Java anti-patterns:Dark knight rises.

Сергей Моренец28 февраля 2013 г.

Agenda

• Design patterns• Anti-patterns overview• Q & A

Design patterns• Design Patterns: Elements of Reusable Object-Oriented

Software• Published on October 21, 1994

• Authors are Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, referred as Gang of Four

• Creational, structural and behavioral patterns.

Hero of the day

Java Anti-docs/*** Rejects current task*/ public void approve() { …}

Inline conversionint recordId = -1;

try { recordId = Integer.parseInt(columnRecordId);} catch (NumberFormatException e) { recordId = 0;}

Exception handleravoidance

ValidationResultCollector vrc = collector.validate();

if(vrc.hasError()) { ValidationResult result = vrc.getError(); errorText = result.getErrorText();} else if(vrc.getErrorCode() == CPFErrorID.getNoCPFError()) {}

Disuse of helper libraries

if (param.getFileName() != null && param.getFileName().length() > 4) {try { if (param.getFileName().contains(".")) { name = param.getFileName().substring(0, param.getFileName().lastIndexOf(".")); } else name = attach.getFileName();} catch (Exception e) { e.printStackTrace();String errorMessage = e.getMessage(); } }

Disuse of helper libraries

String name = FilenameUtils.getBaseName( parameter.getFileName());

String extension = FilenameUtils.getExtension( parameter.getFileName());

Misuse of inheritance• public class SuperMap<K, V> extends

HashMap<K,V>{

• @Override• public V get(Object key) {• …• }• }

Upcastingpublic abstract class BaseClass {

public void do() { ((DerivedClass) this).validate();}}

class DerivedClass extends BaseClass {public void validate() {}}

Lack of synchronization

public class ScreenManager {

private static ScreenManager instance;

public static ScreenManager getInstance() {if (instance == null) { instance = new ScreenManager();}return instance;}}

State inconsistencypublic class AccountManager {private boolean inProgress = false;

public void submit() { if (inProgress) { return; } inProgress = true; … inProgress = false;}}

State inconsistencypublic class AccountManager { private final Lock lock = new ReentrantLock();

public void submit() { lock.lock(); … lock.unlock();}}

State inconsistencypublic class AccountManager {private final Lock lock = new ReentrantLock();

public void submit() {try { lock.lock(); … } finally { lock.unlock();}}}

Platform-dependent API

File tmp = new File("C:\\Temp\\1.tmp");

File exp = new File("export-2013-02-01T12:30.txt");

File f = new File(path +'/'+ filename);

Platform-dependent API

File tmp = File.createTempFile("myapp","tmp");

File exp = new File("export-2013-02-01_1230.txt");

File f = new File(path +File. pathSeparator+ filename);

File dir = new File(path);File f = new File(dir, filename);

Infinitive heapbyte[] pdf = toPdf(file);

Infinitive heapFile pdf = new File(file);InputStream in = new FileInputStream(pdf);

Unbuffered streamsInputStream in = new FileInputStream(file);int b;while ((b = in.read()) != -1) {...}

Unbuffered streamsInputStream in = new BufferedInputStream(new FileInputStream(file))

Wrong propagationtry { …} catch(ParseException e) { LOGGER.error(e.getMessage()); throw new RuntimeException(); throw new RuntimeException(e.toString()); throw new RuntimeException(e.getMessage()); throw new RuntimeException(e);}

Wrong propagationtry {} catch(ParseException e) {throw new RuntimeException(e.getMessage(), e);}

Impossible exceptiontry {... do risky stuff ...} catch(SomeException e) {// never happens}

Impossible exceptiontry {... do risky stuff ...} catch(SomeException e) {// never happens hopefullythrow new IllegalStateException(e.getMessage(), e); }

Unnecessary CalendarCalendar cal = new GregorianCalender(TimeZone.getTimeZone("Europe/Kyiv"));cal.setTime(date);cal.add(Calendar.HOUR_OF_DAY, 8);date = cal.getTime();

Unnecessary Calendardate = new Date(date.getTime() + 8L * 3600L * 1000L);

Misleading calendar Calendar c = Calendar.getInstance();c.set(2009, Calendar.JANUARY, 15);c.getTime() - ?

Misleading calendar Calendar c = new GregorianCalendar(timeZone);c.set(2009, Calendar.JANUARY, 15);

Global constantspublic interface Constants { String version = "1.0"; String dateFormat = "dd.MM.yyyy"; String configFile = ".apprc"; int maxNameLength = 32; String someQuery = "SELECT * FROM ...";}

Static initializersclass Cache {private static final Timer timer = new Timer();}

Static initializersclass Cache {private static Timer timer;

public static startTimer() {timer = new Timer();}}

Reflection overuse

Class beanClass = ... if (beanClass.newInstance() instanceof TestBean) ...

Reflection overuse

Class beanClass = ... if (TestBean.class.isAssignableFrom(beanClass)) ...

Map iterationMap<String, String> map = …;

for (String key : map.keySet()) { String value = map.get(key); …}

Map iterationMap<String, String> map = …;

for (Entry<String, String> entry : map.entrySet()) { String key = entry.getKey(); String value = entry.getValue(); …}

Time measurementlong startTime = System.currentTimeMillis(); ...long elapsedTime = System.currentTimeMillis() - startTime;

Time measurementlong startTime = System.nanoTime();...long elapsedTime = (System.nanoTime() - startTime) / 1000000;

Q&A

• Сергей Моренец, morenets@mail.ru

Collection<User> users = ...if (users != null && !users.isEmpty()) {

int i = 0;for (User user : user) {

if (i > 0)break;

output.setUserId(user.getId());i++;

}}