The current state of the primitives in generics implementation in project valhalla

1. Primitives in Generics
current state
Ivan St. Ivanov

2. Important disclaimer
 This is work in progress
 Likely to change
 I am not an expert
 Based on the State of Specialization
document by Brian Goetz (see
resources)
 Same things apply to Value Types

3. The state of generics
 Only over reference types
 If you want List<int>:
List<Integer> intList = new ArrayList<>();
 Performance penalty:
Heade
r
Pointer
Pointer
Pointer
Pointer
Heade
r
int Heade
r
int Heade
r
int Heade
r
int

4. But why?
 Type erasure in generics
public class Box<T> {
private T value;
public Box(T value) {
this.value = value;
}
public T get() {
return value;
}
}
 At runtime T is erased to Object
 No common “Object” for primitive and
reference types

5. Why erasure at all?
 Use case:
◦ class C uses (or extends) class A
◦ class A is generified
 Source and binary compatibility
◦ If class C does not compile: source
incompatibility
◦ If class C should be recompiled to link:
binary incompatibility
 No flag day allowed!

6. Possible approach
 Project Valhalla
 Compromise #1: current rules for
generics should not be changed
 Compromise #2: use separate
approaches for representing different
types
 Compromise #3: subclass rules:
ArrayList<int> <: List<int> YES
List<int> <: List<Integer> NO
List<int> <: List<?> NO
List<int> <: List NO

7. Proposed syntax
 Introduce any type variable modifier
public class Box<any T> {
private T value;
public Box(T value) {
this.value = value;
}
public T get() {
return value;
}
}
 And then:
Box<int> intBox = new Box<>(42);

8. What about byte code?
ErasedBox (for reference types)
SpecialBox (for primitive types)

9. Side note: translations
 Heterogeneous translation
◦ Different runtime class for each different
parametric type
◦ Hard to achieve data parametricity (Box<?>)
◦ C++ templates
 Homogeneous translation
◦ Same runtime type for all different parametric
types
◦ Java achieved it for reference types
◦ C# for reference and structural types
◦ .NET byte code can range over both types,
Java - not

10. Representation in bytecode
 Erasure of reference types,
specialization for primitives
 Look again at SpecialBox usage site:
 Class name augmented with
specialization info

11. Restrictions for any T
 Cannot be converted or compared to
null
 Cannot be converted to Object or
Object[]
 Cannot be used as lock for
synchronization
 Cannot convert Foo<any T> to Foo<?>
or Foo

12. Available only for <any T>
 Support for new T[size]
 Use it where reference type is allowed
(instanceof)
 Foo<any T>.class

13. Generic methods
 Workaround for not allowing raw types
to be passed when <any T> is
expected
 Implementation:
◦ A version of the <any T> method should be
specialized
◦ Assumption: methods in a class are fixed
◦ invokedynamic is used

14. <any T> generic method
public class SpecializedMethod {
public <any T> T returnValue(T value) { return value; }
public static void main(String[] args) {
SpecializedMethod m = new SpecializedMethod();
m.returnValue(42);
}
}
 Bytecode at call site:

15. Migration challenges
 Wrong assumptions in some classes:
T[] array = (T[]) new Object();
 Problematic overloading
remove(int position);
remove(T element);
 Incompletely generified methods
remove(Object)
 There’s no null value for primitives

16. Possible approach: peeling
 Implement the backward compatibility method in
reference-specific layer
 Add new methods in generic layer
interface List<any T> {
void removeByValue(T element);
void removeByIndex(int pos);
layer<ref T> {
void remove(int pos);
void remove(T element);
default void removeByIndex(int pos) { remove(pos); }
default void removeByValue(T t) { remove(t); }
}
}
 The erased class will have all methods
 The specialized class will only have generic layer
methods

17. Time for experiments
 ArrayList or Optional class that is generic over
<any T>
 Write generic method over <any T> and pass:
reference type, primitive and raw type
 Specialized classes as method parameters,
return types of methods in erased generic or
non-generic classes
 Usage of static member variables and static
methods of specialized classes
 Inner classes combination (specialized in
specialized, specialized in erased, erased in
specialized, specialized in non-generic)
 Create new array of T inside specialized generic
class
 Call instanceof or .class

18. Resources
State of Specialization (by Brian Goetz)
http://cr.openjdk.java.net/~briangoetz/va
lhalla/specialization.html
Stewardship the Sobering Parts
https://www.youtube.com/watch?v=2y5
Pv4yN0b0
Source, binary and behavioral
compatibility
https://blogs.oracle.com/darcy/entry/kin
ds_of_compatibility