Protected access modifier in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a protected variable named data.

Java

protected [1] data;

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

Aint

Bpublic

Cclass

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' or 'void' instead of a data type.

Forgetting to specify a data type.

The protected keyword is an access modifier. Here, int is the correct data type to declare the variable data.

2fill in blank

medium

Complete the code to declare a protected method named showData that returns nothing.

Java

protected [1] showData() { }

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

Aclass

Bint

Cvoid

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using a data type like int or String when the method returns nothing.

Using 'class' which is not a return type.

The method showData does not return any value, so its return type should be void.

3fill in blank

hard

Fix the error in the code by completing the access modifier for the method display so it can be accessed by subclasses.

Java

[1] void display() { System.out.println("Hello"); }

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

Aprivate

Bpublic

Cstatic

Dprotected

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'private' which restricts access to the same class only.

Using 'static' which is not an access modifier.

The protected modifier allows access to subclasses and classes in the same package.

4fill in blank

hard

Fill both blanks to declare a protected constructor for class Example.

Java

class Example {
    [1] [2]() { }
}

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

Aprotected

Bpublic

CExample

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as a constructor return type.

Using 'public' instead of 'protected' for this exercise.

A constructor has the same name as the class and can have the protected access modifier.

5fill in blank

hard

Fill all three blanks to create a protected method named calculate that returns an integer and takes an integer parameter named value.

Java

protected [1] calculate([2] [3]) { return value * 2; }

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

Aint

Cvalue

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using a parameter name different from 'value'.

The method returns an int, takes an int parameter named value, and is declared protected.

Practice

1. Which statement best describes the protected access modifier in Java?

easy

A. It allows access only within the same class.

B. It allows access only within the same package, not subclasses outside.

C. It allows access from any class anywhere.

D. It allows access within the same package and to subclasses outside the package.

Solution

1. Step 1: Understand protected access scope

   The protected modifier allows access to members within the same package and also to subclasses even if they are in different packages.

2. Step 2: Compare with other access levels

   Private restricts access to the same class only, public allows everywhere, and default (package-private) allows only within the same package but not subclasses outside.

3. Final Answer:

   It allows access within the same package and to subclasses outside the package. -> Option D

4. Quick Check:

   Protected = same package + subclasses outside [OK]

Hint: Protected means package + subclass access [OK]

Common Mistakes:

• Confusing protected with private
• Thinking protected is public
• Assuming protected restricts subclasses outside package

2. Which of the following is the correct way to declare a protected method in Java?

easy

A. private protected void myMethod() {}

B. public protected void myMethod() {}

C. protected void myMethod() {}

D. void protected myMethod() {}

Solution

1. Step 1: Recall Java access modifier syntax

   In Java, access modifiers come before the return type and method name. Only one access modifier is allowed per method declaration.

2. Step 2: Identify valid syntax

   protected void myMethod() {} correctly uses protected followed by the return type void and method name. Other options combine modifiers incorrectly or place them in wrong order.

3. Final Answer:

   protected void myMethod() {} -> Option C

4. Quick Check:

   Single access modifier before return type [OK]

Hint: Use only one access modifier before return type [OK]

Common Mistakes:

• Combining multiple access modifiers
• Placing modifiers after return type
• Using invalid modifier combinations

3. Consider the following code in two packages:

package pkg1;
public class Parent {
    protected int value = 10;
}

package pkg2;
import pkg1.Parent;
public class Child extends Parent {
    public int getValue() {
        return value;
    }
}

public class Test {
    public static void main(String[] args) {
        Child c = new Child();
        System.out.println(c.getValue());
    }
}

What is the output when running Test.main?

medium

A. 0

B. 10

C. Compilation error due to access

D. Runtime error

Solution

1. Step 1: Check access of protected member in subclass outside package

   The value field is protected in Parent. The Child class extends Parent and accesses value in getValue(). This is allowed because protected permits subclass access even outside the package.

2. Step 2: Trace the output of getValue()

   The method returns value which is initialized to 10. So, printing c.getValue() outputs 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Protected allows subclass access outside package [OK]

Hint: Protected allows subclass access outside package [OK]

Common Mistakes:

• Thinking protected blocks subclass outside package
• Expecting compilation error
• Confusing default access with protected

4. Identify the error in the following code:

package pkg1;
public class Parent {
    protected int data = 5;
}

package pkg2;
import pkg1.Parent;
public class Other {
    public void printData() {
        Parent p = new Parent();
        System.out.println(p.data);
    }
}

medium

A. Cannot access protected member from non-subclass in different package

B. No error, code runs fine

C. Syntax error in import statement

D. Cannot create instance of Parent class

Solution

1. Step 1: Analyze access of protected member from non-subclass outside package

   The class Other is not a subclass of Parent and is in a different package. Accessing p.data is not allowed because protected members are accessible only within the same package or subclasses outside the package.

2. Step 2: Identify the error type

   This causes a compilation error due to illegal access of a protected member from a non-subclass outside the package.

3. Final Answer:

   Cannot access protected member from non-subclass in different package -> Option A

4. Quick Check:

   Protected blocks non-subclass outside package [OK]

Hint: Protected blocks non-subclass outside package [OK]

Common Mistakes:

• Assuming protected allows all package access
• Confusing subclass and non-subclass access
• Ignoring package differences

5. You have a class Animal in package zoo with a protected method makeSound(). You want to allow only subclasses in any package to use makeSound(), but prevent other classes in the same package from calling it. How can you achieve this?

hard

A. Declare makeSound() as protected and move other classes out of the package.

B. Declare makeSound() as protected and make other classes subclasses of Animal.

C. Declare makeSound() as private and provide a public wrapper method.

D. Declare makeSound() as protected and keep other classes in the same package.

Solution

1. Step 1: Understand protected access within package and subclasses

   Protected members are accessible to all classes in the same package and to subclasses outside the package. So, classes in the same package can call makeSound() even if they are not subclasses.

2. Step 2: Restrict access to only subclasses

   To prevent other classes in the same package from accessing makeSound(), you must move those classes to a different package. Then only subclasses (in any package) can access the protected method.

3. Final Answer:

   Declare makeSound() as protected and move other classes out of the package. -> Option A

4. Quick Check:

   Protected allows package + subclass; move non-subclass classes out [OK]

Hint: Protected allows package access; move classes to restrict [OK]

Common Mistakes:

• Thinking protected blocks same-package classes
• Making other classes subclasses to restrict access
• Using private which blocks subclasses