Private data members in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a private integer variable named age.

Java

class Person {
    [1] int age;
}

Drag options to blanks, or click blank then click option'

Astatic

Bprivate

Cprotected

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' instead of 'private' allows access from anywhere.

Using 'static' changes the variable to belong to the class, not an instance.

The keyword private makes the variable accessible only within the class.

2fill in blank

medium

Complete the code to create a private String variable called name.

Java

class Student {
    [1] String name;
}

Drag options to blanks, or click blank then click option'

Apublic

Bfinal

Cprivate

Dprotected

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' exposes the variable to all classes.

Using 'final' makes the variable constant but does not restrict access.

Use private to keep the variable accessible only inside the class.

3fill in blank

hard

Fix the error in the code by choosing the correct access modifier for the data member.

Java

class Car {
    [1] String model;
}

Drag options to blanks, or click blank then click option'

Aprivate

Bpublic

Cstatic

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' exposes the variable to all classes.

Using 'static' or 'final' does not control access.

The data member should be private to follow encapsulation principles.

4fill in blank

hard

Fill both blanks to declare two private data members: an int id and a String name.

Java

class Employee {
    [1] int id;
    [2] String name;
}

Drag options to blanks, or click blank then click option'

Aprivate

Bpublic

Cprotected

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing access modifiers can cause inconsistent access.

Using 'public' exposes the variables.

Both variables should be declared private to keep them hidden from outside access.

5fill in blank

hard

Fill all three blanks to declare private data members: a String title, an int pages, and a double price.

Java

class Book {
    [1] String title;
    [2] int pages;
    [3] double price;
}

Drag options to blanks, or click blank then click option'

Apublic

Bprivate

Cprotected

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' or 'protected' exposes the data members.

Using 'final' only makes variables constant but does not restrict access.

All three data members should be private to protect the data inside the class.

Practice

1. What is the main purpose of declaring data members as private in a Java class?

easy

A. To allow direct modification of data from other classes

B. To hide the data from outside access and protect it

C. To make the data accessible from anywhere

D. To make the data static and shared

Solution

1. Step 1: Understand private keyword meaning

   The private keyword restricts access to the data member only within the class it is declared.

2. Step 2: Purpose of data hiding

   Hiding data prevents outside code from changing it directly, which protects the data integrity.

3. Final Answer:

   To hide the data from outside access and protect it -> Option B

4. Quick Check:

   Private means hidden and protected [OK]

Hint: Private means only inside class can access it [OK]

Common Mistakes:

• Thinking private allows access from other classes
• Confusing private with public or protected
• Assuming private makes data static

2. Which of the following is the correct way to declare a private integer variable named age inside a Java class?

easy

A. private int age;

B. int private age;

C. private integer age;

D. int age private;

Solution

1. Step 1: Check Java syntax for private variables

   The correct order is the access modifier first, then the type, then the variable name.

2. Step 2: Validate each option

   private int age; follows the correct syntax: private int age;. Others have wrong order or wrong type keyword.

3. Final Answer:

   private int age; -> Option A

4. Quick Check:

   Access modifier + type + name [OK]

Hint: Access modifier comes before type and name [OK]

Common Mistakes:

• Placing 'private' after the type
• Using 'integer' instead of 'int'
• Incorrect order of keywords

3. What will be the output of the following Java code?

class Person {
  private String name = "Alice";
  public String getName() {
    return name;
  }
}
public class Test {
  public static void main(String[] args) {
    Person p = new Person();
    System.out.println(p.getName());
  }
}

medium

A. Compilation error

B. null

C. Alice

D. Runtime error

Solution

1. Step 1: Understand private variable access

   The variable name is private but accessed via the public method getName().

2. Step 2: Trace the method call and output

   The method returns "Alice", so System.out.println prints "Alice".

3. Final Answer:

   Alice -> Option C

4. Quick Check:

   Private data accessed via public method returns value [OK]

Hint: Use public methods to access private data [OK]

Common Mistakes:

• Expecting direct access to private variable
• Thinking private variable is null by default
• Assuming compilation error due to private access

4. Identify the error in the following Java code snippet:

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

medium

A. The setter method does not update the private variable

B. The private variable speed should be public

C. The getter method should return void

D. The class Car should be declared public

Solution

1. Step 1: Analyze the setter method

   The setter method assigns the parameter speed to itself, not to the private variable.

2. Step 2: Understand variable shadowing

   The parameter speed shadows the private variable. To update the private variable, use this.speed = speed;.

3. Final Answer:

   The setter method does not update the private variable -> Option A

4. Quick Check:

   Use 'this' to update private variable in setter [OK]

Hint: Use 'this.' to refer to class variable in setters [OK]

Common Mistakes:

• Not using 'this' keyword in setter
• Making private variable public unnecessarily
• Changing getter return type incorrectly

5. You want to keep a private list of student names inside a class and allow adding names but prevent direct access to the list. Which approach correctly uses private data members and methods?

hard

A. Declare public List<String> students; allow direct access and modification.

B. Declare private List<String> students; no methods to add or access students.

C. Declare private List<String> students; provide public getStudents() returning the list directly.

D. Declare private List<String> students; provide public addStudent(String name) method; no public getter for the list.

Solution

1. Step 1: Understand data hiding and controlled access

   Private list hides data; public method to add controls how data changes.

2. Step 2: Evaluate options for safe access

   Declare private List<String> students; provide public addStudent(String name) method; no public getter for the list. hides list and allows adding names safely. Declare public List<String> students; allow direct access and modification. exposes list directly, unsafe. Declare private List<String> students; provide public getStudents() returning the list directly. exposes list directly via getter, unsafe. Declare private List<String> students; no methods to add or access students. provides no way to add or access data.

3. Final Answer:

   Declare private List<String> students; provide public addStudent(String name) method; no public getter for the list. -> Option D

4. Quick Check:

   Private data + public methods for controlled access [OK]

Hint: Use private list + public add method, no direct getter [OK]

Common Mistakes:

• Making list public and exposing internal data
• Returning private list directly allowing modification
• Not providing any method to modify or access data