Today’s Agenda  More refactoring patterns Software Testing and Maintenance 1

Introduce Null Object (1) class Site … Customer getCustomer () { return _customer; } class Customer … public String getName () { … } public BillingPlan getPlan () { … } public PaymentHistory getHistory () { … } Customer customer = site.getCustomer (); BillingPlan plan; if (customer == null) plan = BillingPlan.basic (); else plan = customer.getPlan (); … String customerName; if (customer == null) customerName = “occupant”; else customerName = customer.getName (); … Software Testing and Maintenance 2

Introduce Null Object (2) class NullCustomer extends Customer { public boolean isNull () { return true; } public String getName () { return “occupant”; } public BillingPlan getPlan () { return BillingPlan.basic (); } class Customer … static Customer newNull () { return new NullCustomer (); } class Site … Customer getCustomer () { return (_customer == null) ? Customer.newNull () : _customer; } Software Testing and Maintenance 3

Introduce Null Object (3) Customer customer = site.getCustomer (); BillingPlan plan; plan = customer.getPlan (); … String customerName; customerName = customer.getName (); … Software Testing and Maintenance 4

Replace Data Value with Object (1) class Order public Order (String customer) { _customer = customer; } public String getCustomer () { return _customer; } public void setCustomer (String customer) { _customer = customer; } private String _customer; Software Testing and Maintenance 5 What if we need to add address or credit rating into a customer?

Replace Data Value with Object (2) class Customer { public Customer (String name) { _name = name; } public String getName () { return _name; } private final String _name; } class Order … public order (String customer) { _customer = new Customer (customer); } public String getCustomer () { return _customer.getName (); } private Customer _customer; public void setCustomer (String customer) { _customer = new Customer (customer); } Software Testing and Maintenance 6

Change Value to Reference class Order { public order (String customer) { _customer = Customer.create (customer); } class Customer { private Customer (String name) { _name = name; } static void loadCustomer () { new Customer (“Bilston Gasworks”).store(); new Customer (“Jeff Bridge”).store(); … } private void store () { _instances.put(this.getName(), this); } public static Customer create (String name) { return (Customer) _instances.get(name); } private static Dictionary _instances = new Hashtable (); } Software Testing and Maintenance 7

Replace Magic Number with Sym. Constant double potentialEnergy (double mass, double height) { return mass * 9.81 * height; } Software Testing and Maintenance 8 double potentialEnergy (double mass, double height) { return mass * GRAVITATIONAL_CONSTANT * height; } Static final double GRAVITATIONAL_CONSTANT = 9.81;

Separate Query from Modifier int getTotalOutstandingAndSetReadyForSummaries () { readyForSummaries = true; return outstanding; } Software Testing and Maintenance 9 int getTotalOutstanding () { return outstanding; } void setReadyForSummaries () { readyForSummaries = true; }

Parameterize Method class Employee { void tenPercentRaise () { salary *= 1.1; } void fivePercentRaise () { salary *= 1.05; } Software Testing and Maintenance 10 class Employee { void raise (double factor) { salary *= (1 + factor); }

Replace Parameter with Explicit Methods void setValue (String name, int value) { if (name.equals(“height”)) { _height = value; return; } if (name.equals(“width”)) { _width = value; return; } Software Testing and Maintenance 11 void setHeight (int arg) { _height = arg; } Void setWidth (int arg) { _width = arg; }

Replace Parameter with Method (1) public double getPrice () { int basePrice = _quantity * _itemPrice; int discountLevel; if (_quantity > 100) discountLevel = 2; else discountLevel = 1; double finalPrice = discountedPrice (basePrice, discountLevel); return finalPrice; } private double discountedPrice (int basePrice, int discountLevel) { if (discountLevel == 2) return basePrice * 0.1; else return basePrice * 0.05; } Software Testing and Maintenance 12

Replace Parameter with Method (2) public double getPrice () { if (getDiscountLevel() == 2) return getBasePrice() * 0.1; else return getBasePrice() * 0.05; } private double getBasePrice () { return _quantity * _itemPrice; } private int getDiscountLevel () { if(_quantity > 100) return 2; else return 1; } Software Testing and Maintenance 13

Replace Constructor with Factory Method (1) class Employee { private int _type; static final int ENGINEER = 0; static final int SALESMAN = 1; static final int MANAGER = 2; Employee (int type) { _type = type; } Software Testing and Maintenance 14 class Employee { static Employee create (int type) { switch (type) { case ENGINEER: return new Engineer (); case SALESMAN: return new Salesman (); case MANAGER: return new Manager (); default: // throw an exception }

Replace Constructor with Factory Method (2) Software Testing and Maintenance 15 static Employee create (String name) { try { return (Employee) Class.forName (name).newInstance(); } catch (Exception ex) { throw new IllegalArgumentException(“Unable to instantiate ” + name); } Employee.create(“Engineer”); Employee.create(“Manager”); …

Encapsulate Downcast Object lastReading () { return readings.lastElement (); } Reding lastReading = (Reading) site.lastReading (); Software Testing and Maintenance 16 Reading lastReading () { return (Reading) readings.lastElement (); } Reding lastReading = site.lastElement();

Replace Error Code with Exception (1) class Account … int withdraw (int amount) { if (amount > _balance) return -1; else { _balance -= amount; return 0; } private int _balance; Software Testing and Maintenance 17

Replace Error Code with Exception (2) class Account … void withdraw (int amount) { if (amount > _balance) throw new IllegalArgumentException (“Amount too large”); _balance -= amount; } private int _balance; Software Testing and Maintenance 18

Replace Error Code with Exception (3) class Account … void withdraw (int amount) throws BalanceException { if (amount > _balance) throw new BalanceException (); _balance -= amount; } private int _balance; Software Testing and Maintenance 19

Pull Up Constructor Body class Manager extends Employee … public Manager (String name, String id, int grade) { _name = name; _id = id; _grade = grade; } Software Testing and Maintenance 20 class Employee … public Employee (String name, string id) { _name = name; _id = id; } class Manager extends Employee … public Manager (String name, String id, int grade) { super (name, id); _grade = grade; }

Extract Subclass (1) class JobItem … public JobItem (int unitPrice, int quantity, boolean isLabor, Employee employee) { _unitPrice = unitPrice; _quantity = quantity; … } public int getTotalPrice () { return getUnitPrice () * quantity; } public int getUnitPrice () { return (_isLabor) ? _employee.getRate() : _unitPrice; } …. class Employee… public Employee (int rate) { _rate = rate; } public int getRate () { return _rate; } private _rate; Software Testing and Maintenance 21

Extract Subclass (2) class JobItem … public JobItem (int unitPrice, int quantity, boolean isLabor) { _unitPrice = unitPrice; _quantity = quantity; _isLabor = isLabor; } public int getUnitPrice () { return _unitPrice; } …. class LaborItem extends JobItem … public LaborItem (int quantity, Employee employee) { super (0, quantity, true); _employee = employee; } public int getUnitPrice () { return _employee.getRate (); } Software Testing and Maintenance 22

Extract Subclass (3)  Modify calls to the constructor of the job item where a labor item is constructed Software Testing and Maintenance 23 JobItem j = new JobItem (0, 5, true, kent); JobItem j = new LaborItem (5, kent);

Form Template Method (1) public String statement () { Enumeration rentals = _rentals.elements (); String result = “Rental Record for ” + getName () + “\n”; while (rentals.hasMoreElements ()) { Rental each = (Rental) rentals.nextElement (); // show figures for this rental result += “\t” + each.getMovie.getTitle() + “\t” + String.valueOf(each.getCharge()) + “\n”; } // add footer lines result += “Amount owed is“ + String.valueOf(getTotalCharge()) + “\n”; result += “You earned “ + String.valueOf(getTotalFrequentRenterPoints()) + “ frequent renter points”; return result; } Software Testing and Maintenance 24

Software Testing and Maintenance 25 Form Template Method (2) public String htmlStatement () { Enumeration rentals = _rentals.elements (); String result = “ Rental Record for ” + getName () + “ \n”; while (rentals.hasMoreElements ()) { Rental each = (Rental) rentals.nextElement (); // show figures for this rental result += each.getMovie.getTitle() + “: ” + String.valueOf(each.getCharge()) + “ \n”; } // add footer lines result += “ You owe “ + String.valueOf(getTotalCharge()) + “ \n”; result += “On this rental you earned “ + String.valueOf(getTotalFrequentRenterPoints()) + “ frequent renter points ”; return result; }

Form Template Method (3) Software Testing and Maintenance 26 CustomerStatement Text Statement HTML Statement

Software Testing and Maintenance 27 Form Template Method (4) class Statement … public String value (Customer customer) { Enumeration rentals = customer.getRentals (); String result = headerString(customer); while (rentals.hasMoreElements ()) { Rental each = (Rental) rentals.nextElement (); // show figures for this rental result += eachRentalString(each); } // add footer lines result += footerString(customer); return result; } abstract String headerString (Customer customer); abstract String eachRentalString (Rental rental); abstract String footerString (Customer customer);

Software Testing and Maintenance 28 Form Template Method (5) class TextStatement … String headerString (Customer customer) { return “Rental Record for “ + customer.getName () + “\n”; } String eachRentalString (Rental rental) { return “\t” + rental.getMovie().getTitle() + “\t” + String.valueOf(rental.getCharge() + “\n”; } String footerString (Customer customer) { return “Amount owed is “ + String.valueOf(customer.getTotalCharge()) + “\n” + “You earned “ + String.valueOf(customer.getTotalFrequentRenterPoints()) + “ frequent renter points”; } // similar for class HtmlStatement

Replace Inheritance with Delegation (1) class MyStack extends Vector { public void push (Object element) { insertElementAt (element, 0); } public Object pop () { Object result = firstElement (); removeElementAt (0); return result; } Software Testing and Maintenance 29

Replace Inheritance with Delegation (2) class MyStack { private Vector _vector = new Vector (); public void push (Object element) { _vector.insertElementAt (element, 0); } public Object pop () { Object result = _vector.firstElement (); _vector.removeElementAt (0); return result; } public int size () { return _vector.size (); } public boolean isEmpty () { return _vector.isEmpty (); } Software Testing and Maintenance 30