Creation Methods and Factory Methods in Java

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Creation Methods and Factory Methods
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What does our industry call a method that creates objects Many programmers would answer "Factory Method," after the name given to a creational pattern in Design Patterns [DP] But are all methods that create objects true Factory Methods Given a broad definition of the term (ie, a method that simply creates objects), the answer would be an emphatic "yes!" But given the way the authors of the Factory Method pattern wrote it in 1995, it's clear that not every method that creates objects offers the kind of loose coupling provided by a genuine Factory Method (eg, see Introduce Polymorphic Creation with Factory Method, 88) To help us be clear when discussing designs or refactorings related to object creation, I'm using the term Creation Method to refer to a static or nonstatic method that creates instances of a class This means that every Factory Method is a Creation Method but not necessarily the reverse It also means that you can substitute the term Creation Method wherever Martin Fowler uses the term "factory method" in Refactoring [F] and wherever Joshua Bloch uses the term "static factory method" in Effective Java [Bloch]
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+ + Communicates what kinds of instances are available better than constructors Bypasses constructor limitations, such as the inability to have two constructors with the same number and type of arguments Makes it easier to find unused creation code Makes creation nonstandard: some classes are instantiated using new, while others use Creation Methods
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Mechanics
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Before beginning this refactoring, identify the catch-all constructor, a full-featured constructor to which other constructors delegate their work If you don't have a catch-all constructor, create one by applying Chain Constructors (340)
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1 Find a client that calls a class's constructor in order to create a kind of instance Apply Extract Method [F] on the constructor call to produce a public, static method This new method is a creation method Now apply Move Method [F] to move the creation method to the class containing the chosen constructor
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Compile and test 2 Find all callers of the chosen constructor that instantiate the same kind of instance as the creation method and update them to call the creation method
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Compile and test 3 If the chosen constructor is chained to another constructor, make the creation method call the chained constructor instead of the chosen constructor You can do this by inlining the constructor, a refactoring that resembles Inline Method [F]
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Compile and test 4 Repeat steps 1 3 for every constructor on the class that you'd like to turn into a Creation Method 5 If a constructor on the class has no callers outside the class, make it non-public
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Compile
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Example
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This example is inspired from the banking domain and a certain loan risk calculator I spent several years writing, extending, and maintaining The Loan class had numerous constructors, as shown in the following code public class Loan public Loan(double commitment, int riskRating, Date maturity) { this(commitment, 000, riskRating, maturity, null); } public Loan(double commitment, int riskRating, Date maturity, Date expiry) {
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this(commitment, 000, riskRating, maturity, expiry); } public Loan(double commitment, double outstanding, int customerRating, Date maturity, Date expiry) { this(null, commitment, outstanding, customerRating, maturity, expiry); } public Loan(CapitalStrategy capitalStrategy, double commitment, int riskRating, Date maturity, Date expiry) { this(capitalStrategy, commitment, 000, riskRating, maturity, expiry); } public Loan(CapitalStrategy capitalStrategy, double commitment, double outstanding, int riskRating, Date maturity, Date expiry) { thiscommitment = commitment; thisoutstanding = outstanding; thisriskRating = riskRating; thismaturity = maturity; thisexpiry = expiry; thiscapitalStrategy = capitalStrategy; if (capitalStrategy == null) { if (expiry == null) thiscapitalStrategy = new CapitalStrategyTermLoan(); else if (maturity == null) thiscapitalStrategy = new CapitalStrategyRevolver(); else thiscapitalStrategy = new CapitalStrategyRCTL(); } }
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Loan could be used to represent seven kinds of loans I will discuss only three of them here A term loan is a loan that must be fully paid by its maturity date A revolver, which is like a credit card, is a loan that signifies "revolving credit": you have a spending limit and an expiry date A revolving credit term loan (RCTL) is a revolver that transforms into a term loan when the revolver expires Given that the calculator supported seven kinds of loans, you might wonder why Loan wasn't an abstract superclass with a subclass for each kind of loan After all, that would have cut down on the number of constructors needed for Loan and its subclasses There were two reasons why this was not a good idea 1 What distinguishes the different kinds of loans is not so much their fields but how numbers, like capital, income, and duration, are calculated To support three different ways to calculate capital for a term loan, we wouldn't want to create three different subclasses of Loan It's easier to support one Loan class and have three differentStrategy classes for a term loan (see the example from Replace Conditional Logic with Strategy, 129) 2 The application that used Loan instances needed to transform loans from one kind of loan to another This transformation was easier to do when it involved changing a few fields on a single Loan instance, rather than completely changing one instance of a Loan subclass into another If you look at the Loan source code presented earlier, you'll see that it has five constructors, the last of which is its catch-all constructor (see Chain Constructors, 340) Without specialized knowledge, it is difficult to know which constructors create term loans, which ones create revolvers, and which ones create RCTLs I happen to know that an RCTL needs both an expiry date and a maturity date, so I know that to create an RCTL, I must call a constructor that lets me pass in both dates Did you know that Do you think the next programmer who reads this code will know it What else is embedded as implicit knowledge in the Loan constructors Plenty If you call the first constructor, which takes three parameters, you'll get back a term loan But if you want a revolver, you'll need to call one of the constructors that take two dates and then
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supply null for the maturity date I wonder if all users of this code will know this Or will they just have to learn by encountering some ugly defects Let's see what happens when I apply the Replace Constructors with Creation Methods refactoring
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1 My first step is to find a client that calls one ofLoan's constructors Here is one such caller that resides in a test case:
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public class CapitalCalculationTests public void testTermLoanNoPayments() { Loan termLoan = new Loan(commitment, riskRating, maturity); }
In this case, a call to the aboveLoan constructor produces a term loan I applyExtract Method [F] on that call to produce a public, static method called createTermLoan: