A Deep Dive into the Void

Transcription

1 A Deep Dive into the Void Java ready Advanced null Type Annotations Stephan Herrmann GK Software

2 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 2 An Old Problem 1965 Tony Hoare introduced Null references in ALGOL W simply because it was so easy to implement Photograph by Rama, Wikimedia Commons, Cc-by-sa-2.0-fr

3 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 3 Still Leading the Charts

4 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 4 An Expensive Problem The Billion Dollar Mistake Photograph by Rama, Wikimedia Commons, Cc-by-sa-2.0-fr

5 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 5 Many Steps to a Solution do nothing: booooooooom recognize patterns booooooom local flow analysis Since ALGOL W Since Eclipse boooom method contracts boom type checking Since Eclipse Juno / Java Since Eclipse Luna / Java 2014

6 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 7 Alternative Strategy Optional a polite way of saying: this value could be missing supports null-less programming doesn't help against existing nulls viable option for from-scratch clean-room new software requires all software to be re-written

7 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # Obviously Correct Typechecking for null analysis Two kinds of types those that include null those that don't This is OK: These are type errors: Dog? barkandreturn(dog dog) { dog.bark(); return null; } Dog barkandreturn(dog? dog) { dog.bark(); return null; }

8 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 9 Typesystem Construction Kit Strategy Richer types add more information Richer type checking new typing rules Stronger guarantees proven absence of errors no methodnotunderstood no ClassCastException JSR 30 org.eclipse.jdt.annotation_2.0 ECJ checks nullness compatibility & flow Stronger guarantees no NPE no SWTExecption(thread)

9 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 10 Ternary or Boolean? get 3 kinds of types green, red, legacy Interfacing with legacy code like using raw types in Java 5+ Null Null type type safety (type (type annotations): The The expression of of type type 'List<String>' needs needs unchecked conversion to to conform to to '@NonNull List<@NonNull String>' Nirwana: per class / interface per package (use package-info.java) let the compiler complain about And back to 2014

10 Where is: Everywhere? DefaultLocation declarations TYPE_USE PARAMETER RETURN_TYPE FIELD TYPE_PARAMETER TYPE_BOUND TYPE_ARGUMENT }) class Foo <T extends Bar> { Foo<String> stringfoo; Bar[][] bars; Bar getbar(string name) { return bar; } } Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 11

11 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 12 Where is: Everywhere? DefaultLocation Not supported non-null by construction new throws, catch argument this receiver in method reference would need runtime evaluation instanceof & cast void void make(@a Foo Foo this) Object::toString no usefull interpretation prefix of static Integer.MAX_VALUE future type declaration

12 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 13 Where is: Everywhere? Wildcards Wildcards and and the the use use of of type type variables variables are are always always excluded excluded from from NonNullByDefault. NonNullByDefault. DefaultLocation Not supported No default local variables better covered by flow analysis on type variables & wildcards these obtain nullness via other channels

13 Where is: Everywhere? DefaultLocation declarations TYPE_USE PARAMETER RETURN_TYPE FIELD TYPE_PARAMETER TYPE_BOUND TYPE_ARGUMENT }) class Foo <T extends Bar> { Foo<String> stringfoo; Bar[][] bars; Bar getbar(string name) { return bar; } } Default default Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 14

14 Where is: Everywhere? DefaultLocation declarations TYPE_USE PARAMETER RETURN_TYPE FIELD TYPE_PARAMETER TYPE_BOUND TYPE_ARGUMENT }) class Foo <T extends Bar> { Foo<String> stringfoo; Bar[][] bars; Bar getbar(string name) { return bar; } } Default default Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 15

15 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 16 Great Expectations ECJ will not make your program null safe It will hurt pin-point design decisions you failed to make contradictory assumptions type checker will need your help a false positive? rules & obligations Application development free to move obligations around Tension in API design strict for effective null checking flexible for wide range of re-use

16 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 17 Generic API The classic declaration Unconstrained type parameter: public interface List<T> { } Client side Free to choose the type argument: List<@NonNull Person> List<@Nullable Person> Implementer No knowledge about free type variable T Must assume the worst need to check on dereference cannot assign null to a T variable

17 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 1 Constraining Type Parameters Admit types: annotated type bound class MyGenericClass1<T Object> legal String Object X X annotated Admit types: type parameter (declaration) we can't say <T Object> class MyGenericClass2<@Nullable T> (interpreted as both upper and lower bound)

18 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 19 Puzzle 1 Legal or class Tricky<T> { } void foo(t t) {} void test() { foo(null); } where apply type parameter? no method parameter? generally yes but not to a type variable Null type mismatch (type annotations): 'null' is is not not compatible to to the the free type variable 'T' 'T' Wildcards Wildcards and and the the use use of of type type variables variables are are always always excluded excluded from from NonNullByDefault. NonNullByDefault.

19 Puzzle 2 What is class Tricky<T> { void foo(@nullable T t) {} void test() { foo(null); } } You said 'T' could mean anything so what T? e.g., if T String? variant of whatever T represents String Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 20

20 Stephan Herrmann: A Deep Dive into the Void - EclipseCon Europe 2014 # 21 DEMO Double Inference External Annotations