Data hiding in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a private variable in a class.

Java

class Person {
    [1] String name;
}

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

Apublic

Bprivate

Cprotected

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using public instead of private, which does not hide the data.

Using static which is unrelated to access control.

The keyword private hides the variable from outside the class, enabling data hiding.

2fill in blank

medium

Complete the code to provide a public method to access the private variable.

Java

class Person {
    private String name;
    public String [1]() {
        return name;
    }
}

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

AgetName

Bname

CsetName

DprintName

Attempts:

3 left

💡 Hint

Common Mistakes

Using setName which is a setter, not a getter.

Using the variable name directly as method name.

The method to access a private variable is called a getter, usually named starting with 'get'.

3fill in blank

hard

Fix the error in the setter method to correctly set the private variable.

Java

class Person {
    private String name;
    public void setName(String [1]) {
        name = [1];
    }
}

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

Athis.name

Bname

CnewName

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for parameter and field without 'this' keyword.

Not assigning the parameter value to the field.

The parameter name should be different from the field name to avoid confusion. Using newName allows setting name = newName;.

4fill in blank

hard

Fill both blanks to correctly implement a setter method using 'this' keyword.

Java

class Person {
    private int age;
    public void setAge([1] age) {
        [2].age = age;
    }
}

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

Aint

Bthis

CPerson

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using class name instead of 'this' to refer to the field.

Omitting the parameter type.

The setter method takes an int parameter and uses this to refer to the current object's field.

5fill in blank

hard

Fill all three blanks to create a class with a private variable and a getter.

Java

class Car {
    private String [1];
    public String [2]() {
        return [3];
    }
}

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

Amodel

BgetModel

DsetModel

Attempts:

3 left

💡 Hint

Common Mistakes

Using setter name instead of getter.

Returning a wrong variable name.

The private variable is model. The getter method is getModel and it returns the variable model.

Practice

1. What is the main purpose of data hiding in Java?

easy

A. To keep class variables private and protect them from outside access

B. To make all variables public for easy access

C. To hide methods from the user interface

D. To encrypt data before storing it

Solution

1. Step 1: Understand data hiding concept

   Data hiding means keeping variables private inside a class to prevent direct access from outside.

2. Step 2: Identify the purpose

   This protects data from unwanted changes and bugs by controlling access through methods.

3. Final Answer:

   To keep class variables private and protect them from outside access -> Option A

4. Quick Check:

   Data hiding = keeping variables private [OK]

Hint: Data hiding means making variables private [OK]

Common Mistakes:

• Thinking data hiding means encrypting data
• Confusing data hiding with making variables public
• Believing data hiding hides methods from UI

2. Which of the following is the correct way to declare a private variable in a Java class?

easy

A. int age;

B. public int age;

C. protected int age;

D. private int age;

Solution

1. Step 1: Recall Java access modifiers

   Private variables are declared with the keyword private to restrict access.

2. Step 2: Identify correct syntax

   Only private int age; correctly declares a private variable.

3. Final Answer:

   private int age; -> Option D

4. Quick Check:

   Private variable = private keyword [OK]

Hint: Use 'private' keyword to hide variables [OK]

Common Mistakes:

• Using public or protected instead of private
• Omitting access modifier defaults to package-private
• Confusing private with protected

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. null

B. Compilation error

C. Alice

D. Runtime error

Solution

1. Step 1: Understand private variable access

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

2. Step 2: Trace the output

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

3. Final Answer:

   Alice -> Option C

4. Quick Check:

   Getter returns private value = Alice [OK]

Hint: Private data accessed via public getter returns value [OK]

Common Mistakes:

• Expecting direct access to private variable
• Thinking code causes compilation error
• Confusing output with null or error

4. Identify the error in this code related to data hiding:

class BankAccount {
  private double balance;
  public void setBalance(double balance) {
    balance = balance;
  }
  public double getBalance() {
    return balance;
  }
}

medium

A. The setter method does not update the private variable

B. The getter method should be private

C. The balance variable should be public

D. The class should not have a setter method

Solution

1. Step 1: Analyze setter method

   The setter uses balance = balance; which assigns the parameter to itself, not the class variable.

2. Step 2: Identify correct assignment

   It should use this.balance = balance; to update the private variable.

3. Final Answer:

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

4. Quick Check:

   Setter must update class variable using 'this' [OK]

Hint: Use 'this' to assign parameter to class variable [OK]

Common Mistakes:

• Forgetting 'this' keyword in setter
• Making getter private by mistake
• Changing variable access to public unnecessarily

5. You want to protect a class's sensitive data but allow controlled updates only if the new value is positive. How would you implement this using data hiding in Java?

hard

A. Make the variable public and check the value before assigning it outside the class

B. Make the variable private and write a setter that updates only if the value is positive

C. Make the variable protected and allow direct access in subclasses

D. Use a public variable and no setter method

Solution

1. Step 1: Use private variable for data hiding

   Keep the sensitive variable private to prevent direct external access.

2. Step 2: Implement setter with condition

   Write a setter method that updates the variable only if the new value is positive, ensuring controlled updates.

3. Final Answer:

   Make the variable private and write a setter that updates only if the value is positive -> Option B

4. Quick Check:

   Private variable + conditional setter = safe updates [OK]

Hint: Use private variable with conditional setter method [OK]

Common Mistakes:

• Making variable public and trusting external checks
• Using protected instead of private for sensitive data
• Not validating data in setter method