Inheritance limitations in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method call with final class inheritance

What is the output of this Java code?

Java

final class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
}

public class Test {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound();
    }
}

A Runtime error

B Animal sound

C Dog barks

D Compilation error due to Dog extending final class Animal

💡 Hint

Recall what the 'final' keyword means when applied to a class.

In Java, a class declared as final cannot be extended. Here, Dog tries to extend final class Animal, causing a compilation error.

❓ Predict Output

intermediate

Output when overriding a final method

What happens when this Java code is compiled and run?

Java

class Vehicle {
    final 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) {
        Vehicle v = new Car();
        v.start();
    }
}

A Car started

B Compilation error due to overriding final method

C Runtime error

D Vehicle started

💡 Hint

Check if final methods can be overridden.

A final method cannot be overridden by subclasses. Here, Car tries to override start(), causing a compilation error.

🔧 Debug

advanced

Identify the inheritance limitation causing error

This code tries to compile but fails. What is the cause of the error?

Java

interface Flyer {
    void fly();
}

class Bird implements Flyer {
    public void fly() {
        System.out.println("Bird is flying");
    }
}

class Penguin extends Bird {
    public void fly() {
        System.out.println("Penguins can't fly");
    }
}

public class Test {
    public static void main(String[] args) {
        Flyer f = new Penguin();
        f.fly();
    }
}

A Compilation error because Penguin cannot override fly() from Bird

B Penguin cannot override fly() because Bird already implements Flyer

C No error; output is 'Penguins can't fly'

D Runtime error due to Penguin fly() method

💡 Hint

Consider if overriding interface methods is allowed.

A subclass can override methods from its superclass even if the superclass implements an interface. Penguin overrides fly() successfully.

📝 Syntax

advanced

Which code causes a compilation error due to multiple inheritance?

Java does not support multiple inheritance with classes. Which option causes a compilation error?

A

class A {}
class B {}
class C extends A, B {}

B

interface A {}
interface B {}
class C implements A, B {}

C

interface A {}
class B {}
class C extends B implements A {}

D

interface A {}
interface B {}
interface C extends A, B {}

💡 Hint

Check how Java handles multiple inheritance with classes and interfaces.

Java does not allow a class to extend more than one class. Option A tries to extend two classes, causing a compilation error.

🚀 Application

expert

How many methods can a class inherit from multiple interfaces with same default method?

Given these interfaces and class, how many distinct default methods named greet() does class Friendly inherit?

Java

interface A {
    default void greet() {
        System.out.println("Hello from A");
    }
}

interface B {
    default void greet() {
        System.out.println("Hello from B");
    }
}

class Friendly implements A, B {
    public void greet() {
        A.super.greet();
        B.super.greet();
        System.out.println("Hello from Friendly");
    }
}

public class Test {
    public static void main(String[] args) {
        Friendly f = new Friendly();
        f.greet();
    }
}

A Two default greet() methods inherited, one from A and one from B

B One default greet() method inherited from interface B

C One default greet() method inherited from interface A

D No default greet() methods inherited because Friendly overrides greet()

💡 Hint

Think about how Java handles default methods with same signature in multiple interfaces.

Class Friendly inherits two default greet() methods, one from each interface A and B. It must override greet() to resolve the conflict and can call both using InterfaceName.super.greet().

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