### new exercise: refactor to introduce strategy pattern

parent c82be143
 ... ... @@ -18,12 +18,12 @@ \usepackage[french,boxed,lined]{algorithm2e} \usepackage{amsmath} \usepackage{siunitx} \xsimsetup{solution/print=false} \xsimsetup{solution/print=true} \usepackage{booktabs} \usepackage{paralist} \lstset{basicstyle=\footnotesize, \lstset{basicstyle=\tiny, keywordstyle=\bfseries, stringstyle=\ttfamily, showstringspaces=\false, ... ... @@ -65,7 +65,8 @@ \newcommand{\code}{\lstinline{#1}} %{\newcommand{\code}{\lstinline{#1}} \newcommand{\code}{{\texttt #1}} \graphicspath{{./img/}} \chapterstyle{ell} ... ... @@ -489,7 +490,7 @@ Follow the instructions below to simplify the method \code{add()}~\cite{Kerievsk \end{exercise} \begin{exercise} Apply the Extract Constant'' operation to replace the magic number 10 '' and introduce an Explaining Variable'' called \code{GROWTH_INCREMENT}. Apply the Extract Constant'' operation to replace the magic number 10 '' and introduce an Explaining Variable'' called \code{GROWTH\_INCREMENT}. \end{exercise} \begin{exercise} ... ... @@ -509,18 +510,33 @@ Follow the instructions below to simplify the method \code{add()}~\cite{Kerievsk \subsection{Replace Conditional Logic with Strategy} \url{http://www.informit.com/articles/article.aspx?p=1398607&seqNum=2} The Strategy pattern defines a family of algorithms, encapsulates each algorithm, and make them interchangeable, letting the algorithm vary independently from clients that use it. We can apply this pattern to classes where several methods have similar structure: a sequence of similar conditions. For instance, let us consider the \code{Loan} class\footnote{\url{http://www.informit.com/articles/article.aspx?p=1398607&seqNum=2}}, from Joshua Kerievsky's book~\cite{Kerievsky:2004}, presented in Listing~\ref{lst:loan}. \lstset{caption=The Loan Class,label=lst:loan,float=htbp} \lstinputlisting{./src/main/java/fr/unantes/refactorings/Loan.java} This class deals with calculating capital for three different kinds of bank loans: \begin{description} \item [Term loan]: a loan from a bank for a specific amount that has a specified repayment schedule and a fixed or floating interest rate. \item [Revolver]: a credit that is automatically renewed as debts are paid off; and \item [Advised line]: a credit that a financial institution approves and maintains for a customer. \end{description} Much of the logic of methods \code{capital()} and \code{duration()} deals with figuring out whether the loan is a term loan, a revolver, or an advised line. For example, a null expiry date and a non-null maturity date indicate a term loan. A null maturity and a non-null expiry date indicate a revolver loan. In this exercise, we will use the Strategy pattern to simplify the calculation of the loan's capital. \begin{exercise} First, create a class called \code{CapitalStrategy} and add a method called \code{capital()}, with an empty body. \end{exercise} \begin{solution} \begin{lstlisting}[caption=cap,label=lst:lab,float=htbp] \begin{lstlisting}[caption=cap,label=lst:lab] public class CapitalStrategy { public double capital(Loan loan) { return 0.0; ... ... @@ -534,14 +550,19 @@ Now, apply the Move Method'' refactoring operation to move the \code{capital() \begin{enumerate} \item Changing the visibility of some private methods: \code{getUnusedPercentage()}, \code{outstandingRiskAmount()}, \code{riskFactor()}, \code{unusedRiskAmount()}, and \code{unusedRiskFactor()}. \item Encapsulating fields \code{commitment}, \code{expiry}, \code{maturity}, and \code{payments}. \item Encapsulating fields \code{commitment}, \code{expiry}, \code{maturity}, \code{riskRating}, \code{today}, \code{start}, and \code{payments}. \item Creating a simple version of method \code{capital()} on class \code{Loan}, which delegates to an instance of \code{CapitalStrategy}. \item Moving the method and replacing all references to \code{this} by \code{loan}. \end{enumerate} \end{exercise} \begin{solution} \begin{lstlisting}[caption=cap,label=lst:lab,float=htbp] \begin{itemize} \item Maybe we should remember that "protected" methods are accessible from classes belonging to the same package. \end{itemize} \begin{lstlisting}[caption=cap,label=lst:label] public class LoanRefactored { // (...) ... ... @@ -595,10 +616,76 @@ public class CapitalStrategy { \end{lstlisting} \end{solution} \begin{exercise} Apply the Move Method'' refactoring operation again to move the \code{duration()} method to class \code{CapitalStrategy}. \end{exercise} \begin{solution} \begin{lstlisting}[caption={duration() method},label=lst:lab] public double duration(LoanRefactored loan) { if (loan.getExpiry() == null && loan.getMaturity() != null) return this.weightedAverageDuration(loan); else if (loan.getExpiry() != null && loan.getMaturity() == null) return loan.strategy.yearsTo(loan.getExpiry(), loan); return 0.0; } \end{lstlisting} \end{solution} \begin{exercise} Since some methods, such as \code{duration()}, \code{weightedAverageDuration()}, \code{yearsTo()}, \code{riskFactor()} and \code{unusedRiskFactor()} are only used by method \code{capital()}, we can move them to class \code{CapitalStrategy} as well. The two constants, \code{MILLIS_PER_DAY} and \code{DAYS_PER_YEAR} can also be moved to class \code{CapitalStrategy}. The two constants, \code{MILLIS\_PER\_DAY} and \code{DAYS\_PER\_YEAR} can also be moved to class \code{CapitalStrategy}. \end{exercise} \begin{solution} \begin{itemize} \item After these refactorings, the code should be: \end{itemize} \lstset{caption={The LoanRefactored Class},label=lst:loan:refactored,float=htbp} \lstinputlisting{./src/main/java/fr/unantes/refactorings/LoanRefactored.java} \lstset{caption=The CapitalStrategy Class,label=lst:capital:strategy,float=htbp} \lstinputlisting{./src/main/java/fr/unantes/refactorings/CapitalStrategy.java} \end{solution} \begin{exercise} Finally, apply the Replace Conditional with Polymorphism '' refactoring operation on method \code{capital()} method. First, create a subclass named \code{CapitalStrategyTermLoan} for the capital calculation for a term loan. Then, move and adapt methods \code{capital()}, \code{duration}, and \code{weightedAverageDuration} to the subclass. \end{exercise} \begin{solution} \begin{lstlisting}[caption=cap,label=lst:lab] public class CapitalStrategyTermLoan extends CapitalStrategy { public double capital(Loan loan) { return loan.getCommitment() * duration(loan) * riskFactorFor(loan); } public double duration(Loan loan) { return weightedAverageDuration(loan); } private double weightedAverageDuration(Loan loan) { double duration = 0.0; double weightedAverage = 0.0; double sumOfPayments = 0.0; Iterator loanPayments = loan.getPayments().iterator(); while (loanPayments.hasNext()) { Payment payment = (Payment)loanPayments.next(); sumOfPayments += payment.amount(); weightedAverage += yearsTo(payment.date(), loan) * payment.amount(); } if (loan.getCommitment() != 0.0) duration = weightedAverage / sumOfPayments; return duration; } \end{lstlisting} In the end, class \code{CapitalStrategy} is abstract: \lstset{caption={The CapitalStrategy Class - Final version},label=lst:capital:strategy,float=htbp} \lstinputlisting{./src/main/java/fr/unantes/refactorings/CapitalStrategyRefactored.java} \end{solution} \bibliographystyle{abbrv} \bibliography{references} ... ...
 ... ... @@ -7,7 +7,7 @@ public class ArrayList { public void add(Object child) { if (!readOnly) { int newSize = elements.length + 1; int newSize = size + 1; if (newSize > elements.length) { Object[] newElements = new Object[elements.length + 10]; for (int i = 0; i < size; i++) { ... ...
 package fr.unantes.refactorings; import java.util.Date; import java.util.Iterator; public class CapitalStrategy { private static final int DAYS_PER_YEAR = 365; private static final int MILLIS_PER_DAY = 86400000; public CapitalStrategy() { } public double capital(LoanRefactored loan) { if (loan.getExpiry() == null && loan.getMaturity() != null) return loan.getCommitment() * loan.duration() * loan.riskFactor(); return loan.getCommitment() * this.duration(loan) * this.riskFactor(loan); if (loan.getExpiry() != null && loan.getMaturity() == null) { if (loan.getUnusedPercentage() != 1.0) return loan.getCommitment() *loan.getUnusedPercentage() * loan.duration() * loan.riskFactor(); return loan.getCommitment() * loan.getUnusedPercentage() * this.duration(loan) * this.riskFactor(loan); else return (loan.outstandingRiskAmount() * loan.duration() * loan.riskFactor()) + (loan.unusedRiskAmount() * loan.duration() * loan.unusedRiskFactor()); return (loan.outstandingRiskAmount() * this.duration(loan) * this.riskFactor(loan)) + (loan.unusedRiskAmount() * this.duration(loan) * this.unusedRiskFactor(loan)); } return 0.0; } public double duration(LoanRefactored loan) { if (loan.getExpiry() == null && loan.getMaturity() != null) return this.weightedAverageDuration(loan); else if (loan.getExpiry() != null && loan.getMaturity() == null) return loan.strategy.yearsTo(loan.getExpiry(), loan); return 0.0; } protected double riskFactor(LoanRefactored loan) { return RiskFactor.getFactors().forRating(loan.getRiskRating()); } protected double unusedRiskFactor(LoanRefactored loan) { return UnusedRiskFactors.getFactors().forRating(loan.getRiskRating()); } protected double weightedAverageDuration(LoanRefactored loan) { double duration = 0.0; double weightedAverage = 0.0; double sumOfPayments = 0.0; Iterator loanPayments = loan.getPayments().iterator(); while (loanPayments.hasNext()) { Payment payment = (Payment) loanPayments.next(); sumOfPayments += payment.amount(); weightedAverage += loan.strategy.yearsTo(payment.date(), loan) * payment.amount(); } if (loan.getCommitment() != 0.0) duration = weightedAverage / sumOfPayments; return duration; } protected double yearsTo(Date endDate, LoanRefactored loanRefactored) { Date beginDate = (loanRefactored.getToday() == null ? loanRefactored.getStart() : loanRefactored.getToday()); return ((endDate.getTime() - beginDate.getTime()) / MILLIS_PER_DAY) / DAYS_PER_YEAR; } }
 package fr.unantes.refactorings; import java.util.Date; public abstract class CapitalStrategyRefactored { private static final int MILLIS_PER_DAY = 86400000; private static final int DAYS_PER_YEAR = 365; public abstract double capital(LoanRefactored loan); protected double riskFactorFor(LoanRefactored loan) { return RiskFactor.getFactors().forRating(loan.getRiskRating()); } public double duration(LoanRefactored loan) { return yearsTo(loan.getExpiry(), loan); } protected double yearsTo(Date endDate, LoanRefactored loan) { Date beginDate = (loan.getToday() == null ? loan.getStart() : loan.getToday()); return ((endDate.getTime() - beginDate.getTime()) / MILLIS_PER_DAY) / DAYS_PER_YEAR; } }
 ... ... @@ -26,18 +26,26 @@ public class Loan { private int riskRating; public double capital() { if (expiry == null && maturity != null) if (expiry == null && maturity != null) // Term Loan return commitment * duration() * riskFactor(); if (expiry != null && maturity == null) { if (getUnusedPercentage() != 1.0) if (getUnusedPercentage() != 1.0) // Revolver return commitment * getUnusedPercentage() * duration() * riskFactor(); else else // Advised Line return (outstandingRiskAmount() * duration() * riskFactor()) + (unusedRiskAmount() * duration() * unusedRiskFactor()); } return 0.0; } public double duration() { if (expiry == null && maturity != null) return weightedAverageDuration(); else if (expiry != null && maturity == null) return yearsTo(expiry); return 0.0; } private double outstandingRiskAmount() { return outstanding; } ... ... @@ -46,13 +54,7 @@ public class Loan { return (commitment - outstanding); } public double duration() { if (expiry == null && maturity != null) return weightedAverageDuration(); else if (expiry != null && maturity == null) return yearsTo(expiry); return 0.0; } private double weightedAverageDuration() { double duration = 0.0; ... ...
 package fr.unantes.refactorings; import java.util.Date; import java.util.Iterator; import java.util.List; public class LoanRefactored { ... ... @@ -9,8 +8,6 @@ public class LoanRefactored { private static String TERM_LOAN = "TL"; private static String REVOLVER = "RC"; private static String RCTL = "RCTL"; private static final int MILLIS_PER_DAY = 86400000; private static final int DAYS_PER_YEAR = 365; private String type; ... ... @@ -24,9 +21,14 @@ public class LoanRefactored { private Date today; private Date start; private int riskRating; final CapitalStrategy strategy; public LoanRefactored() { strategy = new CapitalStrategy(); } public double capital() { return new CapitalStrategy().capital(this); return strategy.capital(this); } protected double outstandingRiskAmount() { ... ... @@ -38,39 +40,7 @@ public class LoanRefactored { } public double duration() { if (getExpiry() == null && getMaturity() != null) return weightedAverageDuration(); else if (getExpiry() != null && getMaturity() == null) return yearsTo(getExpiry()); return 0.0; } private double weightedAverageDuration() { double duration = 0.0; double weightedAverage = 0.0; double sumOfPayments = 0.0; Iterator loanPayments = getPayments().iterator(); while (loanPayments.hasNext()) { Payment payment = (Payment) loanPayments.next(); sumOfPayments += payment.amount(); weightedAverage += yearsTo(payment.date()) * payment.amount(); } if (getCommitment() != 0.0) duration = weightedAverage / sumOfPayments; return duration; } private double yearsTo(Date endDate) { Date beginDate = (today == null ? start : today); return ((endDate.getTime() - beginDate.getTime()) / MILLIS_PER_DAY) / DAYS_PER_YEAR; } protected double riskFactor() { return RiskFactor.getFactors().forRating(riskRating); } protected double unusedRiskFactor() { return UnusedRiskFactors.getFactors().forRating(riskRating); return strategy.duration(this); } protected double getUnusedPercentage() { ... ... @@ -92,4 +62,16 @@ public class LoanRefactored { protected List getPayments() { return payments; } public int getRiskRating() { return riskRating; } protected Date getToday() { return today; } protected Date getStart() { return start; } }
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