Java: Did you Know that?

Transcript

1. @jeanneboyarsky 1 Java: Did You Know That? Jeanne Boyarsky Thursday February 20, 2020 DevNexus speakerdeck.com/boyarsky
2. @jeanneboyarsky About Me • Java Champion • Author • Developer at NYC bank for 17+ years • FIRST Robotics Mentor 2
3. @jeanneboyarsky Pause for a Commercial 3 Java 11 certs •1Z0-815 - Out now •1Z0-816 - April ETA
4. @jeanneboyarsky With Contributions By • Janeice DelVecchio • Elena Felder • Scott Selikoff 4
5. @jeanneboyarsky Removing in a Loop 5
6. @jeanneboyarsky Removing in a Loop 6 List<String> list = new ArrayList<>(); list.add("red"); list.add("alliance"); list.add(“blue"); list.add("alliance"); for(String current : list) { list.remove(current); } System.out.println(list); Output: ConcurrentModificationException
7. @jeanneboyarsky Removing in a Loop 7 List<String> list = new ArrayList<>(); list.add("red"); list.add("alliance"); for(String current : list) { list.remove(current); } System.out.println(list); Output: [alliance]
8. @jeanneboyarsky Behavior Explanation 8 1.List starts as [red, alliance] 2.First iteration through loop, current = red 3.In loop, remove red 4.Now list is [alliance] 5.For loop checks size. 6.Size = 1; already saw one element 7.Done!
9. @jeanneboyarsky Behavior Explanation 9 1.vs List as [red, alliance, blue, alliance] 2.Size = 3 after removal and saw one 3.Not Done! 4.Exception
10. @jeanneboyarsky Removing in a Loop 10 List<String> list = new CopyOnWriteArrayList<>(); list.add("red"); list.add("alliance"); list.add("blue"); list.add("alliance"); for(String current : list) { list.remove(current); } System.out.println(list); Output: []
11. @jeanneboyarsky Or ditch the loop 11 list.clear(); (or) list.removeIf(x -> true); Output: []
12. @jeanneboyarsky Creating a Set 12
13. @jeanneboyarsky Creating a Set 13 String[] words = { "all", "the", “words", "in", "the", "world" }; Set<String> set = new HashSet<>( Arrays.asList(words)); System.out.println(set); Output like: [all, the, world, in, words]
14. @jeanneboyarsky Creating a Set 14 String[] words = { "all", "the", “words", "in", "the", "world" }; Set<String> set = Set.of(words); System.out.println(set); Output: IllegalArgumentException: duplicate element: the
15. @jeanneboyarsky Behavior Explanation 15 1.Set.of() takes varargs. 2.Doc says can’t have duplicates
16. @jeanneboyarsky Backed Collection 16
17. @jeanneboyarsky Collection 17 Map<String, String> map = new HashMap<>(); map.put("Braves", "Atlanta"); map.put("Mets", "NYC"); Set<String> keys = new HashSet<>(map.keySet()); keys.remove("Mets"); System.out.println(map); {Mets=NYC, Braves=Atlanta}
18. @jeanneboyarsky Collection 18 Map<String, String> map = new HashMap<>(); map.put("Braves", "Atlanta"); map.put("Mets", "NYC"); Set<String> keys = map.keySet(); keys.remove("Mets"); System.out.println(map); {Braves=Atlanta}
19. @jeanneboyarsky Behavior Explanation 19 1.keySet(), values(), and entrySet() are backed collections 2.When change any, it affects original map
20. @jeanneboyarsky Overloading 20
21. @jeanneboyarsky Overloading 21 public void check(Number number) { System.out.print(“Number "); } public void check(Integer integer) { System.out.print(“Integer "); } public void delegator(Number number) { check(number); } Integer num = Integer.valueOf(42); Overloading target = new Overloading(); target.check(num); target.delegator(num); Output: Integer Number
22. @jeanneboyarsky Overloading 22 public void check(Number number) { System.out.print(“Number "); } public void check(Integer integer) { System.out.print(“Integer "); } public void delegator(Number number) { check(number); } Number num = Integer.valueOf(42); Overloading target = new Overloading(); target.check(num); target.delegator(num); Output: Number Number
23. @jeanneboyarsky Behavior Explanation 23 1.Method chosen at compile time, not runtime. 2.(vs polymorphism on objects)
24. @jeanneboyarsky == 24
25. @jeanneboyarsky Equality 25 Integer int1 = Integer.valueOf(8); Integer int2 = Integer.valueOf(8); System.out.println((int1 == int2) + " " + int1.equals(int2)); true true
26. @jeanneboyarsky Equality 26 Integer int1 = Integer.valueOf(8_000); Integer int2 = Integer.valueOf(8_000); System.out.println((int1 == int2) + " " + int1.equals(int2)); false true
27. @jeanneboyarsky Behavior Explanation 27 1.Wrapper classes cache small values 2.Lesson: always use equals() for objects
28. @jeanneboyarsky var and inference 28
29. @jeanneboyarsky var and inference 29 List<Integer> x1 = List.of(); List<Integer> x2 = List.of(1, 2, 3); Stream.of(x1, x2) .flatMap(x -> x.stream()) .map(x -> x + 1) .forEach(System.out::print); Output: 234
30. @jeanneboyarsky var and inference 30 List<Integer> x1 = List.of(); var x2 = List.of(1, 2, 3); Stream.of(x1, x2) .flatMap(x -> x.stream()) .map(x -> x + 1) .forEach(System.out::print); Output: 234
31. @jeanneboyarsky var and inference 31 var x1 = List.of(); var x2 = List.of(1, 2, 3); Stream.of(x1, x2) .flatMap(x -> x.stream()) .map(x -> x + 1) .forEach(System.out::print); Compiler error: Operator ‘+’ cannot be applied to capture<?>, int
32. @jeanneboyarsky Behavior Explanation 32 1.var x2 = List.of(1,2,3) 2.x2 is a List<Integer> 3.var x1 = List.of() 4.x1 is a not a List<Integer> 5.Stream.of(x1, x2) is a Stream<List<? extends Object>> 6.flatMap makes Stream<? extends Object> 7.Object doesn’t go with +
33. @jeanneboyarsky Sorting Characters 33
34. @jeanneboyarsky Sorting Characters 34 Stream<String> ohMy = Stream.of( "lions", "tigers", "bears"); Comparator<Character> c = Comparator.naturalOrder(); System.out.println(ohMy.collect( Collectors.groupingBy(String::length, Collectors.mapping(s -> s.charAt(0), Collectors.minBy(c))))); Output: {5=Optional[b], 6=Optional[t]}
35. @jeanneboyarsky Sorting Characters 35 Stream<String> ohMy = Stream.of( "lions", "tigers", "bears"); System.out.println(ohMy.collect( Collectors.groupingBy(String::length, Collectors.mapping(s -> s.charAt(0), Collectors.minBy( Comparator.naturalOrder()))))); Compiler error No suitable method found for….
36. @jeanneboyarsky The actual message 36 Error:(18, 51) java: no suitable method found for groupingBy(String::length,java.util.stream.Collector<java.lang .Object,capture#1 of ?,java.util.Optional<T>>) method java.util.stream.Collectors.<T,K>groupingBy(java.util.function. Function<? super T,? extends K>) is not applicable (cannot infer type-variable(s) T,K (actual and formal argument lists differ in length)) method java.util.stream.Collectors.<T,K,A,D>groupingBy(java.util.func tion.Function<? super T,? extends K>,java.util.stream.Collector<? super T,A,D>) is not applicable (inference variable U has incompatible upper bounds java.lang.Object,java.lang.Comparable<? super T>,T,T)…
37. @jeanneboyarsky The other message 37 Error:(19, 52) java: cannot find symbol symbol: method charAt(int) location: variable s of type java.lang.Object
38. @jeanneboyarsky Behavior Explanation 38 1.char != Character
 Inferred type check fails 2.Propagates error to other call 3.But, this suggests a workaround…
39. @jeanneboyarsky Sorting Characters 39 Stream<String> ohMy = Stream.of( "lions", "tigers", "bears"); System.out.println(ohMy.collect( Collectors.groupingBy(String::length, Collectors.mapping((String s) -> s.charAt(0), Collectors.minBy( Comparator.naturalOrder()))))); Output: {5=Optional[b], 6=Optional[t]}
40. @jeanneboyarsky URL Equality 40
41. @jeanneboyarsky URL Equality 41 URL url1 = new URL("https://google.com"); URL url2 = new URL("https://google.com"); System.out.println(url1.equals(url2)); Output: true
42. @jeanneboyarsky URL Equality 42 URL url1 = new URL("cloudURL"); URL url2 = new URL(“cloudURL"); System.out.println(url1.equals(url2)); Output: false if DNS resolution changes during program
43. @jeanneboyarsky Behavior Explanation 43 1.URL’s equals method calls the URLStreamHandler’s equals method 2.Which uses DNS resolution 3.Cloud URLs change often Instead, use URI https://news.ycombinator.com/ item?id=21765788
44. @jeanneboyarsky Week of the Year 44
45. @jeanneboyarsky Week of Year 45 LocalDate xmasEve = LocalDate.of( 2019, Month.DECEMBER, 24); WeekFields weekFields = WeekFields.of( Locale.getDefault()); int weekNumber = xmasEve.get( weekFields.weekOfWeekBasedYear()); int week = xmasEve.get( weekFields.weekOfYear()); System.out.println(weekNumber + " " + week); Output: 52 52
46. @jeanneboyarsky Week of Year 46 LocalDate newYearsEve = LocalDate.of( 2019, Month.DECEMBER, 31); WeekFields weekFields = WeekFields.of( Locale.getDefault()); int weekNumber = newYearsEve.get( weekFields.weekOfWeekBasedYear()); int week = newYearsEve.get( weekFields.weekOfYear()); System.out.println(weekNumber + " " + week); Output: 1 53
47. @jeanneboyarsky Behavior Explanation 47 1.Most years are 52 weeks + 1 day 2.2020 started on a Wednesday. 3.The week based week starts the Sunday before 4.Whereas weekOfYear() returns 1-54
48. @jeanneboyarsky Week of Year 48 LocalDate xmasEve = LocalDate.of( 2019, Month.DECEMBER, 24); DateTimeFormatter format = DateTimeFormatter.ofPattern( "yyyy-MM-dd"); DateTimeFormatter format2 = DateTimeFormatter.ofPattern( "YYYY-MM-dd"); System.out.println(xmasEve.format(format) + " " + xmasEve.format(format2)); Output: 2019-12-24 2019-12-24
49. @jeanneboyarsky Week of Year 49 LocalDate newYearsEve = LocalDate.of( 2019, Month.DECEMBER, 31); DateTimeFormatter format = DateTimeFormatter.ofPattern( "yyyy-MM-dd"); DateTimeFormatter format2 = DateTimeFormatter.ofPattern( "YYYY-MM-dd"); System.out.println( newYearsEve.format(format) + " " + newYearsEve.format(format2)); Output: 2019-12-31 2020-12-31
50. @jeanneboyarsky Behavior Explanation 50 1.y is year 2.Y is week of year 3.Be careful with week of year!
51. @jeanneboyarsky Closing Resources 51
52. @jeanneboyarsky JDBC 52 PreparedStatement stmt = conn.prepareStatement( "update mytable set updated = now()"); try (stmt) { stmt.executeUpdate(); } “Good”
53. @jeanneboyarsky JDBC 53 PreparedStatement stmt = conn.prepareStatement( "update mytable set updated = now()" + " where key = ?"); stmt.setString(1, "abc"); try (stmt) { stmt.executeUpdate(); } Resource leak!
54. @jeanneboyarsky Behavior Explanation 54 1.What happens if an exception is thrown while calling the PreparedStatement setter? 2.The try-with-resources doesn’t run 3.Resource leak!
55. @jeanneboyarsky JDBC 55 PreparedStatement stmt = conn.prepareStatement( "update mytable set updated = now()" + " where key = ?"); try (stmt) { stmt.setString(1, "abc"); stmt.executeUpdate(); } “good”
56. @jeanneboyarsky IO 56 try (BufferedReader reader = Files.newBufferedReader(path)) { String line = null; while ((line = reader.readLine())!= null) { // process line } } No leak
57. @jeanneboyarsky IO 57 Files.lines(path).count(); Resource leak!
58. @jeanneboyarsky Behavior Explanation 58 1.Resource not closed by terminal operation 2.Affects 1.find() 2.lines() 3.list() 4.newDirectoryStream() 5.walk()
59. @jeanneboyarsky IO 59 try (Stream<String> stream = Files.lines(path)) { stream.count(); } Good
60. @jeanneboyarsky PSA: Free Tools •PMD •FindBugs •CheckStyle •SonarQube - examples: •No return in finally •Close resources •All sorts of bugs 60
61. @jeanneboyarsky Other Favorites? ??? 61