Preparing for the Java 21 Certification and Learning New Features

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https://linktr.ee/jeanneboyarsky 1 Preparing for the Java 21 cert + Learning New Features Jeanne Boyarsky 4/11/24 DevNexus speakerdeck.com/boyarsky

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https://linktr.ee/jeanneboyarsky Pause for a Commercial 2 Java certs: 8/11/17 Book giveaway at end!

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https://linktr.ee/jeanneboyarsky Disclaimer 3 I do not work for Oracle so no safe harbor statement. However, I am not authorized to tell you any non public information or if I know any :) Some of the material in this presentation may appear in our upcoming certification books.

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https://linktr.ee/jeanneboyarsky Strategy 4 Option 1 - wait for Java 21 study materials Option 2: 1. Study from a Java 17 study guide 2. Read about Java 18-21 features 3. Focus on edge cases 4. Do any practice questions you can find This talk helps with option 2. Or if you just want to learn

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https://linktr.ee/jeanneboyarsky Agenda 5 Topics Chances of being in objectives Virtual threads 95% or higher Pattern matching for switch 95% or higher Record patterns 95% or higher Sequenced Collections 10-30% Other topics + open Q&A less than 5%

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https://linktr.ee/jeanneboyarsky 6 Virtual Threads

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https://linktr.ee/jeanneboyarsky Leaps in Concurrency 7 Threads + Executors + Virtual Threads Plus many APIs… 21

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https://linktr.ee/jeanneboyarsky Why we need threads 8 Program Do calculation and call REST API Do calculation and call REST API CPU Network I’m bored… I’m waiting for a reply

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https://linktr.ee/jeanneboyarsky Why we need virtual threads 9 Program Do calculation and call REST API Proceed CPU Network Still bored… Still waiting for a reply Platform Thread 1 Platform Thread n Do calculation and call REST API Send to threads

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https://linktr.ee/jeanneboyarsky Virtual Thread 1 With virtual threads 10 Program Do calculation and call REST API Proceed CPU Network Thanks for the work! Me too! Platform Thread 1 Platform Thread n Send to threads Virtual Thread t Do calculation and call REST API Virtual Thread t+1 Do calculation and call REST API Virtual Thread x Do calculation and call REST API

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https://linktr.ee/jeanneboyarsky When most helpful? 11 • Thousands of threads • Not CPU bound 21

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https://linktr.ee/jeanneboyarsky Comparing 12 Platform (Traditional) Thread Virtual Thread Heavyweight Lightweight Tied to OS threads Runs on OS thread but doesn’t monopolize Often pooled Often shortlived Instance of java.lang.Thread Instance of java.lang.Thread 21

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https://linktr.ee/jeanneboyarsky Fill in the blank 13 try (ExecutorService service = Executors._________) { service.submit(() -> doStuff()); } Examples: • newSingleThreadExecutor() • newFixedThreadPool(5) • newCachedThreadPool() • newVirtualThreadPerTaskExecutor() 21

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https://linktr.ee/jeanneboyarsky Autocloseable 14 Method Java 21 shutdown() No more tasks, but finish ones have shutdownNow() Stop tasks in progress close() Calls shutdown by default 19

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https://linktr.ee/jeanneboyarsky If need directly 15 Platform Thread Virtual Thread Thread.ofPlatform() Thread.ofVirtual() new Thread(…) Not via constructor 21

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https://linktr.ee/jeanneboyarsky What about Scoped Values 16 Preview feature in Java 21 so not on exam 21

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https://linktr.ee/jeanneboyarsky Virtual Threads Mock #1 17 Which is used to create/with a virtual thread? (more than 1 is correct) A. Executors.newVirtualThread() B. Executors.newVirtualThreadExecutor() C. Executors.newVirtualThreadPerTaskExecutor() D. new VirtualThread() E. Thread.ofVirtual() F. Thread.ofVirtualThread() 21

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https://linktr.ee/jeanneboyarsky Virtual Threads Mock #1 18 Which is used to create/with a virtual thread? (more than 1 is correct) A. Executors.newVirtualThread() B. Executors.newVirtualThreadExecutor() C. Executors.newVirtualThreadPerTaskExecutor() D. new VirtualThread() E. Thread.ofVirtual() F. Thread.ofVirtualThread() 21 C, E

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https://linktr.ee/jeanneboyarsky Virtual Threads Mock #2 19 Which is used to create/with a platform thread? (more than 1 is correct) A. Executors.newCachedThreadPool() B. Executors.newPlatformThreadPool() C. Executors.newPlatformThreadPerTaskExecutor() D. new Thread() E. new PlatformThread() F. Thread.ofPlatform() 21

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https://linktr.ee/jeanneboyarsky Virtual Threads Mock #2 20 Which is used to create/with a virtual thread? (more than 1 is correct) A. Executors.newCachedThreadPool() B. Executors.newPlatformThreadPool() C. Executors.newPlatformThreadPerTaskExecutor() D. new Thread() E. new PlatformThread() F. Thread.ofPlatform() 21 A, D, F

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https://linktr.ee/jeanneboyarsky Virtual Threads Mock #3 21 What is true doStuff() at line v1? try (var service = Executors.newVirtualThreadPerTaskExecutor()) { service.submit(() -> doStuff()); } // line v1 A. doStuff() may be running B. doStuff() was killed C. doStuff() completed D. None of the above 21

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https://linktr.ee/jeanneboyarsky Virtual Threads Mock #3 22 What is true doStuff() at line v1? try (var service = Executors.newVirtualThreadPerTaskExecutor()) { service.submit(() -> doStuff()); } // line v1 A. doStuff() may be running B. doStuff() was killed C. doStuff() completed D. None of the above 21 C

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https://linktr.ee/jeanneboyarsky 23 Pattern Matching for Switch

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https://linktr.ee/jeanneboyarsky Evolution 24 Feature Preview Final Switch Expressions 12, 13 14 Pattern Matching for instanceof 14, 15 16 Pattern Matching for switch 17, 18, 19, 20 21 Record Patterns 19, 20 21 Unnamed variables and patterns 21 22

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@jeanneboyarsky What does this print? 14 String place = "Vegas"; switch(place) { case "Vegas" : System.out.println("Table games!"); case "Supermarket" : System.out.println("Lotto!"); default: System.out.println("Maybe?"); } Three lines (missing break) 25

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@jeanneboyarsky Switch Expressions String place = "Vegas"; switch(place) { case "Vegas" -> System.out.println("Table Games!"); case "Supermarket", "Newsstand" -> System.out.println("Lotto!"); default -> System.out.println("anywhere"); } 26 14 Arrow labels No break keyword Multiple values

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@jeanneboyarsky Switch Expressions String place = "Vegas"; var output = switch(place) { case "Vegas" -> "Table Games!"; case "Supermarket", "Newsstand" -> "Lotto!"; default -> "anywhere"; }; System.out.println(output); Switch returns values 27 14 Semicolon

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@jeanneboyarsky Adding Pattern Matching 28 21 return switch (obj) { case null -> ""; case Integer i when i >= 21 -> "can gamble"; case Integer i -> "too young to gamble"; case String s when "poker".equals(s) -> "table game"; case String s when "craps".equals(s) -> "table game"; case String s -> "other game"; default -> throw new IllegalArgumentException("unexpected type"); }; guard clause pattern variable null allowed Still Null Pointer if don’t specify case null

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@jeanneboyarsky Order matters 29 21 return switch (obj) { case null -> ""; case Integer i -> "too young to gamble"; case Integer i when i >= 21 -> "can gamble”; // DOES NOT COMPILE case String s -> "other game"; default -> throw new IllegalArgumentException("unexpected type"); }; Order like catching exceptions!

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@jeanneboyarsky Enums 30 21 enum Suit { HEART, DIAMOND, CLUB, SPADE; } String symbol(Suit suit) { return switch (suit) { case HEART -> "♥"; case Suit.DIAMOND -> "♦"; case CLUB -> "♣"; case Suit.SPADE -> "♠"; }; } Fails compilation if miss one since no default

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@jeanneboyarsky Switch statements 31 21 boolean idCheck(Integer age) { boolean check ; switch (age) { case Integer i when i <= 40 : check = true; break; default : check = false; break; } return check; } Yes, classic switch

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@jeanneboyarsky Pattern variable scope 32 21 boolean idCheck(Integer age) { boolean check ; switch (age) { case Integer i when i <= 21 : check = true; // DOES NOT COMPILE case Integer i when i <= 40 : check = true; default : check = false; } return check; } Can’t use pattern variable if fall thru

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https://linktr.ee/jeanneboyarsky What can’t you do? 33 • In switch(x), x still can’t be • boolean • float • double • long • _ as pattern (coming in Java 22) 21

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https://linktr.ee/jeanneboyarsky Switch Pattern Matching Mock #1 34 What is minimum # lines do you need to remove to make this code compile? double odds(Object obj) { return switch (obj) { case String s -> throw new IllegalArgumentException("unknown game"); case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case Object o -> throw new IllegalArgumentException("unknown game"); default -> throw new IllegalArgumentException("known game"); }; } A. 0 B. 1 21 C. 2 D. 3

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https://linktr.ee/jeanneboyarsky Switch Pattern Matching Mock #1 35 How many lines do you need to remove to make this code compile? double odds(Object obj) { return switch (obj) { case String s -> throw new IllegalArgumentException("unknown game"); case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case Object o -> throw new IllegalArgumentException("unknown game"); default -> throw new IllegalArgumentException("known game"); }; } A. 0 B. 1 21 C. 2 D. 3 C String s & Object o or default

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https://linktr.ee/jeanneboyarsky Switch Pattern Matching Mock #2 36 What are the results of odds(“slots”) and odds(null)? double odds(String str) { return switch (str) { case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case String s -> throw new IllegalArgumentException("unknown game"); }; } A. IllegalArgumentException B. NullPointerException 21 C. .05 D. Does not compile

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https://linktr.ee/jeanneboyarsky Switch Pattern Matching Mock #2 37 What are the results of odds(“slots”) and odds(null)? double odds(String str) { return switch (str) { case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case String s -> throw new IllegalArgumentException("unknown game"); }; } A. IllegalArgumentException B. NullPointerException 21 C. .05 D. Does not compile A, B

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https://linktr.ee/jeanneboyarsky Switch Pattern Matching Mock #3 38 What are the results of odds("slots") and odds(null)? double odds(String str) { return switch (str) { case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case String s -> throw new IllegalArgumentException("unknown game”); case null -> 0.0; }; } A. IllegalArgumentException B. NullPointerException 21 C. 0.0 D. Does not compile

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https://linktr.ee/jeanneboyarsky Switch Pattern Matching Mock #3 39 What are the results of odds("slots") and odds(null)? double odds(String str) { return switch (str) { case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case String s -> throw new IllegalArgumentException("unknown game”); case null -> 0.0; }; } A. IllegalArgumentException B. NullPointerException 21 C. 0.0 D. Does not compile A, C

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https://linktr.ee/jeanneboyarsky 40 Record Patterns

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https://linktr.ee/jeanneboyarsky Record Patterns 41 enum Suit { HEART, DIAMOND, CLUB, SPADE; } enum Rank { NUM_2, NUM_3, NUM_4, NUM_5, NUM_6, NUM_7 NUM_8, NUM_9, NUM_10, JACK, QUEEN, KING, ACE; } record Card(Suit suit, Rank rank) { } if (card instanceof Card c) { System.out.println(c.suit()); System.out.println(c.rank()); } if (card instanceof Card(Suit suit, Rank rank)) { System.out.println(suit); System.out.println(rank); } 21 Deconstructed

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https://linktr.ee/jeanneboyarsky Nested Record Patterns 42 record MultiDeck(int deckNum, Card card) { } if (multi instanceof MultiDeck(int deckNum, Card(Suit suit, Rank rank))) { System.out.println(deckNum); System.out.println(suit); System.out.println(rank); } 21

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https://linktr.ee/jeanneboyarsky Switch pattern matching 43 switch(card) { case Card(Suit suit, Rank rank) when suit == Suit.HEART -> System.out.println("Heart"); case Card c -> System.out.println("other"); } 21

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https://linktr.ee/jeanneboyarsky Switch pattern matching 44 String str = ""; switch (str) { case "heart" -> System.out.println("heart"); } Card card = new Card(Suit.HEART, Rank.NUM_2); switch (card) { case Card(Suit suit, Rank rank) when suit == Suit.HEART -> System.out.println("Heart"); case Card c -> System.out.println("other"); } 21 Must be exhaustive if using “when”

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https://linktr.ee/jeanneboyarsky What can’t you do? 45 • _ for unnamed patterns • Non record class expansion 21

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #1 46 Which of these compile? (more than one) A. if (card instanceof Card (Suit suit, Rank rank)) { } B. if (card instanceof Card (var suit, var rank)) { } C. if (card instanceof Card c) { } D. if (card instanceof Card c.suit, c.rank) { } 21

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #1 47 Which of these compile? (more than one) A. if (card instanceof Card (Suit suit, Rank rank)) { } B. if (card instanceof Card (var suit, var rank)) { } C. if (card instanceof Card c) { } D. if (card instanceof Card c.suit, c.rank) { } 21 A, B, C

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #2 48 What is the output of the following? Card card = new Card(Suit.HEART, Rank.NUM_2); switch (card) { case Card(var suit, var rank) when suit == Suit.HEART && rank == Rank.ACE -> System.out.println("Ace of Hearts"); case Card(var suit, var rank) when suit == Suit.HEART -> System.out.println("Hearts"); } A. Ace of Hearts B. Hearts 21 C. Null Pointer D. Does not compile

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #2 49 What is the output of the following? Card card = new Card(Suit.HEART, Rank.NUM_2); switch (card) { case Card(var suit, var rank) when suit == Suit.HEART && rank == Rank.ACE -> System.out.println("Ace of Hearts"); case Card(var suit, var rank) when suit == Suit.HEART -> System.out.println("Hearts"); } A. Ace of Hearts B. Hearts 21 C. Null Pointer D. Does not compile D (no case Card)

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #3 50 What is the output of the following? Card card = new Card(Suit.HEART, Rank.NUM_2); switch (card) { case Card(var suit, var rank) when suit == Suit.HEART && rank == Rank.ACE -> System.out.println("Ace of Hearts"); case Card(var suit, var rank) when suit == Suit.HEART -> System.out.println("Hearts"); case Card c -> System.out.println("other"); } 21 A. Ace of Hearts B. Hearts C. Null Pointer D. Does not compile

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #3 51 What is the output of the following? Card card = new Card(Suit.HEART, Rank.NUM_2); switch (card) { case Card(var suit, var rank) when suit == Suit.HEART && rank == Rank.ACE -> System.out.println("Ace of Hearts"); case Card(var suit, var rank) when suit == Suit.HEART -> System.out.println("Hearts"); case Card c -> System.out.println("other"); } 21 A. Ace of Hearts B. Hearts C. Null Pointer D. Does not compile B

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #4 52 Given record Card (Suit suit, Rank rank), how many of these compile? case Card(var rank, var suit) when suit == Suit.HEART case Card(Rank rank, Suit suit) when suit == Suit.HEART case Card(Rank rank, var suit) when suit == Suit.HEART case Card(var suit, var rank) when suit == Suit.HEART case Card(Suit suit, Rank rank) when suit == Suit.HEART case Card(var suit, Rank rank) when suit == Suit.HEART A. 0 B. 1 C. 2 21 D. 3 E. 4 F. 5

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https://linktr.ee/jeanneboyarsky Record Patterns Mock #4 53 Given record Card (Suit suit, Rank rank), how many of these compile? case Card(var rank, var suit) when suit == Suit.HEART case Card(Rank rank, Suit suit) when suit == Suit.HEART case Card(Rank rank, var suit) when suit == Suit.HEART case Card(var suit, var rank) when suit == Suit.HEART case Card(Suit suit, Rank rank) when suit == Suit.HEART case Card(var suit, Rank rank) when suit == Suit.HEART A. 0 B. 1 21 C. 2 D. 3 D (last three)

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https://linktr.ee/jeanneboyarsky 54 Sequenced Collections

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https://linktr.ee/jeanneboyarsky What does this output? 55 HashSet set = new HashSet<>(); set.add("Donald"); set.add("Mickey"); set.add("Minnie"); System.out.println(set.iterator().next()); Encounter order undefined(but Mickey on my machine) 21

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https://linktr.ee/jeanneboyarsky Java 17 56 ArrayList, LinkedList, Deque, TreeSet, TreeMap became sequenced collections LinkedHashSet and LinkedHashMap too

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https://linktr.ee/jeanneboyarsky 57 21

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https://linktr.ee/jeanneboyarsky New APIs 58 SequencedCollection SequencedCollection reversed(); void addFirst(E); void addLast(E); E getFirst(); E getLast(); E removeFirst(); E removeLast(); SequencedSet SequencedSet reversed(); SequencedMap SequencedMap reversed(); SequencedSet sequencedKeySet(); SequencedCollection sequencedValues(); SequencedSet> sequencedEntrySet(); V putFirst(K, V); V putLast(K, V); Entry fi rstEntry(); Entry lastEntry(); Entry pollFirstEntry(); Entry pollLastEntry(); 21

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https://linktr.ee/jeanneboyarsky Now defined! 59 SequencedSet set = new LinkedHashSet<>(); set.add("Donald"); set.add("Mickey"); set.add("Minnie"); System.out.println(set.getFirst()); Donald Note: LInkedHashSet not new 21

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https://linktr.ee/jeanneboyarsky Sequenced Collection Mock 60 How many of these lines compile? SequencedCollection seq1 = new HashSet<>(); SequencedSet seq2 = new HashSet<>(); SequencedMap seq3 = new HashSet<>(); SequencedCollection seq4 = new TreeSet<>(); SequencedSet seq5 = new TreeSet<>(); SequencedMap seq6 = new TreeSet<>(); A. One B. Two C. Three D. Four 21

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https://linktr.ee/jeanneboyarsky Sequenced Collection Mock 61 How many of these lines compile? SequencedCollection seq1 = new HashSet<>(); SequencedSet seq2 = new HashSet<>(); SequencedMap seq3 = new HashSet<>(); SequencedCollection seq4 = new TreeSet<>(); SequencedSet seq5 = new TreeSet<>(); SequencedMap seq6 = new TreeSet<>(); A. One B. Two C. Three D. Four B (seq4 and seq 5) 21

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https://linktr.ee/jeanneboyarsky Good to know, but not likely on exam 62 • UTF-8 is the default character set when reading a file • StringBuilder/StringBuffer have a repeat method • String.indexOf(charOrStr, beginIndex, endIndex) • String.splitWithDelimitters(regex, limit) • Finalization deprecated for removal

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https://linktr.ee/jeanneboyarsky Good to know, but not likely on exam 63 Code Snippets in JavaDoc Inline * {@snippet : * if (v.isPresent()) { * System.out.println("v: " + v.get()); * } * } External {@snippet fi le="ShowOptional.java" region=“example"} Example transformation System.out.println("Hello World!"); // @highlight substring="println"

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https://linktr.ee/jeanneboyarsky Book Giveaway 64