Abstract classes

public abstract class Payment {
    private final String orderId;
    protected final double amount;

    protected Payment(String orderId, double amount) {   // abstract classes can have constructors
        if (amount <= 0) throw new IllegalArgumentException("Amount must be positive");
        this.orderId = orderId;
        this.amount = amount;
    }

    public final String process() {            // a fixed sequence of steps (template method)
        validate();
        String ref = charge();                 // the step each subclass fills in
        return "Order " + orderId + " paid, ref " + ref;
    }

    protected void validate() { }             // optional hook with a default
    protected abstract String charge();       // no body: subclasses MUST implement it
}

public class UpiPayment extends Payment {
    private final String vpa;
    public UpiPayment(String orderId, double amount, String vpa) { super(orderId, amount); this.vpa = vpa; }
    @Override protected void validate() { if (!vpa.contains("@")) throw new IllegalArgumentException("Bad UPI ID"); }
    @Override protected String charge() { return "UPI-" + System.nanoTime(); }
}

// Payment p = new Payment("O1", 100);      // compile error: Payment is abstract
Payment p = new UpiPayment("O1", 499, "asha@okbank");
System.out.println(p.process());