Design Patterns in Java (Creational) - Strategy Pattern

Strategy Pattern

• Eliminate conditional statement 

• Behavior encapsulated in class

•  Difficult to add new strategy

• Client aware of strategy

•  Client choose strategy

     

Main

public static void main(String[] args) {

CreditCard amex = new CreditCard(new AmexStrategy());

amex.setNumber("379185883464283");

amex.setDate("04/2020");

amex.setCvv("123");

System.out.println("Amex valid? : " + amex.isValid());

}

Credit Card

public class CreditCard {

private String number;

private String date;

private String cvv;

private ValidationStrategy vs;

public CreditCard(ValidationStrategy vs){

this.vs = vs;

}

public boolean isValid(){

return vs.isValid(this);

}

// getters & setters }

ValidationStrategy

public abstract class ValidationStrategy {

public abstract boolean isValid(CreditCard c);

protected boolean passesLuhn(String ccNumber) {

boolean alternate = false;

int sum = 0;

for (int i = ccNumber.length() - 1; i >= 0; i--) {

int n = Integer.parseInt(ccNumber.substring(i, i + 1));

if (alternate) {

n *= 2;

if (n > 9) {

n = (n % 10) + 1;

}

}

sum += n;

alternate = !alternate;

}

return (sum % 10 == 0);

}

}

Specific Strategy

public class AmexStrategy extends ValidationStrategy {

@Override

public boolean isValid(CreditCard c) {

boolean isValid = true;

isValid = c.getNumber().startsWith("37") || c.getNumber().startsWith("34");

isValid =  passesLuhn(c.getNumber());

return isValid;

}

}