JDT embraces lambda expressions

8
8
Srikanth Sankaran
IBM India
Stephan Herrmann
GK Software
BETA_JAVA8

8 Eclipse and Java™ 8
● Java 8 features available in early access builds:
– JSR308 - Type Annotations.
– JEP120 - Repeating Annotations.
– JEP118 - Method Parameter Reflection.
– JSR269 - Pluggable Annotation Processor API &
javax.lang.model API enhancements for Java 8.
– Support for “code carrying” interface methods.
● Partial support for
– Lambda Expressions & Method/Constructor references
– Overload resolution & Type inference changes still evolving
● had been blocked by incomplete specification until late September
Srikanth Sankaran, Stephan Herrmann: Eclipse and Java 8 - EclipseCon Europe 2013 # 3

8 Eclipse and Java™ 8
● IDE features available in early access builds:
– Type annotations based static null analysis
● substantially complete, open issues exist
– Code completion, code selection/navigation, search engine
● for completed features
– Code formatter and code “reconciler”
– IDE enablement support:
● AST API's, programmatic AST rewriting API support etc
● Full fledged IDE support work in high gear:
– Early access build refreshes very soon.
● JDT team:
– 12 committers + 1 contributor + 1 GSOC student
+ 2 past committers on consulting role.
Java8
Java8

8 New Kinds of Methods
● defender methods
● virtual extension methods
● static interface methods
● default methods

8 New Kinds of Methods
Those are only 2 kinds :)
● Code carrying methods in interfaces:
– static methods
● non-OO programming even in interfaces
– default methods
previous names:
● defender methods
● virtual extension methods

8 Default Methods - Intention
● Intention
– support evolution of interfaces in libraries:
add methods to an interface w/o breaking implementors
– still legal:

8
Default Methods - Consequences
● Consequences
– Java 8 has multiple inheritance, sort-of
– but little help for corner cases
Root
m()
Both
Left
D m()
Right
D m()
resolve:

8
Default Methods - Consequences
● Consequences
– Java 8 has multiple inheritance, sort-of
– but little help for corner cases
Both
Left
D m()
Right
m()
The default method m() inherited from Left conflicts
with another method inherited from Right
Advice: Don't!

8 Default Methods - Compatibility
● Designed for compatibility during evolution
● For the price of compatibility
– Recipe for disaster:
● implement java.util.List → compile against JRE 7 → OK
● upgrade to JRE 8 but compile as 1.7
The type MyList<E> must implement the inherited abstract method Collection<E>.stream()
– Undefined compiler behavior
● 1.7 compiler cannot handle default methods (from .class)
● ecj and javac differ – no plans to assimilate
● depending on compiler implementation details, not specification

8 Default Methods - Compatibility
● Designed for compatibility during evolution
● For the price of compatibility
– Recipe for disaster:
● implement java.util.List → compile against JRE 7 → OK
● upgrade to JRE 8 but compile as 1.7
The type MyList<E> must implement the inherited abstract method Collection<E>.stream()
– Undefined compiler behavior
● 1.7 compiler cannot handle default methods (from .class)
● ecj and javac differ – no plans to assimilate
● depending on compiler implementation details, not specification
Advice: Don't mix -source 1.7 & JRE8!

8
λ
(JSR 335)

8
Introducing Lambda Expressions
● Lambda Expressions, Closures, Anonymous Methods
– what is in a name ?
● A function + “captured state” (may have non-locals)
– accessible even when outside of defining lexical scope
● Theoretical framework dates back several decades:
– Lambda (1930's – Church, Lisp – 1958)
– Closure (1960's - Landin, Scheme - 1975)
– Common idiomatic device in functional programming.
● Paradigm of passing a "code block as data"

8
Java 7 Style of “Code as Data”
● Via anonymous classes

8
Java 7 Style of “Code as Data”
● Via anonymous classes
So if lambda expressions
help model "code as data"
are they just
old wine in a new bottle?
No!

8 Lambda Expressions
● Eliminate thorny issues with anonymous classes
– Verbose/clunky Syntax
– Scoping
● anonymous classes introduce their own scopes
● interplay between names in enclosing scope ↔ inherited names
– Capture
● can only capture explicitly final outer locals
● necessarily capture enclosing class instance (memory leaks)
● Lambdas
– address the above, designed to be flexible & optimizable
– much more pleasing & efficient to program with call backs
– With default methods, enabled Collections overhaul.

8 λ Syntax and Examples

8 Target typing
● Lambda expression is basically
– an anonymous method
– with possibly "captured" enclosing state
● At the grammar level, lambdas are expressions (duh!)
– Can occur anywhere an expression can
– Introduces severe complications in parsing.
● Legally however,
– Lambda needs a context that defines a "target type"
– Everywhere else will have to rejected by the compiler.
– Assignment, initialization, casts, method/ctor invocation,
lambda expression body, conditional expressions and return
statements are contexts that provide target type.

8 Target Typing Examples

8 Functional Interfaces
● The target type must be a functional interface
● Functional interface
– Declares a single abstract method
– Some methods don't count:
● Default & static methods - they are not abstract
● May freely redeclare java.lang.Object's methods
– May be tagged with @FunctionalInterface to express intent
● So the lambda object implements the functional interface
● Another way of constructing objects
– apart from the SE7 ways
(new, reflection, serialization, clone ...)

8 Lambda Syntax examples
– Parameter types
● explicit, or
● inferred from target functional interface methods
– Body
● expression, or
● block.
– ()
● can be omitted if a singleton parameter

8 Lambda Syntax examples
– Parameter types
● explicit, or
Trade-off between
conciseness of expression
● inferred from target functional interface methods
– Body
● expression, or
● block.
– ()
● can be omitted if a singleton parameter
vs
clarity of code
-
in the hands of the programmer

8 Variable capture
● A lambda can refer to enclosing method's locals
– Captured state constitutes the "referencing environment"
– Compiler can infer finality
– Captured state is not mutable.
● Properly: capture of "immutable outer local's value"
– Mutability of outer locals implies race conditions
– Mutability would necessarily impose serial execution.
– Lambda creation & invocation could be far apart.
● may be in different threads, may be after enclosing method return
● This "constraint" actually enables parallelism.
● Enclosing class instance captured only if referred to.

8 Method References
● Similar to lambda expressions
– Require a target type that must be a functional interface
– Serve as instances of the functional interface
– Rather than providing a method body,
they refer to an existing method
– Several variants offered.

8
Lambda code generation
● Not lowered into anonymous classes - i.e not just syntactic sugar
● What does the following print ?
● Lambda object creation under the hood:
– Handled by "Lambda metafactory" API in JRE.
– Compilers generates suitable calls to the metafactory.
– Enables various optimizations.

8 Putting it all together
● Here is a nice example from SOTL[*]
[*] http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html

JDT features in the works 8

8
@
(JSR 308)

8 Annotations in more Places
● Java 5: annotate declarations
– ElementType: packages, classes, fields, methods, locals …
● Java 8: annotate types
– ElementType.TYPE_USE
– ElementType.TYPE_PARAMETER
So what?

8 Why Care About Types?
● Type = Constraints on values
To detect anomalies
– missing capability
– incompatible assignment
– undeclared capability
● Constraint checking avoids errors
– No Such Method / Field
● Basic statically typed OO
– ClassCastException
● Generics
– ??ExcepMtioyn pet example: NullPointerException

Let the Type System 8
Handle Nullity
● Ideally: Java would force explicit choice
– String definitely a String, never null
– String? either a String or null
– Type system ensures: no dereferencing of null
● Nullity as a language feature?
– Heavy weight, incompatible change
– Language change for each new constraint?

8 Pluggable Type System
● Make it easier to add new constraints
– Only one new syntax for all kinds of constraints
● Make it easier to add new type checkers
– Checker Framework (Michael Ernst – U of Washington)
● Examples
– @NonNull
– @Interned equals(== , equals)
– @Immutable value cannot change (Java 5 ?)
– @ReadOnly value cannot change via this reference
– @UI code requires to run on the SWT UI thread

Can't Java 5 Do All This?
@Target(ElementType.PARAMETER)
@interface NonNull5 {}
void java5(@NonNull5 String arg);
arg is qualified to be non-null
@Target(ElementType.TYPE_USE)
void java8(@NonNull8 String arg);
String is qualified to be non-null
String is qualified to be non-null
@Target(ElementType.METHOD)
@NonNull5 String java5();
java5 is qualified to be non-null
8
@Target(ElementType.TYPE_USE)
@NonNull8 String java8();
● We've been lying about the method result
– but we can't lie about everything, e.g.:
void printFirstDog(@NonNull List<Dog> dogs) {
dogs.get(0).bark();
}
NPE?

l1 cannot contain
null elements
Null type mismatch: required '@NonNull String'
but the provided value is null
8
void bad(List<String> unknown, List<@Nullable String> withNulls) {
@NonNull List<@NonNull String> l1 = new ArrayList<>();
l1.add(null);
String first = l1.get(0);
if (first == null) return;
l1 = unknown;
l1 = withNulls;
String canNull = withNulls.get(0);
System.out.println(canNull.toUpperCase());
}
@NonNull List<@Nullable String> l2 = new ArrayList<>();
l2 can contain
null elements
Annotated Generics
void good() {
l1.add("Hello");
for (String elem : l1)
System.out.println(elem.toUpperCase());
l2.add(null);
for (String unknown : l2)
if (unknown != null)
System.out.println(unknown.toUpperCase());
}
Null type mismatch: required '@NonNull String'
but the provided value is null

l1 cannot contain
null elements
Null comparison always yields false:
The variable first cannot be null at this location
8
l1.add(null);
l1 = unknown;
l1 = withNulls;
}
l2 can contain
null elements
Annotated Generics
void good() {
l1.add("Hello");
l2.add(null);
}
Null comparison always yields false:
The variable first cannot be null at this location

l1 cannot contain
null elements
Null type safety (type annotations): The expression of type 'List<String>'
needs unchecked conversion to conform to '@NonNull List<@NonNull String>'
8
l1.add(null);
l1 = unknown;
l1 = withNulls;
}
l2 can contain
null elements
Annotated Generics
void good() {
l1.add("Hello");
l2.add(null);
}
Null type safety (type annotations): The expression of type 'List<String>'
needs unchecked conversion to conform to '@NonNull List<@NonNull String>'

l1 cannot contain
null elements
Null type mismatch (type annotations):
required '@NonNull List<@NonNull String>'
but this expression has type 'List<@Nullable String>'
8
l1.add(null);
l1 = unknown;
l1 = withNulls;
}
l2 can contain
null elements
Annotated Generics
void good() {
l1.add("Hello");
l2.add(null);
}
Null type mismatch (type annotations):
required '@NonNull List<@NonNull String>'
but this expression has type 'List<@Nullable String>'

l1 cannot contain
null elements
Potential null pointer access:
The variable canNull may be null at this location
8
l1.add(null);
l1 = unknown;
l1 = withNulls;
}
l2 can contain
null elements
Annotated Generics
void good() {
l1.add("Hello");
l2.add(null);
}
Potential null pointer access:
The variable canNull may be null at this location

8 Framework Development (1)
● So you want to be smart:
public interface ILib<E> {
@NonNull E work(@Nullable E e);
}
● But will clients like it?
public class LibImpl implements ILib<String> {
@Override
public @Nullable String work(@NonNull String e) { return null; }
● If thou usest annotations know thy co/contravariance
– @Nullable parameter is irreversible
– Design bug in guava's Predicate / Function
}

8 Framework Development (2)
● Avoid overspecification, make interfaces null-generic:
public interface Function<I,O> {
O apply(I in);
}
● Let clients decide
class ToString implements Function<@NonNull Object, @Nullable String> {
@Override
public @Nullable String apply(@NonNull Object o) { return o.toString(); }
No NPE!
}
● Putting the pieces together
<@NonNull I, @Nullable O> Collection<O> map1(Collection<I> in, Function<I, O> f) { … }
<@Nullable I, @NonNull O> Collection<O> map2(Collection<I> in, Function<I, O> f) { … }
...
List<@NonNull Object> in = new ArrayList<>();
Collection<@Nullable String> out = map1(in, new ToString());

8 Caveat: Arrays
void test (@NonNull String [] stringsOrNulls) {
System.out.println(stringsOrNulls[0]);
● Semantics are changing from Java 7 to Java 8
}
array of nonnull elements the array can still be null Þ NPE
void test (String @NonNull [] stringsOrNulls) {
System.out.println(stringsOrNulls[0]);
}
nonnull array NPE- safe (but may print “null”)

8
Status: Annotated Type Analysis
● Implemented EA: @NonNull, @Nullable
– Much analysis already performed
● known & unknown bugs
– Editor may show more bogus errors
● Project > Clean and Problems View
● Planned: @Uninterned
● Proposed: @UiEffect, @Ui …
– by [Colin S. Gordon, Werner Dietl, Michael D. Ernst, and Dan Grossman]
– SWT: bye, bye, “Invalid thread access”

8 Status: Type Annotations
● Supported by
– Compiler
– DOM AST
– APT
● Work in progress
– respect during
CCttrrll CCttrrll 1 CAtlrtl T
● Not yet supported by
– Java model

8 TypeBinding Backstage Story
aka “Symbol”
● Type bindings are “interned”
– OK to use ==
● Broken by encoding type annotations in type bindings
● Solution
– Find/replace == comparisons for T <: TypeBinding
– Tweak our compiler to do this
– Plan: publish the tweak, controlled by @Uninterned

8
Dramatis personæ
● Jay Arthanareeswaran
● Anirban Chakarborty
● Manoj Palat
● Shankha Banerjee
● Manju Mathew
● Noopur Gupta
● Deepak Azad
● Srikanth Sankaran
● Olivier Thomann
● Andy Clement
● Michael Rennie
● Jesper S. Møller
● Walter Harley
● Stephan Herrmann
● Dani Megert
● Markus Keller
● Early Access:
– http://wiki.eclipse.org/JDT_Core/Java8
● We invite you to test - defect reports welcome !

JDT embraces lambda expressions

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Ähnlich wie JDT embraces lambda expressions

Ähnlich wie JDT embraces lambda expressions (20)

Mehr von Eclipse Day India

Mehr von Eclipse Day India (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

JDT embraces lambda expressions