Working effectively with legacy code

Working Effectively with Legacy Code – Part I-ShriKantVashishthaAug 19, 2009

Four Reasons to Change SoftwareAdding a featureFixing a bugImproving the designOptimizing resource usage

A Legacy Code ScenarioWhat changes do we have to make?How will we know that we have done them correctlyHow will we know that we haven’t broken anythingAvoiding change is a bad thing, but what is our alternative?Try harder. Hire more people

Way of doing changes - Edit and Pray

Way of doing changes - Cover and Modify

Why Unit Tests Are Important?Error localizationExecution timeCoverage

Legacy Code DilemmaWhen we change code, we should have tests in place. To put tests in place, we often have to change code.Use Primitivize Parameter and Extract Interface to remove these issues

Fake objects Support Real Testspublic class FakeDisplay implements Display{ private String lastLine = ""; public void showLine(String line) {lastLine = line; } public String getLastLine() { return lastLine; }}import junit.framework.*;public class SaleTest extends TestCase{ public void testDisplayAnItem() {FakeDisplay display = new FakeDisplay(); Sale sale = new Sale(display);sale.scan("1");assertEquals("Milk $3.99", display.getLastLine()); }}

SeamsboolCAsyncSslRec::Init(){ … if (!m_bFailureSent) {m_bFailureSent=TRUE;PostReceiveError(SOCKETCALLBACK, SSL_FAILURE); }CreateLibrary(m_hSslDll1,"syncesel1.dll");CreateLibrary(m_hSslDll2,"syncesel2.dll"); …. return true;}

Object Seamclass CAsyncSslRec{ ... virtual void PostReceiveError(UINT type, UINT errorcode); ...};class TestingAsyncSslRec : public CAsyncSslRec{ virtual void PostReceiveError(UINT type, UINT errorcode) { }};

Seam TypesPreprocessing SeamsLink Seams : static linking in C/C++/Flex and dynamic linking in JavaObject Seams

I Don't Have Much Time and I Have to Change ItSprout MethodSprout ClassWrap MethodWrap Class

Sprout Methodpublic class TransactionGate{ public void postEntries(List entries) { for (Iterator it = entries.iterator(); it.hasNext(); ) { Entry entry = (Entry)it.next();entry.postDate(); }transactionBundle.getListManager().add(entries); } ...}

Sprout Methodpublic class TransactionGate{ public void postEntries(List entries) { List entriesToAdd = new LinkedList(); for (Iterator it = entries.iterator(); it.hasNext(); ) { Entry entry = (Entry)it.next(); if (!transactionBundle.getListManager().hasEntry(entry) {entry.postDate();entriesToAdd.add(entry); } }transactionBundle.getListManager().add(entriesToAdd); } ...}

Sprout Methodpublic class TransactionGate{ ... List uniqueEntries(List entries) { List result = new ArrayList(); for (Iterator it = entries.iterator(); it.hasNext(); ) { Entry entry = (Entry)it.next(); if (!transactionBundle.getListManager().hasEntry(entry) {result.add(entry); } } return result; } ...}public class TransactionGate{ ... public void postEntries(List entries) { List entriesToAdd = uniqueEntries(entries); for (Iterator it = entriesToAdd.iterator(); it.hasNext(); ) { Entry entry = (Entry)it.next();entry.postDate(); }transactionBundle.getListManager().add(entriesToAdd); } ...}

Sprout Classstd::string QuarterlyReportGenerator::generate(){ std::vector<Result> results = database.queryResults(beginDate, endDate); std::string pageText;pageText += "<html><head><title>" "Quarterly Report" "</title></head><body><table>"; if (results.size() != 0) { for (std::vector<Result>::iterator it = results.begin(); it != results.end(); ++it) {pageText += "<tr>";pageText += "<td>" + it->department + "</td>";pageText += "<td>" + it->manager + "</td>"; char buffer [128];sprintf(buffer, "<td>$%d</td>", it->netProfit / 100);pageText += std::string(buffer);sprintf(buffer, "<td>$%d</td>", it->operatingExpense / 100);pageText += std::string(buffer);pageText += "</tr>"; } } else {pageText += "No results for this period"; }pageText += "</table>";pageText += "</body>";pageText += "</html>"; return pageText;}

Wrap Methodpublic class Employee{ ... public void pay() { Money amount = new Money(); for (Iterator it = timecards.iterator(); it.hasNext(); ) { Timecard card = (Timecard)it.next(); if (payPeriod.contains(date)) {amount.add(card.getHours() * payRate); } }payDispatcher.pay(this, date, amount); }}

Wrap Methodpublic class Employee{ private void dispatchPayment() { Money amount = new Money(); for (Iterator it = timecards.iterator(); it.hasNext(); ) { Timecard card = (Timecard)it.next(); if (payPeriod.contains(date)) {amount.add(card.getHours() * payRate); } }payDispatcher.pay(this, date, amount); } public void pay() {logPayment();dispatchPayment(); } private void logPayment() { ... }}

Wrap MethodAdvantagesA good way of getting new, tested functionality into an application when we can't easily write tests for the calling codeIt explicitly makes the new functionality independent of existing functionalityDisadvantagesCan lead to poor names

Wrap Classclass Employee{ public void pay() { Money amount = new Money(); for (Iterator it = timecards.iterator(); it.hasNext(); ) { Timecard card = (Timecard)it.next(); if (payPeriod.contains(date)) {amount.add(card.getHours() * payRate); } }payDispatcher.pay(this, date, amount); } ...}

Wrap Classclass LoggingEmployee extends Employee{ public LoggingEmployee(Employee e) { employee = e; } public void pay() {logPayment();employee.pay(); } private void logPayment() { ... } ...}

Wrap Classclass LoggingPayDispatcher{ private Employee e; public LoggingPayDispatcher(Employee e) {this.e = e; } public void pay() {employee.pay();logPayment(); } private void logPayment() { ... } ...}

Wrap ClassAble to add new behavior into a system without adding it to an existing classWhen there are many calls to the code you want to wrap, it often pays to move toward a decorator-ish wrapperIf new behavior has to happen at couple of places, simple class wrapper is very useful

I Don't Have Much Time and I Have to Change It: SummaryUse Sprout Method when the code you have in the existing method communicates a clear algorithm to the reader. Use Wrap Method when you think that the new feature you’re adding is as important as the work that was there beforeUse Wrap Class when the behavior that I want to add is completely independent

Working effectively with legacy code

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