SOLID + CleanCode. Design objektorientierter Systeme

Transcription

1 SOLID + CleanCode Design objektorientierter Systeme

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3 Style matters and there s beauty in simplicity. msg Monat 2016 Präsentationstitel 3

4 What should matter Clarity Brevity Simplicity Humanity* 4

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7 LE GRILL WHAT THE HELL IS THAT msg Monat 2016 Präsentationstitel 7

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10 Clean Code 10

11 "Clean code can be read, and enhanced by a developer other than its original author. msg Monat 2016 Präsentationstitel 11

12 HOW IT SHOULD BE msg Monat 2016 Präsentationstitel HOW IT USUALLY IS 12

13 Technical Debt 13

14 Technical debt is...well...debt and it may or may not be evil 14

15 Keep the code at top quality and stop it from rotting* *as if code is alive, or is it? 15

16 All sucessful software gets changed Fred Brooks, The mythical man month msg Monat 2016 Präsentationstitel 16

17 S.O.L.I.D.-Prinzipien Fünf Prinzipien für wartbaren Code Nicht an eine Technologie gebunden

18 The Single Responsibility Principle msg Monat 2016 Präsentationstitel 18

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26 The Open/Closed Principle msg Monat 2016 Präsentationstitel 26

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34 The Liskov Substitution Principle msg Monat 2016 Präsentationstitel 34

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41 The Interface Segreggation Principle msg Monat 2016 Präsentationstitel 41

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46 The Dependency Inversion Principle msg Monat 2016 Präsentationstitel 46

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51 NullPointerException 51

52 NullPointerException private Boolean isfinished(string status) { if (status.equalsignorecase("finished") { return true; } } return false; private Boolean isfinished(string status) { if ( Finished.equalsIgnoreCase(status) { return true; } } return false; } 52

53 NullPointerException BigDecimal price = getprice(); System.Out.println(String.valueOf(price)); System.Out.println(price.toString()); //throws java.lang.nullpointer.exception StringUtil.isEmpty StringUtil.isBlank StringUtil.isNumeric //^ are null safe } 53

54 NullPointerException String String in) { return in.touppercase(); //no nullcheck required } void caller(string s) { if (s!=null) System.Out.println(capitalize(s)) //nullcheck required } } 54

55 NullPointerException public Optional<Customer> findcustomerbyname(string name) { //hard work } Optional<Customer> optionalcustomer = findcustomerbyname ("Nelson Diamantu"); if (optionalcustomer.ispresent()) { Customer customer = optional.get(); //do stuff } else { //not there; deal with it! } } 55

56 Exception Handling Don t just hide it catch (NoSuchMethodException e) { return null; //the cause of the exception is now forever lost } Throw specific exceptions in your methods public void foo throws Exception { //avoid this } public void foo throws SpecificException, EvenMoreSpecificException { //atta boy } } 56

57 Exception Handling Don t catch Exception try { somemethod(); } catch (Exception e) { Logger.log( call failed, e); } Wrap exceptions in custom exceptions catch (NoSuchMethodException e) { throw new MyServiceException ( Info: + e.getmessage()); //incorrect, stack trace lost } catch (NoSuchMethodException e) { throw new MyServiceException ( Info:, e); //correct } 57

58 Exception Handling Log or throw. Never both catch (NoSuchMethodException e) { Logger.log( call failed, e); throw e; } Never throw exceptions from finally try { somemethod(); //throws exception } finally (Exception e) { cleanup(); throw e; //exception will be lost if we throw here } } 58

59 Exception Handling Only catch exceptions you can handle catch (NoSuchMethodException e) { throw e; //throwing this doesn t really help } Use finally if you re not going to handle the exception try { somemethod(); //throws exception } finally { cleanup(); } } 59

60 Throw early, catch late! 60

61 Exception Handling Always clean up after handling exceptions Throw only relevant exceptions wrap them if necessary Exceptions should not dictate your flow One single log entry per exception 61

62 Ommit needless words msg Monat 2016 Präsentationstitel 62

63 "Classes should be immutable unless there's a very good reason to make them mutable...if a class cannot be made immutable, limit its mutability as much as possible. msg Monat 2016 Präsentationstitel 63

64 Immutable = an object cannot be modified once created 64

65 How to implement All attributes should be final Object reference must not be exposed Any modification results in a new (immutable) object Class should be marked as final 65

66 Advantages Thread safe Boosts performance and simplifies development Improve reusability 66

67 Immutable Objects in Java java.lang.string java.math.biginteger java.math.bigdecimal java.io.file 67

68 java.lang.string class S{ public static void main(string [] args){ String x = "abc"; //1. use a literal String y = new String("abc"); //2. use constructor String a = "abcd"; String b = "abcd"; System.out.println(a == b); // True System.out.println(a.equals(b)); // True } } String c = new String("abcd"); String d = new String("abcd"); System.out.println(c == d); // False System.out.println(c.equals(d)); // True 68

69 java.lang.string class S{ public static void main(string [] args){ String s = "Java"; s.concat(" SE 6"); s.replace('6','7'); System.out.print(s); } } 69

70 java.lang.string class S{ public static void main(string [] args){ String a = new String("abc"); if (a == "abc") { System.out.println("egal"); } else { System.out.println("nu e"); } } } 70

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72 java.lang.string class S{ public static void main(string [] args){ String result=""; for(string s: arr){ result = result + s; } } } 72

73 StringBuffer, StringBuilder StringBuffer is mutable All public methods are synchronized, therefore it is thread-safe but also very slow StringBuilder is a copy of StringBuffer but without synchronization use it whenever possible 73

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