Day 16: Abstraction in Java

What is Abstraction?

Abstraction means:

Showing only the necessary data and hiding the unwanted or internal details.

In simple words, abstraction focuses on what an object does, not how it does it.

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Why Do We Need Abstraction?

• To reduce complexity
• To hide implementation details
• To improve security
• To make code easy to understand and maintain

Real-life example:
When we use an ATM, we only know actions like withdraw, check balance, etc.
We don’t know how the ATM internally processes the transaction — that part is hidden.

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How to Achieve Abstraction in Java?

Abstraction is achieved using the abstract keyword.

The abstract keyword can be used at:

• Class level
• Method level

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Important Rules of Abstraction

• An abstract class can have:

  • Abstract methods
  • Non-abstract (normal) methods

• We cannot create an object for an abstract class

• An abstract class can have a constructor

• Abstract methods do not have a body

• Child class must override all abstract methods

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Abstract Class Example (From Class)

Abstract Class: ATM

public abstract class ATM {

    public abstract void withdraw();

    public void welcomeMessage() {
        System.out.println("Welcome");
    }

    ATM() {
        System.out.println("Constructor");
    }

    public static void main(String[] args) {
        // ATM atm = new ATM();  // Not allowed
    }
}

Explanation:

• ATM is an abstract class
• withdraw() is an abstract method
• welcomeMessage() is a non-abstract method
• Constructor is allowed in abstract class
• Object creation of ATM is not allowed

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Child Class Implementing Abstract Method

Class: IciciBank

public class IciciBank extends ATM {

    public static void main(String[] args) {
        IciciBank icici = new IciciBank();
    }

    @Override
    public void withdraw() {
        System.out.println("withdraw");
    }
}

Explanation:

• IciciBank extends ATM
• It implements the abstract method withdraw()
• Without implementing withdraw(), this class would also become abstract
• Object creation is allowed for IciciBank

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Using the Implemented Methods

Class: User

public class User extends IciciBank {

    public static void main(String[] args) {

        User user = new User();
        user.welcomeMessage();
        user.withdraw();
    }
}

Explanation:

• User class indirectly inherits from ATM

• It can access:

  • welcomeMessage() (non-abstract method)
  • withdraw() (implemented method)

• Output is achieved using abstraction without knowing internal logic

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Flow of Execution

1. Object of User is created
2. Abstract class constructor executes first
3. welcomeMessage() method is called
4. withdraw() method implementation is executed

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Key Points to Remember

• Abstraction hides internal implementation
• Achieved using abstract keyword
• Abstract class can have constructor
• Object creation for abstract class is not allowed
• Child class must override abstract methods
• Abstraction focuses on what, not how

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Difference Between Abstract and Normal Class

Feature	Abstract Class	Normal Class
Object creation	❌ Not allowed	✔ Allowed
Abstract methods	✔ Allowed	❌ Not allowed
Constructor	✔ Allowed	✔ Allowed
Inheritance	✔ Required	Optional

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When to Use Abstraction?

Use abstraction when:

• You want to hide implementation details
• Multiple classes follow a common structure
• You want to enforce method implementation
• You want loose coupling in your application

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