Abstract classes in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an abstract class named Vehicle.

Java

public [1] class Vehicle {}

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

Afinal

Bprivate

Cstatic

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' instead of 'abstract' which prevents inheritance.

Using 'static' which is not valid for top-level classes.

Using 'private' which is an access modifier, not for abstract classes.

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

2fill in blank

medium

Complete the code to declare an abstract method named startEngine in the Vehicle class.

Java

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

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

Afinal

Babstract

Cstatic

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' which means the method cannot be overridden.

Using 'static' which is not allowed for abstract methods.

Using 'private' which prevents subclass access.

The abstract keyword is used to declare a method without a body inside an abstract class.

3fill in blank

hard

Fix the error in the code by completing the class declaration to make it abstract.

Java

public [1] {
    public abstract void startEngine();
}

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

Aabstract class Car

Bclass Car

Cfinal class Car

Dstatic class Car

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving out 'abstract' causes a compilation error.

Using 'final' prevents subclassing which is needed for abstract methods.

Using 'static' is invalid for top-level classes.

The class must be declared abstract if it contains abstract methods.

4fill in blank

hard

Fill both blanks to complete the subclass Car that extends the abstract class Vehicle and implements the startEngine method.

Java

public class Car [1] Vehicle {
    @Override
    public void startEngine() [2]
}

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

Aextends

Bimplements

C()

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' which is for interfaces, not classes.

Using parentheses instead of braces for method body.

The subclass uses extends to inherit from the abstract class. The method body is enclosed in {}.

5fill in blank

hard

Fill all three blanks to complete the abstract class Shape with an abstract method area, and a concrete method displayArea that prints the area.

Java

public abstract class Shape {
    public abstract double [1]();

    public void [2]() {
        System.out.println("Area: " + [3]());
    }
}

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

Aarea

BdisplayArea

DprintArea

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the method calls causing errors.

Not matching method names exactly.

The abstract method is named area. The concrete method is displayArea which calls area() to get the value.

Practice

1. Which statement about abstract classes in Java is true?

easy

A. All methods in an abstract class must be abstract.

B. Abstract classes cannot have any methods with code.

C. Abstract classes are the same as interfaces.

D. You cannot create an object directly from an abstract class.

Solution

1. Step 1: Understand abstract class instantiation

   Abstract classes cannot be instantiated directly, meaning you cannot create objects from them using new.

2. Step 2: Check method rules in abstract classes

   Abstract classes can have both abstract methods (without body) and regular methods (with code). So, not all methods must be abstract.

3. Final Answer:

   You cannot create an object directly from an abstract class. -> Option D

4. Quick Check:

   Abstract class instantiation = not allowed [OK]

Hint: Remember: abstract classes can't make objects directly [OK]

Common Mistakes:

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

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

easy

A. abstract class MyClass {}

B. class abstract MyClass {}

C. abstract MyClass class {}

D. class MyClass abstract {}

Solution

1. Step 1: Recall Java syntax for abstract classes

   The keyword abstract must come before the keyword class in the declaration.

2. Step 2: Check each option's order

   Only abstract class MyClass {} has the correct order: abstract class MyClass {}. Others have incorrect keyword order.

3. Final Answer:

   abstract class MyClass {} -> Option A

4. Quick Check:

   abstract class syntax = 'abstract class' [OK]

Hint: abstract keyword always 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 code?

abstract class Animal {
    abstract void sound();
    void sleep() {
        System.out.println("Sleeping");
    }
}

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();
        a.sleep();
    }
}

medium

A. Sleeping\nBark

B. Compilation error

C. Bark\nSleeping

D. Runtime error

Solution

1. Step 1: Understand method calls on abstract class reference

   The variable a is of type Animal but refers to a Dog object. Calling sound() calls Dog's implementation, printing "Bark".

2. Step 2: Call the concrete method from abstract class

   Calling sleep() uses the method defined in Animal, printing "Sleeping".

3. Final Answer:

   Bark Sleeping -> Option C

4. Quick Check:

   Dog sound then Animal sleep = Bark then Sleeping [OK]

Hint: Abstract ref calls subclass method, regular method runs as is [OK]

Common Mistakes:

• Expecting compilation error for abstract class reference
• Confusing method call order
• Thinking abstract class methods can't be called

4. Identify the error in the following code:

abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    void draw() {
        System.out.println("Drawing Circle");
    }
}

class Square extends Shape {
}

public class Test {
    public static void main(String[] args) {
        Shape s = new Square();
        s.draw();
    }
}

medium

A. Square class must implement the abstract method draw()

B. Cannot create object of abstract class Shape

C. Method draw() in Circle should be abstract

D. No error, code runs fine

Solution

1. Step 1: Check abstract method implementation in subclasses

   The abstract method draw() in Shape must be implemented by all non-abstract subclasses.

2. Step 2: Verify Square class implementation

   The Square class does not implement draw() and is not declared abstract, causing a compilation error.

3. Final Answer:

   Square class must implement the abstract method draw() -> Option A

4. Quick Check:

   All abstract methods must be implemented in concrete subclasses [OK]

Hint: All abstract methods must be implemented or class must be abstract [OK]

Common Mistakes:

• Forgetting to implement abstract methods
• Thinking abstract class objects can be created
• Marking implemented methods as abstract

5. You want to design a system where different types of vehicles share a common method startEngine() but each vehicle starts differently. Which approach using abstract classes is best?

hard

A. Make Vehicle a concrete class with startEngine() implemented, subclasses override it if needed.

B. Make an abstract class Vehicle with an abstract method startEngine(), then subclasses implement it.

C. Make Vehicle an interface with startEngine() method, implemented by subclasses.

D. Make Vehicle a final class with startEngine() method.

Solution

1. Step 1: Understand the need for shared method with different implementations

   Since startEngine() must be shared but implemented differently, an abstract method enforces subclasses to provide their own version.

2. Step 2: Choose abstract class with abstract method

   Declaring Vehicle as abstract with abstract startEngine() ensures all subclasses implement it, sharing the concept but customizing behavior.

3. Final Answer:

   Make an abstract class Vehicle with an abstract method startEngine(), then subclasses implement it. -> Option B

4. Quick Check:

   Abstract class with abstract method enforces implementation [OK]

Hint: Use abstract method to force subclass-specific behavior [OK]

Common Mistakes:

• Using concrete method without forcing override
• Confusing interfaces with abstract classes
• Making class final prevents subclassing