Private access modifier in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a private variable named age of type int.

Java

private [1] age;

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

Aint

BString

Cvoid

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using String instead of int for age.

Forgetting to specify the data type.

The private keyword restricts access, and int is the correct type for the variable age.

2fill in blank

medium

Complete the code to create a private method named getAge that returns an int.

Java

private [1] getAge() { return age; }

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

Avoid

Bint

CString

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as return type when the method returns a value.

Using String instead of int.

The method getAge returns an integer, so its return type must be int.

3fill in blank

hard

Fix the error in the code by completing the blank to access the private variable name inside the class.

Java

public String getName() { return this.[1]; }

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

Aname

BgetName

CName()

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase Name instead of name.

Trying to call name() as a method.

The private variable is named name with lowercase, so this.name accesses it correctly.

4fill in blank

hard

Fill both blanks to create a private constructor that initializes the private variable id.

Java

private [1]([2] id) { this.id = id; }

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

AMyClass

Bint

CString

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as constructor return type.

Using wrong parameter type like String.

The constructor name must match the class name MyClass, and the parameter type for id is int.

5fill in blank

hard

Fill all three blanks to create a private setter method named setScore that updates the private variable score.

Java

private [1] setScore([2] score) { this.[3] = score; }

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

Avoid

Bint

Cscore

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using a return type other than void.

Using wrong parameter type like String.

Using incorrect variable name in assignment.

The setter method returns nothing (void), takes an int parameter named score, and assigns it to the private variable score.

Practice

1. What does the private access modifier do in Java?

easy

A. It restricts access to the member only within its own class.

B. It allows access from any class in the same package.

C. It allows access from any class in any package.

D. It allows access only from subclasses.

Solution

1. Step 1: Understand the meaning of private

   The private modifier restricts access to the member so that only code inside the same class can use it.

2. Step 2: Compare with other access levels

   Other modifiers like public or protected allow wider access, but private is the most restrictive.

3. Final Answer:

   It restricts access to the member only within its own class. -> Option A

4. Quick Check:

   Private means only inside class [OK]

Hint: Private means only accessible inside the same class [OK]

Common Mistakes:

• Confusing private with protected or public
• Thinking private allows subclass access
• Assuming private allows package access

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

easy

A. private count int;

B. int private count;

C. private int count;

D. count private int;

Solution

1. Step 1: Recall Java variable declaration syntax

   In Java, the access modifier comes first, then the type, then the variable name.

2. Step 2: Check each option

   private int count; follows the correct order: private int count;. Others have wrong order or syntax.

3. Final Answer:

   private int count; -> Option C

4. Quick Check:

   Access modifier + type + name [OK]

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

Common Mistakes:

• Placing 'private' after the type or variable name
• Mixing order of keywords
• Missing semicolon

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

class Test {
  private int number = 5;
  public void printNumber() {
    System.out.println(number);
  }
}
public class Main {
  public static void main(String[] args) {
    Test t = new Test();
    t.printNumber();
  }
}

medium

A. Runtime error

B. Compilation error

C. 0

D. 5

Solution

1. Step 1: Understand private member access inside class

   The variable number is private but accessed inside its own class method printNumber(), which is allowed.

2. Step 2: Check main method output

   The main method creates an object and calls printNumber(), which prints the value 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Private accessible inside class method [OK]

Hint: Private fields can be used inside their own class methods [OK]

Common Mistakes:

• Thinking private prevents access even inside class
• Expecting compilation error due to private
• Confusing default value with actual initialization

4. Identify the error in the following Java code:

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

medium

A. Missing constructor in Person class.

B. Cannot access private field name from outside the class.

C. Syntax error in class declaration.

D. No error; code runs fine.

Solution

1. Step 1: Check access to private field from outside class

   The field name is private in Person class, so it cannot be accessed directly from Main class.

2. Step 2: Understand Java access rules

   Accessing p.name from Main causes a compilation error due to private access.

3. Final Answer:

   Cannot access private field name from outside the class. -> Option B

4. Quick Check:

   Private fields block outside access [OK]

Hint: Private fields can't be accessed outside their class [OK]

Common Mistakes:

• Assuming private fields are accessible everywhere
• Thinking missing constructor causes error here
• Ignoring access modifiers

5. You want to keep a user's password private in a Java class but allow other classes to check if a password matches. Which is the best way to do this?

hard

A. Make the password field private and provide a public method to check the password.

B. Make the password field public so other classes can read it directly.

C. Make the password field protected and access it from other classes.

D. Make the password field private and provide a public method to return the password.

Solution

1. Step 1: Protect sensitive data with private fields

   Passwords should be private to prevent direct access and protect security.

2. Step 2: Provide controlled access via public method

   A public method can check if a given password matches without revealing the actual password.

3. Final Answer:

   Make the password field private and provide a public method to check the password. -> Option A

4. Quick Check:

   Private data + public check method = secure design [OK]

Hint: Keep data private; use public methods to interact safely [OK]

Common Mistakes:

• Making password public exposes sensitive data
• Returning password directly breaks security
• Using protected exposes data to subclasses unnecessarily