What Is Left of List<String> by the Time the Program Runs

Two variables, declared with different type arguments, and a comparison of their classes that says they are the same.

java

List<String> a = new ArrayList<>();
List<Integer> b = new ArrayList<>();

System.out.println(a.getClass() == b.getClass());

text

true

Type arguments are checked by the compiler and then erased. The bytecode contains one ArrayList class, holding Object references, exactly as it did before generics existed.

What the compiler does and what the runtime sees
Written	At compile time	At runtime
`List<String>`	A list that only accepts strings	A list
`refs.get(0)`	Produces a `String`	Produces an `Object`, cast to `String`
`T` in a generic class	A real type	`Object`

Erasure was chosen so that code written before Java 5 kept running unchanged on the same virtual machine. It worked, and the price is paid by every strange generics rule you will meet.

So when the compiler refuses something about generics that looks perfectly reasonable, the reason is nearly always that the information you are relying on is gone by the time it would be needed.