core13AbstractionDemo

1package pkg1core;2 3/*4 * core13AbstractionDemo.java5 * --------------------6 * Abstract classes: partial implementation + enforced contract.7 *8 * EXPLANATION:9 *  - An abstract class cannot be instantiated; it may hold state and concrete10 *    methods plus abstract methods that subclasses MUST implement.11 *  - Use an abstract class (vs interface) when you need shared state/behavior.12 *  - Template Method pattern: a concrete method orchestrates abstract steps.13 */14public class core13AbstractionDemo {15 16    abstract static class Payment {17        protected final double amount;18        Payment(double amount) { this.amount = amount; }19 20        // Template method: fixed algorithm, variable steps21        final void process() {22            validate();23            System.out.println("Charging " + amount + " via " + method());24            authorize();25            System.out.println("Done.\n");26        }27        void validate() { if (amount <= 0) throw new IllegalArgumentException("amount<=0"); }28 29        // Steps subclasses must define30        abstract String method();31        abstract void authorize();32    }33 34    static class CardPayment extends Payment {35        CardPayment(double a){ super(a); }36        String method() { return "Credit Card"; }37        void authorize() { System.out.println("  contacting card network..."); }38    }39    static class UpiPayment extends Payment {40        UpiPayment(double a){ super(a); }41        String method() { return "UPI"; }42        void authorize() { System.out.println("  verifying VPA..."); }43    }44 45    public static void main(String[] args) {46        Payment[] payments = { new CardPayment(120.0), new UpiPayment(45.5) };47        for (Payment p : payments) p.process();48        // new Payment(10);  // ERROR: cannot instantiate abstract class49    }50}