Abstract classes in Java

Introduction

Abstract classes let you create a base blueprint for other classes. They help share common code but cannot be used to make objects directly.

When you want to define common behavior for related classes but don't want to create objects of the base class.

When some methods should be shared by all subclasses, but others must be defined differently by each subclass.

When you want to force subclasses to implement certain methods.

When you want to provide some default code and some methods that subclasses must complete.

When designing a system with a clear hierarchy and shared features.

Syntax

Java

abstract class Animal {
    // Abstract method (no body)
    abstract void makeSound();

    // Regular method
    void sleep() {
        System.out.println("Sleeping...");
    }
}

An abstract class is declared with the keyword abstract.

It can have both abstract methods (without body) and regular methods (with body).

Examples

Here, Vehicle is abstract and Car provides the method body.

Java

abstract class Vehicle {
    abstract void startEngine();
}

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

Abstract method area() must be implemented by Circle.

Java

abstract class Shape {
    abstract double area();
}

class Circle extends Shape {
    double radius;
    Circle(double radius) {
        this.radius = radius;
    }
    double area() {
        return Math.PI * radius * radius;
    }
}

Abstract class can have regular methods like plugIn() and abstract methods like turnOn().

Java

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

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

Sample Program

This program shows an abstract class Animal with an abstract method makeSound() and a regular method sleep(). The subclasses Dog and Cat implement makeSound(). We cannot create an Animal object directly.

Java

abstract class Animal {
    abstract void makeSound();

    void sleep() {
        System.out.println("Sleeping...");
    }
}

class Dog extends Animal {
    void makeSound() {
        System.out.println("Bark");
    }
}

class Cat extends Animal {
    void makeSound() {
        System.out.println("Meow");
    }
}

public class Main {
    public static void main(String[] args) {
        // Animal animal = new Animal(); // Error: Cannot instantiate abstract class

        Dog dog = new Dog();
        Cat cat = new Cat();

        System.out.println("Dog says:");
        dog.makeSound();
        dog.sleep();

        System.out.println("Cat says:");
        cat.makeSound();
        cat.sleep();
    }
}

Important Notes

Time complexity depends on the methods implemented in subclasses, not the abstract class itself.

Abstract classes use memory only when instantiated through subclasses.

Common mistake: Trying to create an object of an abstract class directly causes a compile error.

Use abstract classes when you want to share code and force subclasses to implement certain methods. Use interfaces if you only want to define method signatures without any code.

Summary

Abstract classes cannot be instantiated but can have both abstract and regular methods.

Subclasses must implement all abstract methods.

They help organize code by sharing common behavior and enforcing method implementation.

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