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  1. Dashboard
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  4. L-World
  5. LW2

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changes.mady.by.user David Simms

Saved on Jul 01, 2019

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changes.mady.by.user Karen Kinnear

Saved on Jul 01, 2019

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Comment: minor edits

LW2 is an iteration of a previous prototype, adding further language support and JDK API support for "inline - types" (formerly referred to as "value types"). There are also subtle type system, runtime and JIT changes

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  • Requires source level >= JDK13 (since LW2, the prototype is based on mainline JDK, whose version string is JDK 14 at the time of writing)
  • A class declaration of inline type is made by using the "inline class" modifier, or (for IDE syntax parsing convenience) with "@__inline__" annotation (since LW2)
    • Interfaces, annotation types, enums can not be inline types
    • Top level, inner, nested, local classes may be inline types. Inline types may declare inner, nested, local types
    • Inline Types are implicitly final, so cannot be abstract
  • Inline Types may not declare an explicit super class (except Object). They implicitly extend java.lang.Object akin to enums, annotation types and interfaces
  • Inline Types may declare explicit interfaces
  • Inline Types constructors may not pass the "this" handle until all instance fields are definitely assigned.
  • All instance fields of a inline class are implicitly finalInline types may not to declare fields of its own type directly.
    There is a method to do this "indirectly" using the "?" operator, but it's value is still final. (since LW2)
  • "Indirect" projections of inline types via the "?" operator (since LW2)
    • E.g. the projection of Point is Point?
    • Point? IS also an inline type which allows null for backward compatibility
    • maintains type of a field,  array element, or generic parametric argument, but they may be null
    • Point is a subtype of Point?
    • cast between inline type and indirect type
    • Is implicitly a "NullableType"
    • enables compatibility with current generics via erasure to null-object object references
    • Arrays subtyping relationships: InlineType[] <: IndirectType[] <: Object[] <: Object
      • Widening conversions may be carried out from in the order above
      • Narrowing from an array of IndirectType to an InlineType is not possible.
      • This array covariance enables compatibility with existing API, especially generic type erasure
  • Inline types may not declare fields of its own type directly.  If A and B are inline classes, A may not contain a field B if B contains a field A.
    • There is a method to do this "indirectly" using the "?" operator, but it's value is still final. (since LW2
  • java.lang.Object methods:
    • javac automatically generates hashCode, equals, longHashCode and toString computed solely from the instance's state and not from its identity
    • javac does not clone(), finalize(), wait*), notify*() on inline type receivers
  • javac allows comparison of inline type using ==, != (since LW2)
    • By default this implies a test in the instance state (i.e. field by field "substitutability" test), as inline type types have no identity (i.e. not a reference test).
  • Inline Types can not be assigned null, null can not be cast to or compared with inline types
    • With the exception if of an "indirect" reference using the "?" operator. (since LW2)
  • Indirect Types cannot be type arguments in generic type parameterizations, type witnesses in generic method invocations, wildcard bounds
    • With the exception if of an "indirect" reference using the "?" operator. (since LW2) 
  • Type migration, including partial recompilation is not supported.

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  • Class new /modified API:
    • isInlineClass()
    • asPrimaryType()
    • asIndirectType() / isIndirectType()
    • asNullableType() / isNullableType()
    • getName() reflects the Q or L type signatures for arrays of inline types (since LW2)
      • In the .class file, the null-free inline type today is represented by a Q type signature, and the indirect projection is represented by an L type descriptor
    • newInstance() on a an inline type throws NoSuchMethodException
  • setAccessible() on a inline type throws InaccessibleObjectException
  • Initial core Reflection and VarHandles support in place (since LW2)
  • Attempts to build Reference objects with inline type will result in IllegalArgumentException

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  • IllegalMonitorException thrown on attempts to synchronize or call wait(*) or notify*() on a an inline type
  • ClassCircularityError thrown if loading an instance field of a inline type which declares its own type either directly
  • Attempts to serialize an inline type will throw NotSerializableException
  • Cast from indirect type to inline type may result in NullPointerException (since LW2)
  • Attempts to store "null" into Inline type array will throw NullPointerException
    • Where as Whereas null into indirect type array works as normal (since LW2)

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  • Use cases that worked for you, including any positive performance or footprint information
  • Bugs or questions about surprise behavior
  • We have performance holes, if you have a problem, please provide a use case for us to consider

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