how casting this object to generic type work?

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Category:Languages

From my prerequisites, generic types are invariant, so I can say if we have B subtype of A, then List<B> has no relationship with List<A>. So casting won't work on List<A> and List<B>

From effective java third edition we have this snippet of code:

// Generic singleton factory pattern private static UnaryOperator<Object> IDENTIFY_FN = (t) -> t;  @SuppressWarnings("unchecked") public static <T> UnaryOperator<T> identifyFunction() {     return (UnaryOperator<T>) IDENTIFY_FN; //OK But how, why? }  public static void main(String[] args) {     String[] strings = {"a", "b", "c"};     UnaryOperator<String> sameString = identifyFunction();     for (String s : strings) {         System.out.println(sameString.apply(s));     } } 

Here I am confused, we have casted IDENTIFY_FN which its type is UnaryOperator<Object> to UnaryOperator<T> which has another type parameter.

When type erasure arrives String is a subtype of Object, but from what I know UnaryOperator<String> is not a subtype of UnaryOperator<Object>.

Do Object and T relate somehow? And how does casting succeed in this case?

 


Generics don't exist at runtime. At runtime, every UnaryOperator<T> is a UnaryOperator<Object>. The cast is necessary to placate the compiler at compile-time. At runtime it's meaningless.

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