Data hiding in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code demonstrating data hiding?

Consider the following Java class that uses private fields to hide data. What will be printed when the main method runs?

Java

public class Person {
    private String name = "Alice";
    private int age = 30;

    public String getName() {
        return name;
    }

    public void setName(String newName) {
        name = newName;
    }

    public static void main(String[] args) {
        Person p = new Person();
        System.out.println(p.name);
    }
}

A Runtime error

B Alice

C null

D Compilation error: name has private access in Person

💡 Hint

Private fields can be accessed directly from main inside the same class using an instance reference.

The name field is private, but access from within the same class (even static main via instance p) is allowed. It prints "Alice".

❓ Predict Output

intermediate

What is the output when accessing a private field via a public method?

Given the class below, what will be printed when the main method runs?

Java

public class BankAccount {
    private double balance = 1000.0;

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount account = new BankAccount();
        System.out.println(account.getBalance());
    }
}

A 1000.0

B Compilation error: getBalance() not found

C 0.0

D Runtime error

💡 Hint

Private fields can be accessed inside the class. Public methods can expose them safely.

The private field balance is accessed via the public method getBalance(). This is the correct way to read private data from outside the class.

🔧 Debug

advanced

Why does this code fail to update the private field?

Look at this Java class. The setAge method tries to update the private field age. Why does the age not change after calling setAge(40)?

Java

public class Employee {
    private int age = 25;

    public void setAge(int age) {
        age = age;
    }

    public int getAge() {
        return age;
    }

    public static void main(String[] args) {
        Employee e = new Employee();
        e.setAge(40);
        System.out.println(e.getAge());
    }
}

A The parameter 'age' shadows the field 'age', so the assignment does nothing

B The field 'age' is final and cannot be reassigned

C The method setAge is never called

D Private fields cannot be changed once set

💡 Hint

Check the difference between the parameter and the field names inside the method.

The method parameter age has the same name as the field age. The assignment age = age; assigns the parameter to itself, not the field. To fix, use this.age = age;.

📝 Syntax

advanced

Which option correctly declares a private field with a public getter in Java?

Choose the correct code snippet that declares a private integer field count and a public method getCount that returns it.

A

private int count;
public int getCount() { return Count; }

B

int private count;
public int getCount() { return count; }

C

private int count;
public int getCount() { return count; }

D

private int count;
public int getCount() { return this.count; }

💡 Hint

Remember Java syntax for access modifiers and case sensitivity.

Option D correctly declares a private field and returns it using this.count. Option D is also valid but option D is preferred for clarity. Option D has invalid modifier order. Option D uses wrong case Count.

🚀 Application

expert

How many objects can access the private field directly in this scenario?

Given the following two classes in the same package, how many objects can directly access the private field secret of SecretBox?

public class SecretBox {
    private String secret = "hidden";
}

public class Spy {
    public void tryAccess() {
        SecretBox box = new SecretBox();
        System.out.println(box.secret);
    }
}

A Zero objects can access the private field directly

B Only objects of class Spy can access it

C Only objects of class SecretBox can access it

D All objects in the same package can access it

💡 Hint

Recall what private means in Java regarding access from other classes.

Private fields are accessible only within the class they are declared in. No other class, even in the same package, can access them directly.