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>
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.
jdk1.7.0_91 at hand, but I tested with jdk1.7.0_13, jdk1.7.0_40, jdk1.7.0_51, jdk1.7.0_80, and Eclipse (with Java7 conformity). None of them accepted the code of the question.
Java 8 changed a lot about type inference and related subjects. As such, it's not surprising if some edge cases (such as shady casts) suddenly become invalid.
Whatever the reason is, you can still cast your list, but it's slightly more ugly than before:
(List<? extends MyEntity>) (List) new ArrayList<>(removedObjs);
As pointed out by Peter Lawrey's comment, this can be written even shorter as
(List) new ArrayList<>(removedObjs);
<>you should get just a warning. If it doesn't work in Java 8, then it probably shouldn't have worked in Java 7.removedObjs.add("42");between the two lines to see why.