Erasure and reified arrays

Type erasure: generics exist at compile time only. ArrayList<String> and ArrayList<Integer> are the same class at runtime — one ArrayList class, checked at the door by the compiler, unverified inside. The compiler inserts casts where values leave generic boundaries and generates bridge methods when overrides need to match an erased signature.

List<String> a = new ArrayList<>();
List<Integer> b = new ArrayList<>();
a.getClass() == b.getClass()   // true — one erased class

Arrays are the opposite: reified and covariant.

Object[] objs = new String[1];   // legal — arrays are covariant
objs[0] = 42;                    // ArrayStoreException AT RUNTIME

The array knows its element type and enforces it with a runtime check; the generic list cannot even know its type argument. This is why new T[] is illegal, why instanceof List<String> cannot be written, and why String[].class != Integer[].class while the two lists share a class. Covariance + reification = the runtime store check; invariance + erasure = the compile-time door check. Two different safety strategies.