgumentException("GPA must be between 0.0 and 4.0.");
        this.gpa = gpa;
    }

    public void setEnrolled(boolean enrolled) { this.isEnrolled = enrolled; }

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

    // Instance method (user requested)
    public void displayDetails() {
        System.out.println(name + " - " + age);
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("=== 1. User Requested Snippet with toString() ===");
        Student student = new Student("Ravi", 20, 3.8);
        student.displayDetails();
        System.out.println("toString(): " + student);

        System.out.println("
=== 2. Encapsulation Getters & Setters ===");
        student.setGpa(3.95);
        System.out.printf("Updated GPA via setter: %.2f%n", student.getGpa());

        System.out.println("
=== 3. Validation in Setter (Protected State) ===");
        try {
            student.setAge(200); // Invalid!
        } catch (IllegalArgumentException e) {
            System.out.println("Validation caught: " + e.getMessage());
        }

        try {
            student.setGpa(5.5); // Invalid GPA!
        } catch (IllegalArgumentException e) {
            System.out.println("Validation caught: " + e.getMessage());
        }

        System.out.println("
=== 4. toString() in String Concatenation ===");
        System.out.println("Student object info: " + student);

        System.out.println("
=== 5. Fluent Method Chaining Example ===");
        // Simulate building report header
        System.out.println("Profile Card: [" + student.getName() + " | Age: " + student.getAge() + " | GPA: " + student.getGpa() + "]");
    }
}

class BankAccount {
    private final String accountId;
    private String holderName;
    private double balance;

    public BankAccount(String accountId, String holderName, double initialDeposit) {
        this.accountId  = accountId;
        this.holderName = holderName;
        this.balance    = Math.max(0, initialDeposit);
    }

    public void deposit(double amount) {
        if (amount <= 0) throw new IllegalArgumentException("Deposit amount must be positive!");
        balance += amount;
    }

    public void withdraw(double amount) {
        if (amount <= 0 || amount > balance)
            throw new IllegalArgumentException("Invalid withdrawal: $" + amount);
        balance -= amount;
    }

    public double getBalance()    { return balance; }
    public String getHolderName() { return holderName; }

    @Override
    public String toString() {
        return String.format("BankAccount{id='%s', holder='%s', balance=$%.2f}",
                accountId, holderName, balance);
    }
}

public class PracticalApplication {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount("ACC-2026-001", "Ravi Kumar", 5000.0);
        System.out.println("=== Bank Account Operations ===");
        System.out.println("Initial: " + acc);
        acc.deposit(2000.0);
        acc.withdraw(800.0);
        System.out.println("Final  : " + acc);
    }
}