Strategy

1// Strategy interface
2interface PaymentStrategy {
3    void pay(double amount);
4    boolean validate();
5}
6
7// Concrete strategies
8class CreditCardPayment implements PaymentStrategy {
9    private String cardNumber;
10    private String cvv;
11    private String expiryDate;
12    
13    public CreditCardPayment(String cardNumber, String cvv, String expiryDate) {
14        this.cardNumber = cardNumber;
15        this.cvv = cvv;
16        this.expiryDate = expiryDate;
17    }
18    
19    @Override
20    public boolean validate() {
21        System.out.println("Validating credit card...");
22        return cardNumber != null && cardNumber.length() == 16;
23    }
24    
25    @Override
26    public void pay(double amount) {
27        System.out.println("Paid $" + amount + " using Credit Card");
28    }
29}
30
31class PayPalPayment implements PaymentStrategy {
32    private String email;
33    private String password;
34    
35    public PayPalPayment(String email, String password) {
36        this.email = email;
37        this.password = password;
38    }
39    
40    @Override
41    public boolean validate() {
42        System.out.println("Validating PayPal account...");
43        return email != null && email.contains("@");
44    }
45    
46    @Override
47    public void pay(double amount) {
48        System.out.println("Paid $" + amount + " using PayPal");
49    }
50}
51
52class CryptoPayment implements PaymentStrategy {
53    private String walletAddress;
54    
55    public CryptoPayment(String walletAddress) {
56        this.walletAddress = walletAddress;
57    }
58    
59    @Override
60    public boolean validate() {
61        System.out.println("Validating crypto wallet...");
62        return walletAddress != null && walletAddress.length() > 20;
63    }
64    
65    @Override
66    public void pay(double amount) {
67        System.out.println("Paid $" + amount + " using Cryptocurrency");
68    }
69}
70
71// Context
72class ShoppingCart {
73    private List<Double> items = new ArrayList<>();
74    private PaymentStrategy paymentStrategy;
75    
76    public void addItem(double price) {
77        items.add(price);
78    }
79    
80    public void setPaymentStrategy(PaymentStrategy strategy) {
81        this.paymentStrategy = strategy;
82    }
83    
84    public void checkout() {
85        double total = items.stream().mapToDouble(Double::doubleValue).sum();
86        
87        if (paymentStrategy.validate()) {
88            paymentStrategy.pay(total);
89        } else {
90            System.out.println("Payment validation failed!");
91        }
92    }
93}
94
95// Usage
96public class Main {
97    public static void main(String[] args) {
98        ShoppingCart cart = new ShoppingCart();
99        cart.addItem(100.00);
100        cart.addItem(50.00);
101        
102        // Pay with credit card
103        cart.setPaymentStrategy(
104            new CreditCardPayment("1234567890123456", "123", "12/25"));
105        cart.checkout();
106        
107        // Same cart, different payment
108        cart.setPaymentStrategy(
109            new PayPalPayment("user@email.com", "password"));
110        cart.checkout();
111    }
112}