/**

* Day 10 - Encapsulation: Access Modifiers + Validation

*

* <p><b>Concept</b>

* Encapsulation is about protecting an object's internal state (fields)

* and exposing only a controlled API (methods).

*

* <p><b>What this file demonstrates</b>

* <ul>

* <li>Private fields</li>

* <li>Getters and setters with validation</li>

* <li>Domain-style methods (e.g., {@code isOnHonorRoll()})</li>

* <li>Keeping an object in a valid state via invariants</li>

* </ul>

*

* <p><b>Real-life analogy</b>

* A bank account does not allow direct access to balance.

* You must use deposit/withdraw, which enforce rules.

*/

public class EncapsulationExample {

/**

* A simple Student class demonstrating proper encapsulation.

*

* <p>Invariants:

* <ul>

* <li>name is non-blank</li>

* <li>age is within a reasonable range</li>

* <li>gpa is between 0.0 and 4.0</li>

* </ul>

*/

static class Student {

private String name;

private int age;

private double gpa;

/**

* Constructs a Student in a valid state.

*

* @param name student's name (non-blank)

* @param age student's age (5..40)

* @param gpa student's GPA (0.0..4.0)

*/

Student(String name, int age, double gpa) {

// Constructor should fail fast if object cannot be created validly.

// We'll reuse setters so validation stays in one place.

setName(name);

setAge(age);

setGPA(gpa);

if (this.name == null) {

throw new IllegalArgumentException("name must be non-blank");

}

}

/** @return student's name */

public String getName() {

return name;

}

/**

* Updates the name.

*

* <p>Teaching note: Here we "reject" invalid values. In production you might

* prefer throwing an exception instead of printing.

*/

public void setName(String name) {

if (name != null && !name.isBlank()) {

this.name = name;

} else {

System.out.println("ERROR: Name cannot be blank");

}

}

/** @return student's age */

public int getAge() {

return age;

}

/** Updates age if within range. */

public void setAge(int age) {

if (age >= 5 && age <= 40) {

this.age = age;

} else {

System.out.println("ERROR: Age must be between 5 and 40");

}

}

/** @return student's GPA */

public double getGPA() {

return gpa;

}

/** Updates GPA if within [0.0, 4.0]. */

public void setGPA(double gpa) {

if (gpa >= 0.0 && gpa <= 4.0) {

this.gpa = gpa;

} else {

System.out.println("ERROR: GPA must be between 0.0 and 4.0");

}

}

/** @return true if the student qualifies for honor roll */

public boolean isOnHonorRoll() {

return gpa >= 3.5;

}

@Override

public String toString() {

return "Student{name='" + name + "', age=" + age + ", gpa=" + gpa + "}";

}

}

/**

* Program entry point.

*

* @param args CLI args (not used)

*/

public static void main(String[] args) {

System.out.println("--- Creating Student with Encapsulation ---\n");

Student student = new Student("Tejas", 20, 3.8);

System.out.println("Initial: " + student);

System.out.println("\n--- Testing Getters ---");

System.out.println("Name: " + student.getName());

System.out.println("Age: " + student.getAge());

System.out.println("GPA: " + student.getGPA());

System.out.println("\n--- Testing Setters with Valid Values ---");

student.setName("Raj");

student.setAge(21);

student.setGPA(3.9);

System.out.println("After valid updates: " + student);

System.out.println("\n--- Testing Setters with INVALID Values ---");

student.setName(" "); // invalid

student.setAge(-5); // invalid

student.setGPA(5.0); // invalid

System.out.println("\nAfter invalid attempts (object still valid):");

System.out.println(student);

System.out.println("\n--- Testing Domain Method ---");

System.out.println("On honor roll: " + student.isOnHonorRoll());

System.out.println("\n--- Encapsulation Benefits ---");

System.out.println("1) Data protection: invalid values rejected");

System.out.println("2) Validation centralized in one place");

System.out.println("3) Object remains in a valid state (invariants)");

System.out.println("4) Implementation can change without breaking callers");

}

}