Inheritance limitations in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class that cannot be extended.

Java

public [1] class Vehicle {}

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

Astatic

Bfinal

Cabstract

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'abstract' instead of 'final' to prevent inheritance.

Using final before a class means it cannot be inherited by other classes.

2fill in blank

medium

Complete the code to prevent a method from being overridden in subclasses.

Java

public class Animal {
    public [1] void sound() {
        System.out.println("Animal sound");
    }
}

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

Aabstract

Bstatic

Cprivate

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' which changes method behavior but does not prevent overriding.

The final keyword before a method means subclasses cannot override it.

3fill in blank

hard

Fix the error in the code by choosing the correct keyword to allow inheritance.

Java

[1] class Shape {}

public class Circle extends Shape {}

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

Apublic

Babstract

Cfinal

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' which prevents inheritance.

The class must be public (or default) and not final to allow inheritance.

4fill in blank

hard

Fill both blanks to declare an abstract class with an abstract method.

Java

public [1] class Device {
    public [2] void start();
}

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

Aabstract

Bfinal

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' for abstract methods or classes.

An abstract class can have abstract methods without body.

5fill in blank

hard

Fill all three blanks to override a method and prevent further overriding.

Java

public class Parent {
    public void greet() {
        System.out.println("Hello from Parent");
    }
}

public class Child extends Parent {
    @Override
    public [1] void greet() {
        System.out.println("Hello from Child");
    }
}

public class GrandChild extends Child {
    @Override
    public void greet() {
        [2].greet();
        System.out.println("Hello from GrandChild");
    }
}

// To prevent GrandChild from overriding greet(), add [3] keyword in Child class method.

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

Afinal

Bsuper

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to override a final method causes a compile error.

Marking the Child's greet() method as final prevents GrandChild from overriding it. The super keyword calls the parent's method.

Practice

1.

Which of the following is not allowed in Java inheritance?

• Extending multiple classes
• Extending a final class
• Overriding a final method
• All of the above

easy

A. Extending multiple classes

B. Extending a final class

C. Overriding a final method

D. All of the above

Solution

1. Step 1: Understand Java inheritance rules

   Java supports only single inheritance of classes, so extending multiple classes is not allowed.

2. Step 2: Check restrictions on final classes and methods

   Classes declared as final cannot be extended, and final methods cannot be overridden.

3. Final Answer:

   All of the above -> Option D

4. Quick Check:

   Java disallows multiple inheritance, final class extension, and final method overriding [OK]

Hint: Remember: final means no inheritance or override allowed [OK]

Common Mistakes:

• Thinking Java supports multiple class inheritance
• Trying to override final methods
• Assuming final classes can be extended

2.

Which of the following class declarations is correct in Java?

public class Animal {}
public class Dog extends Animal {}
public class Cat extends Dog, Animal {}

easy

A. All classes are correctly declared

B. Only Dog class declaration is correct

C. Cat class declaration is correct

D. Animal class declaration is incorrect

Solution

1. Step 1: Check single inheritance rule

   Java allows a class to extend only one class. Dog extends Animal correctly.

2. Step 2: Analyze Cat class declaration

   Cat tries to extend Dog and Animal simultaneously, which is invalid syntax in Java.

3. Final Answer:

   Only Dog class declaration is correct -> Option B

4. Quick Check:

   Single inheritance means one parent only [OK]

Hint: Java classes extend only one class at a time [OK]

Common Mistakes:

• Trying to extend multiple classes in one declaration
• Confusing interfaces with classes for multiple inheritance
• Assuming all class declarations are valid

3.

What will be the output of the following Java code?

final class Vehicle {
    void start() { System.out.println("Vehicle started"); }
}

class Car extends Vehicle {
    void start() { System.out.println("Car started"); }
}

public class Test {
    public static void main(String[] args) {
        Car c = new Car();
        c.start();
    }
}

medium

A. Compilation error

B. Vehicle started

C. Runtime error

D. Car started

Solution

1. Step 1: Identify final class usage

   The class Vehicle is declared final, so it cannot be extended by any class including Car.

2. Step 2: Check inheritance and compilation

   Since Car tries to extend final Vehicle, the compiler will throw an error.

3. Final Answer:

   Compilation error -> Option A

4. Quick Check:

   final class cannot be subclassed [OK]

Hint: final classes cannot be extended, causing compile errors [OK]

Common Mistakes:

• Assuming final class can be extended
• Expecting runtime error instead of compile error
• Thinking method overriding causes error here

4.

Find the error in the following code snippet:

class Parent {
    final void show() {
        System.out.println("Parent show");
    }
}

class Child extends Parent {
    void show() {
        System.out.println("Child show");
    }
}

medium

A. Parent class cannot have final methods

B. Child class must declare show() as final

C. Child class cannot override final method show()

D. No error, code is valid

Solution

1. Step 1: Understand final method behavior

   Methods declared final in a parent class cannot be overridden in child classes.

2. Step 2: Analyze Child class method

   Child class tries to override final method show(), which causes a compile-time error.

3. Final Answer:

   Child class cannot override final method show() -> Option C

4. Quick Check:

   final methods block overriding [OK]

Hint: final methods cannot be overridden in subclasses [OK]

Common Mistakes:

• Thinking final methods can be overridden
• Assuming no error in overriding final methods
• Confusing final methods with abstract methods

5.

You want to prevent any class from extending your class SecureData, but still allow other classes to use its methods. Which is the best way to do this?

hard

A. Declare the class SecureData as final

B. Make all methods in SecureData final

C. Make SecureData an abstract class

D. Declare SecureData methods as private

Solution

1. Step 1: Understand the effect of final class

   Declaring a class as final prevents any other class from extending it, but allows normal usage of its methods.

2. Step 2: Evaluate other options

   Making methods final prevents overriding but not extending; abstract class requires subclassing; private methods are inaccessible outside the class.

3. Final Answer:

   Declare the class SecureData as final -> Option A

4. Quick Check:

   final class blocks inheritance but allows usage [OK]

Hint: Use final class to block inheritance but allow method use [OK]

Common Mistakes:

• Confusing final methods with final classes
• Using abstract class which requires subclassing
• Making methods private, blocking access