Abstract classes in Java - Step-by-Step Execution

Concept Flow - Abstract classes

Define abstract class

↓

Declare abstract method(s)?

Yes→No method body

↓

Provide concrete method(s)

↓

Create subclass

↓

Implement abstract method(s)

↓

Instantiate subclass object

↓

Use methods (abstract implemented + concrete)

An abstract class defines methods without full code, subclasses must complete them before creating objects.

Execution Sample

Java

abstract class Animal {
    abstract void sound();
    void breathe() {
        System.out.println("Breathing...");
    }
}

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

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.sound();
        d.breathe();
    }
}

Defines an abstract Animal class with an abstract sound method and a concrete breathe method; Dog class implements sound; then Dog object calls both methods.

Execution Table

Step	Action	Evaluation	Result
1	Define abstract class Animal with abstract method sound()	Animal cannot be instantiated	Animal is a blueprint with abstract method sound()
2	Define concrete method breathe() in Animal	Method breathe() has implementation	breathe() prints 'Breathing...' when called
3	Create subclass Dog extends Animal	Dog inherits Animal's methods	Dog must implement abstract method sound()
4	Implement sound() in Dog	sound() prints 'Bark'	Dog now concrete, can be instantiated
5	Instantiate Dog d = new Dog()	Object d created	d is a Dog object
6	Call d.sound()	Calls Dog's sound()	Output: Bark
7	Call d.breathe()	Calls Animal's breathe()	Output: Breathing...
8	Try to instantiate Animal (not shown)	Compilation error	Cannot instantiate abstract class Animal

💡 Execution stops after Dog object calls methods; Animal cannot be instantiated directly.

Variable Tracker

Variable	Start	After Step 5	After Step 6	After Step 7	Final
d	null	Dog object created	Method sound() called	Method breathe() called	Dog object with methods used

Key Moments - 3 Insights

Why can't we create an object of the abstract class Animal?

Because Animal has an abstract method sound() without implementation, Java prevents creating objects from incomplete classes (see execution_table step 1 and 8).

How does Dog class use the breathe() method without defining it?

Dog inherits breathe() from Animal, which has a concrete implementation, so Dog objects can use it directly (see execution_table step 7).

What happens if Dog does not implement the abstract method sound()?

Dog would also become abstract and cannot be instantiated, so Java requires Dog to implement sound() to create objects (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is printed when d.sound() is called at step 6?

A "Breathing..."

B "Bark"

C Nothing, it causes an error

D "sound() method called"

💡 Hint

Check execution_table row for step 6 where d.sound() is called and output is shown.

At which step does the Dog class become concrete and instantiable?

A Step 4

B Step 5

C Step 3

D Step 1

💡 Hint

Look at execution_table step 4 where Dog implements sound(), making it concrete.

If we try to create new Animal(), what happens according to the execution table?

A Runtime error occurs

B Object is created successfully

C Compilation error: Cannot instantiate abstract class

D Animal's sound() method is called

💡 Hint

See execution_table step 8 about instantiating Animal.

Concept Snapshot

abstract class ClassName {
  abstract void methodName(); // no body
  void concreteMethod() { ... } // optional
}

- Abstract classes cannot be instantiated.
- Subclasses must implement all abstract methods.
- Can have concrete methods usable by subclasses.
- Used to define a common template with some methods left to subclasses.

Full Transcript

An abstract class in Java is a class that cannot be instantiated directly because it may contain abstract methods without implementation. It acts as a blueprint for other classes. Abstract methods are declared without a body and must be implemented by subclasses. Concrete methods in the abstract class can be used by subclasses as is. In the example, Animal is abstract with an abstract method sound() and a concrete method breathe(). Dog extends Animal and implements sound(), making Dog concrete and instantiable. The Dog object can call both sound() and breathe(). Trying to instantiate Animal directly causes a compilation error. This structure helps organize code by defining common behavior and forcing subclasses to provide specific implementations.

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