Partial abstraction 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 {
    // class body
}

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

Aprivate

Bfinal

Cstatic

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' instead of 'abstract' will prevent the class from being subclassed.

Using 'static' or 'private' keywords here is incorrect for class declaration.

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'

Aabstract

Bstatic

Cfinal

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' or 'static' keywords with abstract methods is not allowed.

Forgetting the semicolon at the end of the method declaration.

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

3fill in blank

hard

Fix the error in the subclass Car that extends Vehicle by correctly overriding the abstract method startEngine.

Java

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

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

AstartEngine()

BstartEngine(void)

CstartEngine

DstartEngine[]

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses after the method name causes a syntax error.

Using incorrect parameter syntax like 'void' inside parentheses.

The method name must include parentheses () to correctly override the abstract method.

4fill in blank

hard

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

Java

public [1] class Shape {
    public [2] void draw();
}

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

Aabstract

Bfinal

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' or 'static' keywords instead of 'abstract' for class or method.

Providing a method body for an abstract method.

The class must be declared abstract and the method abstract without a body.

5fill in blank

hard

Fill all three blanks to create a subclass Circle that extends Shape and implements the draw method.

Java

public class Circle extends [1] {
    @Override
    public void [2] {
        System.out.println([3]);
    }
}

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

AShape

Bdraw()

C"Drawing a circle"

DShape()

Attempts:

3 left

💡 Hint

Common Mistakes

Using constructor syntax 'Shape()' instead of class name in extends.

Omitting parentheses in method override.

Forgetting to put the message inside quotes.

The subclass extends the abstract class Shape, overrides the method draw(), and prints a message.

Practice

1.

What does partial abstraction mean in Java?

easy

A. An abstract class can have both abstract and concrete methods.

B. An abstract class can only have abstract methods.

C. An abstract class can be instantiated directly.

D. All methods in an abstract class must be static.

Solution

1. Step 1: Understand abstract class capabilities

   Partial abstraction means an abstract class can have some methods with implementation (concrete) and some without (abstract).

2. Step 2: Compare options with definition

   Only An abstract class can have both abstract and concrete methods. correctly states this; others are incorrect or false statements.

3. Final Answer:

   An abstract class can have both abstract and concrete methods. -> Option A

4. Quick Check:

   Partial abstraction = abstract + concrete methods [OK]

Hint: Abstract class can mix method types, not only abstract [OK]

Common Mistakes:

• Thinking abstract classes must have only abstract methods
• Believing abstract classes can be instantiated
• Confusing static methods with abstract methods

2.

Which of the following is the correct way to declare an abstract method inside an abstract class?

public abstract class Shape {
    ?
}

easy

A. public void draw();

B. void draw() {}

C. abstract void draw();

D. public abstract void draw();

Solution

1. Step 1: Recall abstract method syntax

   Abstract methods must be declared with the 'abstract' keyword and no body, and usually have a visibility modifier.

2. Step 2: Check each option

   public abstract void draw(); correctly declares 'public abstract void draw();'. public void draw(); misses 'abstract', C misses visibility, D has a method body which is invalid for abstract methods.

3. Final Answer:

   public abstract void draw(); -> Option D

4. Quick Check:

   Abstract method = 'public abstract' + no body [OK]

Hint: Abstract methods have no body and use 'abstract' keyword [OK]

Common Mistakes:

• Omitting the 'abstract' keyword
• Providing a method body for abstract methods
• Missing visibility modifier

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. Compilation error

B. Sleeping\nBark

C. Bark\nSleeping

D. Runtime error

Solution

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

   Variable 'a' is of type Animal but refers to Dog instance. Calling 'sound()' invokes Dog's override, printing 'Bark'. Calling 'sleep()' uses Animal's concrete method, printing 'Sleeping'.

2. Step 2: Check output order

   First 'a.sound()' prints 'Bark', then 'a.sleep()' prints 'Sleeping'.

3. Final Answer:

   Bark Sleeping -> Option C

4. Quick Check:

   Abstract ref calls subclass method then superclass concrete method [OK]

Hint: Abstract class ref calls subclass override methods [OK]

Common Mistakes:

• Expecting abstract class to instantiate directly
• Confusing method call order
• Thinking abstract methods have implementation

4.

Identify the error in the following code snippet:

abstract class Vehicle {
    abstract void move();
}

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

medium

A. Car class must implement the move() method or be abstract.

B. Vehicle class cannot have abstract methods.

C. Car class cannot have methods other than move().

D. No error, code is correct.

Solution

1. Step 1: Check abstract method implementation

   Vehicle declares abstract method move(). Any concrete subclass must implement it or be declared abstract.

2. Step 2: Analyze Car class

   Car does not implement move() and is not abstract, so this causes a compilation error.

3. Final Answer:

   Car class must implement the move() method or be abstract. -> Option A

4. Quick Check:

   Concrete subclass must implement all abstract methods [OK]

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

Common Mistakes:

• Forgetting to implement abstract methods in subclass
• Thinking abstract class can't have abstract methods
• Believing subclass can skip abstract methods without being abstract

5.

Given the abstract class below, which subclass implementation correctly uses partial abstraction?

abstract class Appliance {
    abstract void turnOn();
    void plugIn() {
        System.out.println("Plugged in");
    }
}

Choose the correct subclass:

hard

A. class Fan extends Appliance {}

B. class Fan extends Appliance {
void turnOn() {
System.out.println("Fan is on");
}
}

C. class Fan extends Appliance {
void turnOn() {
System.out.println("Fan is on");
}
void plugIn() {
System.out.println("Fan plugged in");
}
}

D. class Fan extends Appliance {
void plugIn() {
System.out.println("Fan plugged in");
}
}

Solution

1. Step 1: Check abstract method implementation

   Subclass must implement abstract method turnOn() to be concrete.

2. Step 2: Analyze each subclass option

   class Fan extends Appliance { void turnOn() { System.out.println("Fan is on"); } } implements turnOn() only, using inherited plugIn() as is, which is valid partial abstraction. class Fan extends Appliance { void plugIn() { System.out.println("Fan plugged in"); } } misses turnOn(), causing error. class Fan extends Appliance { void turnOn() { System.out.println("Fan is on"); } void plugIn() { System.out.println("Fan plugged in"); } } overrides plugIn() unnecessarily but is valid. class Fan extends Appliance {} misses turnOn(), causing error.

3. Final Answer:

   class Fan extends Appliance { void turnOn() { System.out.println("Fan is on"); } } -> Option B

4. Quick Check:

   Implement abstract methods, inherit concrete ones [OK]

Hint: Implement abstract methods, inherit concrete ones unchanged [OK]

Common Mistakes:

• Not implementing abstract methods in subclass
• Overriding concrete methods unnecessarily
• Leaving subclass empty without abstract method implementation