Fixing a billion dollar mistake

A presentation regarding so called "Billion Dollar Mistake", which is introducing of Null Reference. Presentation covers different ways of handling that problem in Java and JVM projects. Talk was presented during Quality Meetup #15 in Katowice, Poland.

Transcript

1. Fixing a billion
   dollar mi$take
   Piotr Wittchen
   1
   

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2. What is a billion dollar mistake?
   2
   

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3. null
   3
   

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4. A bit of history
   https://en.wikipedia.org/wiki/Tony_Hoare
   https://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare
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5. Bad practices nowadays according to Uncle Bob
   Passing null
   Returning null
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6. Passing null
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7. What could happen if we pass null to our method?
   public void processData(String data) {
   data.contains("foo");
   }
   processData(null);
   NullPointerException
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8. Null-checks in application code
   public void processData(String data) {
   if(data != null) {
   data.contains("foo");
   }
   }
   processData(null);
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9. Null-checks in library API
   public void processData(String data) {
   if(data == null) {
   throw new NullPointerException("data is null!");
   }
   data.contains("foo");
   }
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10. Objects.requireNonNull(...) method introduced in Java 7
    public void processData(String data) {
    Objects.requireNonNull(data, "data cannot be null");
    data.contains("foo");
    }
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11. Guava and Preconditions
    public void processData(String data) {
    Preconditions.checkNotNull(data ,"data cannot be null");
    data.contains("foo");
    }
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12. @Nonnull Annotation
    public void processData(@Nonnull String data) {
    data.contains("foo");
    }
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13. Returning null
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14. Dealing with collections
    public List createList() {
    List list = new ArrayList<>();
    ...
    if(canReturnList) {
    return list;
    }
    return null;
    }
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15. Dealing with collections
    public List createList() {
    List list = new ArrayList<>();
    ...
    if(canReturnList) {
    return list;
    }
    return list;
    }
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16. Dealing with collections
    public List createList() {
    List list = new ArrayList<>();
    ...
    if(canReturnList) {
    return list;
    }
    return new ArrayList<>();
    }
    16
    

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17. Null Object Pattern
    interface NullableObject {
    boolean isNull();
    Object getObject();
    }
    class RealObject implements NullableObject {
    @Override public boolean isNull() {
    return false;
    }
    @Override public Object getObject() {
    return new Object();
    }
    }
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18. Null Object Pattern
    interface NullableObject {
    boolean isNull();
    Object getObject();
    }
    class NullObject implements NullableObject {
    @Override public boolean isNull() {
    return true;
    }
    @Override public Object getObject() {
    return null;
    }
    }
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19. Null Object Pattern
    NullableObject object;
    Object returnedValue;
    if(!object.isNull()) {
    returnedValue = object.getObject();
    }
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20. Optional type in Guava
    final Optional optional = Optional.of(new Object());
    if (optional.isPresent()) {
    Object notNull = optional.get();
    }
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21. Optional type in Guava
    final Optional optional = Optional.absent();
    final boolean isPresent = optional.isPresent();
    assertThat(isPresent).isFalse();
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22. Optional type in Guava
    final Optional optional = Optional.fromNullable(null);
    final boolean isPresent = optional.isPresent();
    assertThat(isPresent).isFalse();
    https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained
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23. Optional type in Java 8
    Optional optional = Optional.of(new Object());
    if (optional.isPresent()) {
    Object notNull = optional.get();
    }
    This example is the same as Guava Optional
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24. Optional type in Java 8
    Optional optional = Optional.empty();
    Object returnedValue = optional.orElse(StringUtils.EMPTY);
    assertThat(returnedValue).isInstanceOf(String.class);
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25. Optional type in Java 8
    Optional optional = Optional.of(new Object());
    optional.ifPresent(object -> assertThat(object).isNotNull());
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26. Optional type in Java 8
    final Optional optional = Optional.of(new Object());
    final String value = optional
    .map(o -> o.toString())
    .orElse(StringUtils.EMPTY);
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27. Enhancements of Optional type in Java 9
    final Optional optional = Optional.of(new Object());
    optional.ifPresentOrElse(object -> assertThat(object).isNotNull(),
    Assert::fail);
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28. Enhancements of Optional type in Java 9
    final Optional optional = Optional.empty();
    Optional newValue = optional.or(() -> Optional.of(new Object()));
    assertThat(newValue.isPresent()).isTrue();
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29. Enhancements of Optional type in Java 9
    List> optionals;
    optionals
    .stream()
    .flatMap(Optional::stream)
    .collect(Collectors.toList());
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30. We have even better Option (in Vavr)
    Option option = Option.of(new Object());
    boolean defined = option.isDefined();
    assertThat(defined).isTrue();
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31. We have even better Option (in Vavr)
    Option option = Option.none();
    boolean defined = option.isDefined();
    assertThat(defined).isFalse();
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32. We have even better Option (in Vavr)
    Option option = Option.of(null);
    boolean defined = option.isDefined();
    assertThat(defined).isFalse();
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    // and no exception is thrown here!
    

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33. We have even better Option (in Vavr)
    Option option = Option.of(new Object());
    Object returnedValue = option.getOrElse(() -> StringUtils.EMPTY);
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34. We have even better Option (in Vavr)
    final Option option = Option.of(new Object());
    option.forEach(object -> {
    if (option.isDefined()) {
    // handle object here
    } else {
    // object does not exist
    }
    });
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35. We have even better Option (in Vavr)
    final Option option = Option.of(new Object());
    option
    .peek(object -> /* handle object here*/)
    .onEmpty(() -> /* object does not exist*/);
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36. We have even better Option (in Vavr)
    private Object getObject() {
    throw new NullPointerException("Surprise! There's no Object!");
    }
    final Object object = Try .of(this::getObject)
    .toOption()
    .getOrElse(new Object());
    assertThat(object).isNotNull();
    // and no exception is thrown here!
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37. We have even better Option (in Vavr)
    Option option = Option.of("my string");
    String result = Match(option).of(
    Case($Some($()), String::toUpperCase),
    Case($None(), () -> StringUtils.EMPTY)
    );
    assertThat(result).isEqualTo("MY STRING");
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38. Option type and Vavr
    References:
    ● http://www.vavr.io
    ● http://www.vavr.io/vavr-docs/#_option
    ● https://softwaremill.com/do-we-have-better-option-here
    ● https://dev.to/koenighotze/in-praise-of-vavrs-option
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39. Optional won’t save us from NPEs
    Optional optional = Optional.of(new Object());
    ...
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    NullPointerException
    optional = null;
    optional.isPresent();
    

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40. Let’s talk about immutability
    Object object = new Object();
    object = null;
    object.toString();
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    NullPointerException!
    

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41. Let’s talk about immutability
    final Object object = new Object();
    object = null;
    object.toString();
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    Error: java: cannot assign a value to final variable object
    

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42. Let’s talk about immutability
    final Object object = new Object();
    object = null;
    object.toString();
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    Error: java: cannot assign a value to final variable object
    

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43. Let’s talk about immutability
    final Object object = new Object();
    object = null;
    object.toString();
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    Error: java: cannot assign a value to final variable object
    

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44. Let’s talk about immutability
    final Object object = new Object();
    object = null;
    object.toString();
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    Error: java: cannot assign a value to final variable object
    

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45. Let’s talk about immutability
    final Object object = new Object();
    object.toString();
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    Don’t modify the state of the data if you don’t have to
    

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46. Null safety in Kotlin
    value.getObject() // IntelliJ will highlight this as an error if there could be NPE!
    if (value != null) { // null-check
    value.getObject()
    }
    value?.getObject() // safe call
    val l: Int = if (b != null) b.length else -1
    val l = b?.length ?: -1; // Elvis operator
    value!!.getObject() // unsafe call - can produce NPE!
    46
    https://kotlinlang.org/docs/reference/null-safety.html
    

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47. Maybe type in RxJava
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    Maybe is lazy and asynchronous
    

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48. Maybe type in RxJava
    Maybe maybe = createMaybe();
    maybe
    .subscribeOn(Schedulers.io())
    .subscribe(object -> System.out.println(object.toString()));
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49. Maybe type in RxJava
    Maybe maybe = createMaybe();
    maybe
    .subscribeOn(Schedulers.io())
    .subscribe(new MaybeObserver() {
    @Override public void onSubscribe(Disposable d) {
    }
    @Override public void onSuccess(Object o) {
    }
    @Override public void onError(Throwable e) {
    }
    @Override public void onComplete() {
    }
    });
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50. Compile time analysis
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51. Error Prone
    Error Prone is a static analysis tool for Java developed at
    Google that catches common programming mistakes at
    compile-time.
    https://github.com/google/error-prone
    51
    

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52. Null Away
    NullAway is a tool developed at Uber built as an Error Prone
    plugin to help eliminate NullPointerExceptions (NPEs) in
    your Java code.
    https://github.com/uber/NullAway
    52
    

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53. Null Away
    static void log(Object x) {
    System.out.println(x.toString());
    }
    static void foo() {
    log(null);
    }
    warning: [NullAway] passing @Nullable parameter 'null' where @NonNull is required
    log(null);
    ^
    53
    

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54. Null Away
    Integration with Gradle-based Java project
    https://github.com/uber/NullAway#java-non-android
    Integration with Gradle-based Android project
    https://github.com/uber/NullAway#android
    http://wittchen.io/2017/09/15/integrating-nullaway-with-an-android-project/
    54
    

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55. Keeping up good practices in greenfield projects
    ● Choose one approach or several approaches
    ● Apply chosen solutions to your work routine and CI server
    ● Perform code reviews carefully
    ● Be consistent
    55
    

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56. How about legacy code?
    ● Apply proper rules to the new code (see previous slide)
    ● Perform null-checks and write more unit tests for the old code
    ● Keep backward compatibility when necessary
    ● Gradually deprecate old code and introduce new rules
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57. Are there languages without null?
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58. Are there languages without null/nil/None/etc.?
    58
    

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59. Languages without null
    Languages with Option type:
    ● Rust
    ● Ocaml
    ● Standard ML
    ● F#
    ● Scala
    Languages with Maybe type:
    ● Elm
    ● Haskell
    and others I could miss...
    Crystal has nil, but compiler prevents NPEs at compile time
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60. Summary
    ● Do not pass null
    ● Do not return null
    ● Return empty collections instead of null
    ● Use Optional/Option type for returning “nullable” sync data
    ● Use RxJava Maybe type for returning “nullable” async data
    ● Make data in your code as much immutable as possible
    ● Use compile time analysis and static code analysis
    ● Use language without null or with null-safety features when you can
    ● Be careful during code reviews
    ● Be consistent during applying good practices
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61. Fixing a billion dollar mi$take
    Thank you for attention! Questions?
    Piotr Wittchen
    wittchen.io
    github.com/pwittchen
    61
    

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