Java if ternary operator and Collections.emptyList()

Question

Could you please explain why with the first return type the code can't be compiled? The message is : Type mismatch: cannot convert from List<capture#1-of ? extends Object> to List<String>.

Is there inserted an explicit cast in the second case ?

public class GenericsTest {

        private String getString() {
            return null;
        }

        public List<String> method() {
            String someVariable = getString();
            //first return type
            //return someVariable == null ? Collections.emptyList() : Collections.singletonList(someVariable);
            //second return type
            if (someVariable == null) {
                return Collections.emptyList();
            } else {
                return Collections.singletonList(someVariable);
            }
        }
    }

Answer 1

Because of type inference rules. I don't know why exactly (you should check the JSL, the ternary operator section), but it appears the ternary expression does not infer the type parameter from the return type.

In other words, the type of the ternary expression depends on the types of its operands. But one of the operands has undetermined type parameter (Collections.emptyList()). At that point the ternary expression still does not have a type, so it cannot influence the type parameter. There are two types to be inferred - one is the result of the ternary expression, and the other is the type parameter of the .emptyList() method.

Use Collections.<String>emptyList() to explicitly set the type

Answer 2

The type of the expression flag ? trueCase : falseCase is the most common type of the two cases.

In this case the most common type of Collections.emptyList() and Collections.singletonList(someVariable) is List<? extends Object> because it cannot "see in the future" that Collections.emptyList() should return List<String> in the expression.

---

When you do:

return Collections.emptyList();

the compiler can be smart and detect the type by the return type and check the correctness (inferred).