Java Abstraction: Abstract Classes, Abstract Methods and Contracts

Abstraction is the fundamental OOP principle that hides complex internal implementation details and exposes only the essential operational features to the outside world. In Java, abstraction is achieved via Abstract Classes (0% to 100% partial abstraction) and Interfaces (100% pure abstraction).

What is an Abstract Class?

An abstract class is a class declared with the abstract keyword. It represents a conceptual base entity that cannot be instantiated directly. An abstract class can contain:

• Abstract Methods: Methods with no body (no implementation) that must be implemented by concrete subclasses.
• Concrete Methods: Fully implemented methods that provide shared default logic for all subclasses.
• Instance & Static Variables: State fields that subclasses inherit.
• Constructors: Initializers called when subclasses execute super().

Practical Example: Payment Processing Architecture

// Abstract Superclass
abstract class PaymentGateway {
    protected double amount;

    // Constructor in abstract class
    public PaymentGateway(double amount) {
        this.amount = amount;
    }

    // Concrete Method: Shared auditing logic
    public void logTransaction(String status) {
        System.out.println("[AUDIT LOG] Transaction for $" + amount + " Status: " + status);
    }

    // Abstract Method: Contract that subclasses MUST implement
    public abstract boolean processPayment();
}

// Concrete Subclass 1: Credit Card
class CreditCardPayment extends PaymentGateway {
    private String cardNumber;

    public CreditCardPayment(double amount, String cardNum) {
        super(amount);
        this.cardNumber = cardNum;
    }

    @Override
    public boolean processPayment() {
        System.out.println("Contacting Visa/Mastercard network for Card: " + cardNumber);
        logTransaction("SUCCESS_CREDIT_CARD");
        return true;
    }
}

// Concrete Subclass 2: UPI
class UpiPayment extends PaymentGateway {
    private String upiId;

    public UpiPayment(double amount, String upiId) {
        super(amount);
        this.upiId = upiId;
    }

    @Override
    public boolean processPayment() {
        System.out.println("Sending payment request to UPI VPA: " + upiId);
        logTransaction("SUCCESS_UPI");
        return true;
    }
}