Java 8 Lambda Expressions

Java 8 introduced a change named Java's biggest ever -- lambda expressions. In this talk we'll explore lambda expressions, when and how to use them, then dive even deeper and see how they work under the hood.

Transcript

1. Java 8 λs
   IL JUG, April 2014
   Niv Steingarten, Takipi
   

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2. Outline
   ● What are lambda expressions?
   ● Java 8 lambdas + Stream API
   ● Syntax
   ● Under the hood
   ● Questions
   

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3. Lambda expressions
   

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4. Lambda expressions
   Anonymous implementation
   of a
   functional interface
   

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5. Lambda expressions
   List list = Arrays.asList(
   "Say", "hello", "to", "Java 8", "Lambdas");
   Collections.sort(list, new Comparator() {
   @Override
   public int compare(String x, String y) {
   return x.compareTo(y);
   }
   });
   System.out.println(list);
   

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6. Lambda expressions
   List list = Arrays.asList(
   "Say", "hello", "to", "Java 8", "Lambdas");
   Collections.sort(list, new Comparator() {
   @Override
   public int compare(String s1, String s2) {
   return s1.compareTo(s2);
   }
   });
   System.out.println(list);
   

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7. Lambda expressions
   Java < 8
   Collections.sort(list, new Comparator() {
   @Override
   public int compare(String x, String y) {
   return x.compareTo(y);
   }
   });
   

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8. Lambda expressions
   Java < 8
   Collections.sort(list, new Comparator() {
   @Override
   public int compare(String x, String y) {
   return x.compareTo(y);
   }
   });
   Java 8
   list.sort((x, y) -> x.compareTo(y));
   

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9. Lambda expressions
   List list =
   Arrays.asList(3, 9, 12, -1);
   int factor = getFactor();
   list.forEach(x -> System.out.println(x * factor));
   

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10. When can we use λs?
    

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11. When can we use λs?
    “SAM”
    Single Abstract Method
    

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12. When can we use λs?
    @FunctionalInterface
    public interface Comparator {
    int compare(T o1, T o2);
    }
    

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13. When can we use λs?
    @FunctionalInterface
    public interface Comparator {
    int compare(T o1, T o2);
    }
    @FunctionalInterface
    public interface Predicate {
    boolean test(T t);
    }
    

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14. When can we use λs?
    Java < 8
    new Thread(new Runnable() {
    @Override
    public void run() {
    System.out.println("Lambdas can have");
    System.out.println("multiple statements");
    }
    }).start();
    

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15. When can we use λs?
    Java 8
    new Thread(() -> {
    System.out.println("Lambdas can have");
    System.out.println("multiple statements");
    }).start();
    

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16. Streams + Lambdas
    

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17. Streams + Lambdas
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    List newPrices = new ArrayList();
    for (Double p : prices) {
    System.out.println(p);
    if (p > 300) {
    newPrices.add(p * (1.0 - dailyDiscount));
    }
    }
    Collections.sort(newPrices);
    for (Double p : newPrices) {
    System.out.println(p);
    }
    

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18. Streams + Lambdas
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    prices.stream()
    .peek(p -> System.out.println(p))
    .filter(p -> (p > 300))
    .map(p -> p * (1.0 - dailyDiscount))
    .sorted()
    .forEach(p -> System.out.println(p));
    

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19. Streams + Lambdas
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    prices.stream()
    .peek(p -> System.out.println(p))
    .filter(p -> (p > 300))
    .map(p -> p * (1.0 - dailyDiscount))
    .sorted()
    .forEach(p -> System.out.println(p));
    Consumer:
    T → void
    

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20. Streams + Lambdas
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    prices.stream()
    .peek(p -> System.out.println(p))
    .filter(p -> (p > 300))
    .map(p -> p * (1.0 - dailyDiscount))
    .sorted()
    .forEach(p -> System.out.println(p));
    Predicate:
    T → boolean
    

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21. Streams + Lambdas
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    prices.stream()
    .peek(p -> System.out.println(p))
    .filter(p -> (p > 300))
    .map(p -> p * (1.0 - dailyDiscount))
    .sorted()
    .forEach(p -> System.out.println(p));
    Function:
    T → R
    

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22. Streams + Lambdas
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    prices.stream()
    .peek(p -> System.out.println(p))
    .filter(p -> (p > 300))
    .map(p -> p * (1.0 - dailyDiscount))
    .sorted()
    .forEach(p -> System.out.println(p));
    Requires:
    T implements Comparable
    

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23. Streams + Lambdas
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    prices.stream()
    .peek(p -> System.out.println(p))
    .filter(p -> (p > 300))
    .map(p -> p * (1.0 - dailyDiscount))
    .sorted()
    .forEach(p -> System.out.println(p));
    Consumer: (+ terminal operation)
    T → void
    

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24. Syntax
    

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25. Syntax
    ● (int x) -> { int y = x + 1; return y; }
    

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26. Syntax
    ● (int x) -> { int y = x + 1; return y; }
    ● (int x) -> { return x + 1; }
    

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27. Syntax
    ● (int x) -> { int y = x + 1; return y; }
    ● (int x) -> { return x + 1; }
    ● (int x) -> x + 1
    

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28. Syntax
    ● (int x) -> { int y = x + 1; return y; }
    ● (int x) -> { return x + 1; }
    ● (int x) -> x + 1
    ● (x) -> x + 1
    

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29. Syntax
    ● (int x) -> { int y = x + 1; return y; }
    ● (int x) -> { return x + 1; }
    ● (int x) -> x + 1
    ● (x) -> x + 1
    ● x -> x + 1
    

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30. Syntax
    ● (int x) -> { int y = x + 1; return y; }
    ● (int x) -> { return x + 1; }
    ● (int x) -> x + 1
    ● (x) -> x + 1
    ● x -> x + 1
    ● (x, y) -> x * y
    

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31. Off-topic:
    Method references
    List prices = Arrays.asList(1000.0, 150.0, 499.0);
    prices.stream()
    .peek(p -> System.out.println(p))
    .filter(p -> (p > 300))
    .map(Double::toString)
    .sorted()
    .forEach(p -> System.out.println(p));
    

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32. Under the hood
    

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33. Java bytecode
    

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34. Java bytecode
    ALOAD 0
    GETFIELD java/util/ArrayList.elementData : [Ljava/lang/Object;
    GETSTATIC java/util/ArrayList.EMPTY_ELEMENTDATA : [Ljava/lang/Object;
    IF_ACMPEQ L1
    ICONST_0
    GOTO L2
    BIPUSH 10
    ISTORE 2
    ILOAD 1
    ILOAD 2
    IF_ICMPLE L4
    ALOAD 0
    ILOAD 1
    INVOKESPECIAL java/util/ArrayList.ensureExplicitCapacity (I)V
    RETURN
    

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35. Java bytecode
    ● Designed for Java 1
    

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36. Java bytecode
    ● Designed for Java 1
    ● Strict Object-Oriented specification
    

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37. Java bytecode
    ● Designed for Java 1
    ● Strict Object-Oriented specification
    ● Highly-performant on modern JVMs
    

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38. Under the hood
    List list =
    Arrays.asList("Alice", "Bob", "Eve");
    list.forEach(s -> System.out.println(s));
    

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39. Under the hood
    List list =
    Arrays.asList("Alice", "Bob", "Eve");
    list.forEach(s -> System.out.println(s));
    

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40. Under the hood
    ...
    aload_0
    invokedynamic
    #0:accept:()LConsumer;
    invokeinterface
    List.forEach:(LConsumer;)V
    ...
    

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41. Under the hood
    Step 1: Desugaring
    

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42. Desugaring
    class A {
    public void foo() {
    List list = ...
    list.forEach(s -> System.out.println(s));
    }
    }
    

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43. Desugaring
    class A {
    public void foo() {
    List list = ...
    list.forEach([await link of lambda$1 Consumer]);
    }
    private static void lambda$1(String s) {
    System.out.println(s);
    }
    }
    

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44. Desugaring
    class B {
    public void foo() {
    List list = ...
    int factor = ...
    list.map(n -> n * factor);
    }
    }
    

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45. Desugaring
    class B {
    public void foo() {
    List list = ...
    int factor = ...
    list.map([await link of lambda$1 as Function]);
    }
    private static Integer lambda$1(int factor, Integer n)
    {
    return n * factor;
    }
    }
    

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46. Under the hood
    Step 2: The Lambda Metafactory
    

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47. The Lambda Metafactory
    metaFactory(MethodHandles.Lookup caller,
    String invokedName,
    MethodType invokedType,
    MethodHandle descriptor,
    MethodHandle impl)
    

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48. The Lambda Metafactory
    

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49. The Lambda Metafactory
    ● “invokedynamic” encountered
    

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50. The Lambda Metafactory
    ● “invokedynamic” encountered
    ● Metafactory is invoked once
    

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51. The Lambda Metafactory
    ● “invokedynamic” encountered
    ● Metafactory is invoked once
    ● ...returns a specific “Lambda Factory”
    

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52. The Lambda Metafactory
    ● “invokedynamic” encountered
    ● Metafactory is invoked once
    ● ...returns a specific “Lambda Factory”
    ● Call site linked to the specific factory forever
    

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53. Under the hood
    Step 3: The Lambda Factory
    

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54. The Lambda Factory
    class A {
    public void foo() {
    List list = ...
    list.forEach([lambda$1 factory]);
    }
    private static void lambda$1(String s) {
    System.out.println(s);
    }
    }
    

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55. The Lambda Factory
    class lambda$1Factory {
    public Consumer create(...) {
    ...
    }
    }
    

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56. The Lambda Factory
    class A {
    public void foo() {
    List list = ...
    list.forEach([lambda$1Factory]);
    }
    }
    

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57. Under the hood
    Step 4: The Lambda
    

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58. The Lambda
    class Lambda$1Impl implements Consumer {
    @Override
    public void accept(String s) {
    A.lambda$1bridge(s);
    }
    }
    

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59. The Lambda
    class Lambda$1Impl implements Consumer {
    @Override
    public void accept(String s) {
    A.lambda$1bridge(s);
    }
    }
    

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60. But why?
    

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61. But why?
    ● Try not to touch the bytecode specification
    

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62. But why?
    ● Try not to touch the bytecode specification
    ● Delegates lambda handling to the JVM
    

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63. But why?
    ● Try not to touch the bytecode specification
    ● Delegates lambda handling to the JVM
    ● While allowing the compiler minimal control
    

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64. But why?
    ● Try not to touch the bytecode specification
    ● Delegates lambda handling to the JVM
    ● While allowing the compiler minimal control
    

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65. But why?
    ● Try not to touch the bytecode specification
    ● Delegates lambda handling to the JVM
    ● While allowing the compiler minimal control
    ● Enabled runtime optimization and analysis
    

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66. But why?
    ● Try not to touch the bytecode specification
    ● Delegates lambda handling to the JVM
    ● While allowing the compiler minimal control
    ● Enabled runtime optimization and analysis
    ● Allow flexibility in internal implementation
    

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67. Thank you!
    Now go and be awesome.
    niv@takipi.com
    @nivstein / @takipid
    

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