Partial abstraction in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of partial abstraction with abstract and concrete methods

What is the output of this Java code that uses partial abstraction with an abstract class and a concrete subclass?

Java

abstract class Vehicle {
    abstract void start();
    void stop() {
        System.out.println("Vehicle stopped");
    }
}

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

public class Main {
    public static void main(String[] args) {
        Vehicle v = new Car();
        v.start();
        v.stop();
    }
}

A Car started\nVehicle stopped

B Vehicle started\nVehicle stopped

C Car started

D Compilation error due to abstract method

💡 Hint

Remember that abstract classes can have both abstract and concrete methods, and the subclass must implement abstract methods.

The abstract class Vehicle has an abstract method start() and a concrete method stop(). The subclass Car implements start(). When we call start() on the Vehicle reference pointing to Car, Car's start() runs. The stop() method is inherited and runs as is.

🧠 Conceptual

intermediate

Understanding partial abstraction in Java

Which statement best describes partial abstraction in Java?

A A class that cannot have any methods.

B A class that has only concrete methods and can be instantiated.

C A class that has only abstract methods and can be instantiated.

D A class that has both abstract and concrete methods and cannot be instantiated.

💡 Hint

Think about what makes a class abstract and what partial abstraction means.

Partial abstraction means a class has some methods with implementation (concrete) and some without (abstract). Such a class is abstract and cannot be instantiated directly.

🔧 Debug

advanced

Identify the error in partial abstraction usage

What error will this code produce?

Java

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

class Dog extends Animal {
    // Missing implementation of sound()
}

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

A Compilation error: Dog must implement abstract method sound()

B Output:\nSleeping

C Runtime error: NullPointerException

D Compilation error: Cannot instantiate abstract class Animal

💡 Hint

Check if all abstract methods are implemented in the subclass.

The Dog class does not implement the abstract method sound(), so it must be declared abstract or implement the method. Since it is not abstract and does not implement sound(), compilation fails.

📝 Syntax

advanced

Correct syntax for partial abstraction

Which option shows the correct syntax for declaring a partially abstract class with one abstract and one concrete method?

A abstract class Shape { abstract void draw() {} void erase(); }

B class Shape { abstract void draw(); void erase() { System.out.println("Erased"); } }

C abstract class Shape { abstract void draw(); void erase() { System.out.println("Erased"); } }

D abstract class Shape { void draw(); abstract void erase() { System.out.println("Erased"); } }

💡 Hint

Remember abstract methods cannot have a body and abstract classes must be declared with the abstract keyword.

Option C correctly declares an abstract class with an abstract method draw() (no body) and a concrete method erase() with a body. Option C is invalid because abstract methods cannot be in non-abstract classes. Options C and D have syntax errors with abstract methods having bodies or concrete methods without bodies.

🚀 Application

expert

Number of methods in a partially abstract class hierarchy

Given this code, how many methods can be called on the object 'obj' of type Parent?

Java

abstract class Parent {
    abstract void methodA();
    void methodB() { System.out.println("B"); }
}

class Child extends Parent {
    void methodA() { System.out.println("A"); }
    void methodC() { System.out.println("C"); }
}

public class Test {
    public static void main(String[] args) {
        Parent obj = new Child();
        // How many methods can be called on obj?
    }
}

A Cannot call any methods on obj

B 2 methods: methodA() and methodB()

C 1 method: methodA() only

D 3 methods: methodA(), methodB(), and methodC()

💡 Hint

Remember the reference type determines accessible methods, not the object type.

The variable obj is of type Parent, so only methods declared in Parent are accessible: methodA() (abstract, implemented in Child) and methodB() (concrete). methodC() is declared only in Child and is not accessible via Parent reference.