Java Method-Local Inner Classes are Unsound
summary
Claim: Java’s combination of method-local inner classes plus erased generics and class-based visibility lets a non-generic local class implicitly tied to a method type parameter escape one invocation and be reused under a different instantiation, producing a value typed as Foo<String> that actually contains an Integer and later throws ClassCastException. JVM erasure makes instanceof and casts succeed (no compiler warnings), raw types and overload-driven checkcasts mask the error, and languages like Scala/Kotlin mitigate it by warning or rejecting such checks.
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chapters (9)
- 0:00Method-local inner classes
- 1:47What is a method-local inner class
- 3:13Java generics interaction
- 4:20Unsoundness example
- 11:06Simpler unsoundness example
- 12:08JVM bytecode analysis
- 14:54Other JVM languages
- 18:22Core issue and solutions
- 20:07Q&A
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