Private data members in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing private data member directly

What will be the output of the following Java code?

Java

class Person {
    private String name = "Alice";
}

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

A Compilation error

B null

C Alice

D Runtime error

💡 Hint

Private members cannot be accessed directly outside their class.

The field 'name' is private in class Person. Trying to access it directly from Main causes a compilation error.

❓ Predict Output

intermediate

Output when accessing private member via getter

What will be the output of this Java program?

Java

class Person {
    private String name = "Bob";
    public String getName() {
        return name;
    }
}

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

A Bob

B null

C Compilation error

D Runtime error

💡 Hint

Private members can be accessed inside the class and exposed via public methods.

The private field 'name' is accessed through the public method getName(), so it prints 'Bob'.

❓ Predict Output

advanced

Effect of private data member shadowing

What is the output of this Java code?

Java

class Animal {
    private String type = "Animal";
    public String getType() {
        return type;
    }
}

class Dog extends Animal {
    private String type = "Dog";
    public String getType() {
        return type;
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        System.out.println(a.getType());
    }
}

A Animal

B Dog

C Compilation error

D Runtime error

💡 Hint

Method overriding works even if private fields are shadowed.

The variable 'a' is of type Animal but refers to a Dog object. The getType() method is overridden in Dog and returns Dog's private 'type'.

🔧 Debug

advanced

Identify the error with private data member access

Which option correctly identifies the error in this code snippet?

Java

class Car {
    private int speed;
    public void setSpeed(int speed) {
        speed = speed;
    }
    public int getSpeed() {
        return speed;
    }
}

public class Main {
    public static void main(String[] args) {
        Car c = new Car();
        c.setSpeed(50);
        System.out.println(c.getSpeed());
    }
}

A The code will not compile because 'speed' is private.

B The private field 'speed' cannot be accessed in getSpeed method.

C The setSpeed method does not set the private field 'speed' because of variable shadowing.

D The setSpeed method should be static to modify 'speed'.

💡 Hint

Look at the assignment inside setSpeed method.

Inside setSpeed, the parameter 'speed' shadows the field 'speed'. The assignment 'speed = speed;' assigns the parameter to itself, leaving the field unchanged.

🧠 Conceptual

expert

Why use private data members in Java?

Which option best explains the main reason for declaring data members as private in Java?

A To improve the performance of the program by hiding data.

B To make the data members accessible only within the same package.

C To allow subclasses to access the data members directly.

D To prevent other classes from accessing or modifying the data directly, ensuring controlled access.

💡 Hint

Think about encapsulation and data protection.

Private data members hide the internal state of an object and force access through methods, allowing validation and control.