Abstract methods in Java - Step-by-Step Execution

Concept Flow - Abstract methods

Define abstract class

↓

Declare abstract method (no body)

↓

Subclass extends abstract class

↓

Subclass must override abstract method

↓

Create subclass object

↓

Call overridden method

↓

Execute subclass method body

Abstract methods are declared without a body in an abstract class. Subclasses must provide the method's body.

Execution Sample

Java

abstract class Animal {
  abstract void sound();
}

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();
  }
}

This code defines an abstract method sound() in Animal, implements it in Dog, then calls it.

Execution Table

Step	Action	Evaluation	Result
1	Define abstract class Animal with abstract method sound()	No method body	Animal cannot be instantiated
2	Define subclass Dog extending Animal	Dog must override sound()	Dog provides sound() method with body
3	Create Dog object d	Dog is concrete, so instantiation allowed	Object d created
4	Call d.sound()	Calls Dog's overridden sound()	Prints 'Bark'
5	End of execution	No more statements	Program ends

💡 Program ends after calling overridden method in subclass

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
d (Dog object)	null	null	null	Dog instance created	Dog instance exists

Key Moments - 3 Insights

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

Because Animal has an abstract method without a body (see step 1 in execution_table), Java forbids creating its objects.

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

The code will not compile because Dog must provide a body for all abstract methods inherited (step 2).

When we call d.sound(), which method runs?

The overridden method in Dog runs, printing 'Bark' (step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the result of step 4?

A Compilation error

B Prints nothing

C Prints 'Bark'

D Runtime error

💡 Hint

Check step 4 in execution_table where d.sound() is called

At which step is the Dog object created?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at variable_tracker and execution_table step 3 for object creation

If we remove the method body from Dog's sound(), what happens?

A Compilation error because Dog must override abstract method

B Runtime error when calling sound()

C Program runs and prints 'Bark'

D Animal can be instantiated

💡 Hint

Refer to key_moments about overriding abstract methods

Concept Snapshot

abstract class ClassName {
  abstract returnType methodName(); // no body
}

Subclass must override abstract methods.
Cannot instantiate abstract class.
Calling overridden method runs subclass code.

Full Transcript

Abstract methods are declared in abstract classes without a body. You cannot create objects of abstract classes. Subclasses must provide the method body by overriding the abstract method. When you create an object of the subclass and call the method, the subclass's version runs. This ensures a common method signature with different implementations.