Encapsulation best practices in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Encapsulation with Private Fields

What is the output of this Java code that uses encapsulation with private fields and public getters/setters?

Java

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        if (age > 0) {
            this.age = age;
        }
    }

    public static void main(String[] args) {
        Person p = new Person("Alice", 30);
        p.setAge(-5);
        System.out.println(p.getName() + " is " + p.getAge() + " years old.");
    }
}

A Compilation error due to private fields.

B Alice is 30 years old.

C Alice is -5 years old.

D Runtime error due to invalid age.

💡 Hint

Check how the setAge method handles negative values.

The setAge method only updates the age if the new value is positive. Since -5 is not positive, the age remains 30.

🧠 Conceptual

intermediate

Why Use Private Fields in Encapsulation?

Why is it a best practice to declare class fields as private when using encapsulation in Java?

A To allow subclasses to access fields directly without getters or setters.

B To make the fields accessible from any class without restrictions.

C To prevent direct access and modification from outside the class, ensuring control over data.

D To improve the speed of the program by avoiding method calls.

💡 Hint

Think about how encapsulation protects data integrity.

Private fields restrict direct access, so the class controls how data is accessed or changed, which helps maintain data integrity.

🔧 Debug

advanced

Identify the Encapsulation Violation

Which option shows a violation of encapsulation best practices in Java?

Java

public class BankAccount {
    public double balance;

    public BankAccount(double balance) {
        this.balance = balance;
    }

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }
}

A Making the balance field public allows direct modification from outside the class.

B Providing a deposit method to update balance.

C Checking if deposit amount is positive before adding.

D Using a constructor to set the initial balance.

💡 Hint

Encapsulation means hiding data from outside direct access.

The balance field is public, so any code can change it directly, bypassing validation and control.

📝 Syntax

advanced

Correct Setter Method Syntax for Encapsulation

Which setter method correctly follows encapsulation best practices in Java?

Java

private int score;

A public void setScore(int score) { this.score = score; }

B public int setScore(int score) { this.score = score; return score; }

C public void setScore() { score = 10; }

D void setScore(int score) { score = score; }

💡 Hint

A setter should accept a parameter and update the private field correctly.

Option A correctly takes a parameter and assigns it to the private field using 'this' to avoid confusion.

🚀 Application

expert

Number of Accessible Fields After Encapsulation

Given this class, how many fields are accessible directly from outside the class without using getters or setters?

Java

public class Car {
    private String model;
    protected int year;
    public String color;
    String owner;
}

A 4

B 2

C 3

D 1

💡 Hint

Consider Java's access levels: private, protected, public, and default (package-private).

Only the public field 'color' is accessible directly from any outside class. 'year' is protected (accessible in subclasses and package), 'owner' is package-private (default), and 'model' is private.