Java 8 Incompatible Types Error

Question

Set<Object> removedObjs = new HashSet<>();
List<? extends MyEntity> delObjs = (List<? extends MyEntity>) new ArrayList<>(removedObjs);

MyEntity is marker interface.

Above code is working fine in java-7(java version "1.7.0_91", to be precise) but not in java-8(java version "1.8.0_77")

In Java8, I am getting the following exception:

incompatible types: ArrayList<Object> cannot be converted to List< ? extends MyEntity>

Answer 1

Your code should neither work in Java 7 nor in Java 8, because you are trying to cast an ArrayList<Object> (type returned from the constructor) to a List<? extends MyEntity> and it has no sense even at compile time (see this question). That's why the compiler is complaining.

The following code will compile:

Set<Object> removedObjs = new HashSet<>();
List<? extends Integer> delObjs = (List) new ArrayList<>(removedObjs);

However, the compiler will throw a warning since you are performing an unsafe conversion.

EDIT: As @Tom points out:

"Your code should not work in java 7" It shouldn't, but it worked, because the compiler checks were still a bit faulty in that version. This has been fixed in Java 8, that is why it now fails.