Partial abstraction in Java - Mini Project: Build & Apply

Partial Abstraction in Java

📖 Scenario: You are creating a simple program to represent different types of vehicles. Some vehicles share common features, but each type has its own way of showing details.

🎯 Goal: Build a Java program using partial abstraction with an abstract class and concrete subclasses to show vehicle details.

📋 What You'll Learn

Create an abstract class called Vehicle with one abstract method showDetails() and one concrete method start().

Create two subclasses called Car and Bike that extend Vehicle.

Implement the showDetails() method in both subclasses with specific messages.

Create objects of Car and Bike and call their start() and showDetails() methods.

💡 Why This Matters

🌍 Real World

Partial abstraction is used in software to define common behavior for a group of related objects while allowing each object to have its own specific details.

💼 Career

Understanding partial abstraction helps in designing clean, reusable code in object-oriented programming, which is essential for many software development jobs.

1

Create the abstract class Vehicle

Create an abstract class called Vehicle with one abstract method showDetails() and one concrete method start() that prints "Vehicle is starting".

Java

abstract class Vehicle {
    // Your code here
}

Hint

Use the abstract keyword for the class and the method showDetails(). The method start() should print the message.

2

Create subclasses Car and Bike

Create two classes called Car and Bike that extend the abstract class Vehicle.

Java

abstract class Vehicle {
    abstract void showDetails();
    void start() {
        System.out.println("Vehicle is starting");
    }
}

// Your code here

Hint

Use extends Vehicle to inherit. Implement the showDetails() method in both classes with the exact print statements.

3

Create objects and call methods

In the main method, create one object of Car called myCar and one object of Bike called myBike. Call the start() and showDetails() methods on both objects.

Java

abstract class Vehicle {
    abstract void showDetails();
    void start() {
        System.out.println("Vehicle is starting");
    }
}

class Car extends Vehicle {
    void showDetails() {
        System.out.println("This is a Car");
    }
}

class Bike extends Vehicle {
    void showDetails() {
        System.out.println("This is a Bike");
    }
}

public class Main {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Create objects with new and call the methods exactly as shown.

4

Display the output

Run the program and print the output of the start() and showDetails() methods for both myCar and myBike.

Java

abstract class Vehicle {
    abstract void showDetails();
    void start() {
        System.out.println("Vehicle is starting");
    }
}

class Car extends Vehicle {
    void showDetails() {
        System.out.println("This is a Car");
    }
}

class Bike extends Vehicle {
    void showDetails() {
        System.out.println("This is a Bike");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();
        Bike myBike = new Bike();
        myCar.start();
        myCar.showDetails();
        myBike.start();
        myBike.showDetails();
    }
    // Output is shown when program runs
}

Hint

Check the console output matches the expected lines exactly.

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