What is Abstraction in Java?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Identify the essential behaviour

   Decide what an object must do for its callers, independent of how it will be built.

2. Step 2

   Declare an abstract type

   Create an interface or an abstract class exposing only those essential method signatures.

3. Step 3

   Leave implementation open

   Mark abstract methods without a body so concrete subclasses must supply the details.

4. Step 4

   Provide concrete implementations

   Write subclasses or implementing classes that fill in the hidden how for each method.

5. Step 5

   Program to the abstraction

   Have callers depend on the abstract type so implementations can change freely.

Code Example

abstract class Payment {
    // Essential behaviour exposed, implementation hidden
    abstract void pay(double amount);

    void receipt(double amount) {
        System.out.println("Paid: " + amount);
    }
}

class CardPayment extends Payment {
    @Override
    void pay(double amount) {
        System.out.println("Charging card: " + amount);
    }
}

public class Demo {
    public static void main(String[] args) {
        Payment p = new CardPayment(); // program to the abstraction
        p.pay(99.0);
        p.receipt(99.0);
    }
}

Follow-up Questions

• How is abstraction different from encapsulation in Java?
• When would you choose an abstract class over an interface for abstraction?
• Can an abstract class have a constructor?
• Why can't you instantiate an abstract class directly?
• How do interfaces support abstraction after Java 8 default methods?