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11. Generics and type erasure

Type erasure​

Java compiles generic types to their erased form: List<String> becomes List in bytecode. The JVM has no runtime knowledge of the type parameter. Reflection on Method.getGenericReturnType() returns the pre-erasure generic type string if the class was compiled with debug information; otherwise it returns the erased type.

GenericSignature​

typemap.GenericSignature parses JVM generic type descriptor strings of the form ClassName<Param1,Param2>. It supports:

  • Raw types (List with no params, IsRaw() == true)
  • Single-param types (List<String>, Optional<Integer>)
  • Two-param types (Map<String,Integer>)
  • Nested generics (Map<String,List<Integer>>, handled by splitTopLevelParams which tracks bracket depth)
  • Wildcard detection (WildcardErased() == true when any param contains ?)

EraseType / ErasedParams​

reflect.EraseType(name string) string strips the <...> suffix from a Java type name, returning the raw class name. ErasedParams returns the top-level type parameter strings. NormaliseTypeName converts JVM internal notation ($ inner-class separator and [] array suffix) to the form used in the reflection surface JSON.

Monomorphisation strategy​

For the type table, MEP-67 monomorphises only the supported generic containers:

  • List<T>: T must be in the closed table or the method is skipped with SkipUncheckedGeneric.
  • Map<K,V>: K and V must both be in the closed table.
  • Optional<T>: T must be in the closed table.
  • CompletableFuture<T>: T must be in the closed table.

This conservative strategy avoids the complexity of full parametric polymorphism while covering the most common generic APIs in Maven Central.