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• Requires source level >= JDK14 (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 an inline class are implicitly final
• "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 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 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 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 an inline type throws NoSuchMethodException
• setAccessible() on a an 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 an inline type
• ClassCircularityError thrown if loading an instance field of a an 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
  • Whereas null into indirect type array works as normal (since LW2)

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• platforms: x64 Linux, x64 Mac OS X, and x64 Windows
• no support for atomic fields containing indirect types
• no support for @Contended inline type fields
• no AOT, CDS, ZGC, serviceability agent, JVMTI, limited JNI
• -Xint and C2 only, no C1, no tiered-compilation, no Graal
• unsafe field and array accessor APIs are not supported for inline types
  • low-level unsafe APIs are UNSAFE and will not be changed to support inline types
  • risks: If a an inline type has been flattened in a container, unsafe does not know the layout
    • getObject could return the first flattened element rather than the expected reference
• interpreter is not optimized, focus is on JIT optimization
• Some major changes to JIT compiler are still a work in progress

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