Object semantics, equals and hashCode contracts, generics, exceptions, and immutability.
3 questions in Java & Spring.
Generic type arguments exist only for the compiler; the bytecode keeps a raw type plus a signature string, which is why there is no T.class and no new T[]. Wildcards then follow producer-extends, consumer-super: read through ? extends, write through ? super.
Equal objects must produce equal hash codes. Break that and a HashMap sends lookups to a bucket the entry was never placed in, so the map holds a value you can no longer retrieve and a set silently admits duplicates. The same failure appears later if a key is mutated after insertion.
Boxing produces objects, so == compares references. The specification requires small values to come from a cache, which is why comparisons on small numbers appear to work and fail above the cached range. The other costs are a NullPointerException on unboxing and overload resolution preferring the primitive method.