Why does this code with generics throw a ClassCastException in Java 11?

Question

The code below succeeds in Java 8 but throws a ClassCastException in Java 11. Why did the behavior change?

I could not find any related changes in OpenJDK's Java 9, Java 10 or Java 11 feature sets.

public class GenericsExample {

    public static void main(String[] args) {
        Set<Car> set = new HashSet<>();
        set.add(getAnimal());
    }

    static <T extends Animal> T getAnimal() {
        return (T) new Animal() {};
    }

    interface Animal {}

    class Car {}
}

Answer 1

Indeed it was a bug in Java 8, which was fixed in Java 9 - bugfix. In some scenarios, the javac CHECKCAST instruction was skipped.

If you are curious, consider those 2 lines of code:

Set<Car> set = new HashSet<>(); // line 11
set.add(getAnimal());           // line 12

Java 8 bytecode will look like this:

 LINENUMBER 11 L1
 ALOAD 1
 ALOAD 0
 INVOKEVIRTUAL UserManagerTest.getAnimal ()LUserManagerTest$Animal;
 INVOKEINTERFACE java/util/Set.add (Ljava/lang/Object;)Z (itf)
 POP

But Java 9 will look like this:

LINENUMBER 11 L1
ALOAD 1
ALOAD 0
INVOKEVIRTUAL UserManagerTest.getAnimal ()LUserManagerTest$Animal;
CHECKCAST UserManagerTest$Car
INVOKEINTERFACE java/util/Set.add (Ljava/lang/Object;)Z (itf)
POP

The only difference is CHECKCAST instruction, which (according to JavaDoc) states that the named class, array, or interface type is resolved. If object can be cast to the resolved class, array, or interface type, the operand stack is unchanged; otherwise, the checkcast instruction throws a ClassCastException.