Partial abstraction in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is partial abstraction in Java?

Partial abstraction means a class has some methods with implementation and some without. It is done using <code>abstract</code> classes that can have both abstract and concrete methods.

beginner

How do you declare an abstract method in Java?

Use the abstract keyword before the method signature and do not provide a body. For example: abstract void display();

beginner

Can you create an object of an abstract class?

No, you cannot create an object of an abstract class directly. You must create a subclass that implements the abstract methods and then create an object of that subclass.

intermediate

Why use partial abstraction instead of full abstraction?

Partial abstraction lets you share common code in the abstract class while forcing subclasses to provide specific details. It helps avoid code duplication and provides a template for subclasses.

beginner

Example of a partial abstraction in Java?

An abstract class Animal with an abstract method sound() and a concrete method sleep(). Subclasses like Dog implement sound().

Which keyword is used to declare a class with partial abstraction in Java?

A abstract

B partial

C interface

D final

The abstract keyword declares a class that can have both abstract and concrete methods.

Can an abstract class have implemented methods?

A Yes, it can have both abstract and concrete methods

B No, all methods must be abstract

C Only static methods can be implemented

D Only private methods can be implemented

Abstract classes can have both abstract methods (without body) and concrete methods (with body).

What happens if a subclass does not implement all abstract methods?

A It will compile without errors

B It becomes a concrete class

C It must be declared abstract

D It automatically implements methods

If a subclass does not implement all abstract methods, it must also be declared abstract.

Which of these is NOT true about partial abstraction?

A It uses abstract classes

B It cannot be instantiated directly

C It allows some methods to have code

D It requires all methods to be abstract

Partial abstraction means some methods have code; not all methods are abstract.

Why might you choose partial abstraction over interfaces?

A Because interfaces cannot have methods

B To provide common method implementations

C To force all methods to be abstract

D To allow object creation of the abstract class

Partial abstraction allows sharing common code in abstract classes, unlike interfaces which traditionally only declare methods.

Explain partial abstraction in Java and how it differs from full abstraction.

Think about classes that have some methods with code and some without.

Describe a simple example of partial abstraction using an abstract class and a subclass.

Use an animal or vehicle example.

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