Abstract vs concrete classes in Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an abstract class named Vehicle.

Java

public [1] class Vehicle {
    public abstract void start();
}

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

Astatic

Bfinal

Cabstract

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' instead of 'abstract' to declare the class.

Forgetting to use any keyword and making it a concrete class.

The keyword abstract is used to declare an abstract class in Java.

2fill in blank

medium

Complete the code to make the Car class inherit from the abstract Vehicle class.

Java

public class Car [1] Vehicle {
    @Override
    public void start() {
        System.out.println("Car started");
    }
}

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

Aextends

Binherits

Cimplements

Duses

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' instead of 'extends' for class inheritance.

Using a non-existent keyword like 'inherits'.

In Java, a class inherits from another class using the extends keyword.

3fill in blank

hard

Fix the error in the code by completing the method declaration in the abstract class.

Java

public abstract class Animal {
    public [1] void makeSound();
}

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

Afinal

Babstract

Cstatic

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' or 'static' keywords with abstract methods.

Forgetting to mark the method as abstract.

Abstract methods in an abstract class must be declared with the abstract keyword and have no body.

4fill in blank

hard

Fill both blanks to create a concrete class Dog that extends Animal and implements the makeSound method.

Java

public class Dog [1] Animal {
    @Override
    public void makeSound() [2] {
        System.out.println("Bark");
    }
}

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

Aextends

B()

C{}

Dimplements

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' instead of 'extends' for class inheritance.

Using parentheses instead of curly braces for method body.

The class extends the abstract class, and the method body is enclosed in {}.

5fill in blank

hard

Fill all three blanks to create a concrete class Cat that extends Animal, implements makeSound, and adds a new method purr.

Java

public class Cat [1] Animal {
    @Override
    public void makeSound() [2] {
        System.out.println("Meow");
    }
    public void purr() [3] {
        System.out.println("Purr");
    }
}

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

Aextends

B()

C{}

Dimplements

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' instead of 'extends' for class inheritance.

Using parentheses instead of curly braces for method bodies.

The class extends Animal, and both methods have bodies enclosed in {}.

Practice

1. Which statement best describes an abstract class in Java?

easy

A. It is the same as an interface and cannot have any methods with code.

B. It must have all methods fully implemented and can be instantiated.

C. It can have methods without implementation and cannot be instantiated directly.

D. It is a class that can only contain static methods.

Solution

1. Step 1: Understand abstract class definition

   An abstract class can have methods without implementation (abstract methods) and cannot create objects directly.

2. Step 2: Compare with other options

   Concrete classes have full method implementations and can be instantiated. Interfaces differ from abstract classes. Static-only classes are unrelated.

3. Final Answer:

   It can have methods without implementation and cannot be instantiated directly. -> Option C

4. Quick Check:

   Abstract class = no direct objects [OK]

Hint: Abstract classes can't create objects directly [OK]

Common Mistakes:

• Thinking abstract classes can be instantiated
• Confusing abstract classes with interfaces
• Assuming all methods must be implemented

2. Which of the following is the correct way to declare an abstract class in Java?

easy

A. abstract class Vehicle {}

B. class abstract Vehicle {}

C. Vehicle abstract class {}

D. class Vehicle abstract {}

Solution

1. Step 1: Recall Java syntax for abstract classes

   The keyword 'abstract' comes before 'class' followed by the class name.

2. Step 2: Check each option

   Only 'abstract class Vehicle {}' matches correct syntax. The other options have incorrect keyword order.

3. Final Answer:

   abstract class Vehicle {} -> Option A

4. Quick Check:

   abstract keyword before class name [OK]

Hint: Put 'abstract' before 'class' keyword [OK]

Common Mistakes:

• Placing 'abstract' after 'class'
• Mixing keyword order
• Omitting 'abstract' keyword

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

abstract class Animal {
    abstract void sound();
}

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

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

medium

A. Runtime error

B. Animal sound

C. Compilation error

D. Bark

Solution

1. Step 1: Understand class hierarchy and method overriding

   Animal is abstract with abstract method sound(). Dog extends Animal and implements sound() printing "Bark".

2. Step 2: Analyze main method execution

   Animal reference points to Dog object. Calling a.sound() runs Dog's sound(), printing "Bark".

3. Final Answer:

   Bark -> Option D

4. Quick Check:

   Abstract method overridden = Dog's output [OK]

Hint: Abstract method calls run subclass implementation [OK]

Common Mistakes:

• Expecting abstract class method to run
• Thinking abstract class can be instantiated
• Confusing compile and runtime errors

4. Identify the error in the following code snippet:

abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    // No draw() method implemented
}

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

medium

A. Circle must implement the abstract method draw() or be declared abstract.

B. Cannot create object of class Circle.

C. Abstract class Shape cannot have abstract methods.

D. No error, code runs fine.

Solution

1. Step 1: Check subclass implementation of abstract methods

   Circle extends Shape but does not implement abstract method draw().

2. Step 2: Understand Java rules for abstract methods

   A concrete class must implement all abstract methods or be declared abstract itself. Circle is concrete but missing draw().

3. Final Answer:

   Circle must implement the abstract method draw() or be declared abstract. -> Option A

4. Quick Check:

   Concrete subclass must implement all abstract methods [OK]

Hint: Concrete class must implement all abstract methods [OK]

Common Mistakes:

• Thinking abstract methods can be skipped
• Assuming abstract class can't have abstract methods
• Believing object creation is the error

5. You want to design a system where different types of employees calculate their salary differently. Which approach best uses abstract and concrete classes?

hard

A. Create only concrete classes for each employee type without any abstract class.

B. Create an abstract class Employee with an abstract method calculateSalary(), then create concrete subclasses like Manager and Developer implementing it.

C. Use an interface with no methods and concrete classes implementing it.

D. Create a concrete Employee class with a fixed calculateSalary() method used by all employees.

Solution

1. Step 1: Identify need for shared rules with different implementations

   Employee types share concept of salary calculation but differ in details.

2. Step 2: Use abstract class with abstract method

   Abstract class Employee defines calculateSalary() abstractly. Subclasses implement specific logic.

3. Step 3: Evaluate other options

   Create only concrete classes for each employee type without any abstract class. lacks shared abstraction. Use an interface with no methods and concrete classes implementing it. uses interface with no methods, so no contract. Create a concrete Employee class with a fixed calculateSalary() method used by all employees. fixes salary calculation, no variation.

4. Final Answer:

   Create an abstract class Employee with an abstract method calculateSalary(), then create concrete subclasses like Manager and Developer implementing it. -> Option B

5. Quick Check:

   Abstract class sets rules, subclasses do work [OK]

Hint: Abstract class for rules, concrete classes for details [OK]

Common Mistakes:

• Not using abstraction for shared behavior
• Using concrete class with fixed method only
• Interfaces without methods don't enforce contracts