如何实现策略设计模式

Question

Can anyone tell how can we implement Strategy design pattern in the following code. I have gone through various links but not fully clear about implementing it. Question is: Calculate points and display the report in PDF or HTML using Strategy Design Pattern. Below is the code already available. Customer, Movie and Rental are classes already created:

1. Customer.java

    public class Customer { private String _name; private Vector _rentals = new Vector(); public Customer(String name) { _name = name; }; public void addRental(Rental arg) { _rentals.addElement(arg); } public String getName() { return _name; } public static void main(String[] args) { Customer c = new Customer("ABC"); Movie m = new Movie("Title", 1); Rental r = new Rental(m, 10); c.addRental(r); // Rental calculation String s = c.statement(); System.out.println("s: " + s); } public String statement() { double totalAmount = 0; int frequentRenterPoints = 0; Enumeration rentals = _rentals.elements(); String result = "Rental Record for " + getName() + "\n"; while (rentals.hasMoreElements()) { double thisAmount = 0; Rental each = (Rental) rentals.nextElement(); // determine amounts for each line switch (each.getMovie().getPriceCode()) { case Movie.REGULAR: thisAmount += 2; if (each.getDaysRented() > 2) thisAmount += (each.getDaysRented() - 2) * 1.5; break; case Movie.NEW_RELEASE: thisAmount += each.getDaysRented() * 3; break; case Movie.CHILDRENS: thisAmount += 1.5; if (each.getDaysRented() > 3) thisAmount += (each.getDaysRented() - 3) * 1.5; break; } // add frequent // renter points frequentRenterPoints++; // add bonus for a two day new release rental if ((each.getMovie().getPriceCode() == Movie.NEW_RELEASE) && each.getDaysRented() > 1) frequentRenterPoints++; // show figures for this rental result += "\t" + each.getMovie().getTitle() + "\t" + String.valueOf(thisAmount) + "\n"; totalAmount += thisAmount; } // add footer lines result += "Amount owed is " + String.valueOf(totalAmount) + "\n"; result += "You earned " + String.valueOf(frequentRenterPoints) + " frequent renter points"; return result; } }

2. Movie.java

    public class Movie { public static final int CHILDRENS = 2; public static final int REGULAR = 0; public static final int NEW_RELEASE = 1; private String _title; private int _priceCode; public Movie(String title, int priceCode) { _title = title; _priceCode = priceCode; } public int getPriceCode() { return _priceCode; } public void setPriceCode(int arg) { _priceCode = arg; } public String getTitle() { return _title; }; }

3. Rental.java

    public class Rental { private Movie _movie; private int _daysRented; public Rental(Movie movie, int daysRented) { _movie = movie; _daysRented = daysRented; } public int getDaysRented() { return _daysRented; } public Movie getMovie() { return _movie; } }

Answer 1

Your task is "display the report in PDF or HTML using Strategy Design Pattern", so strategy will be applied here. Before display Report you should have all computed data (keep the work as was done) and after just pass data and choose appropriate report type.

public interface IReport 
{
    public void printReport(String data);
}

public class ReportHTML implements IReport
{
    @Override
    public void printReport(String data) {
        System.out.println("Html Report with data="+data);  
    }
}

public class ReportPDF implements IReport{
    @Override
    public void printReport(String data) {
        System.out.println("PDF Report with data="+data);   
    }
}

public class Test {
    public static void main(String[] args)
    {
        //do whatever computation concerning business logic
        //after for print pass computed data and chose report type as needed
        IReport ir = new ReportHTML();
        ir.printReport("[Customers, Movie, Rental]");
        ir = new ReportPDF();
        ir.printReport("[Customers, Movie, Rental]");
    }
}

Output:

Html Report with data=[Customers, Movie, Rental]
PDF Report with data=[Customers, Movie, Rental]

Answer 2

HTML and PDF are display formats. A strategy is just a pluggable algorithm, usually defined by an interface. Since the task is to display the report and the pluggable part is how to display it (the algorithm), you need a display strategy. As for names, many people would call the interface ReportDisplayStrategy and that's fine, but often I think the design pattern name is just pollution, so I'd consider something more like ReportRenderer. If the output is always a file (there's a lot more you could do here), the interface might look like:

interface ReportRenderer
{ 
    File render(Report report)
}

The rest is matter of making your report object and writing the renderer implementations for PDF and HTML.