条件运算符中的类型不匹配（java）

Question

I don't understand why the following would NOT cause an error.

boolean condition = <whatever>;
return condition ? Collections.singleton("a field") : Collections.EMPTY_LIST;

Collections.singleton() returns a java.util.Set which obviously is NOT the same type as EMPTY_LIST .

Should that really NOT cause a type-mismatch error?

Answer 1

Both Collections.singleton() and Collections.EMPTY_LIST implement Collection (and extend Object for that matter). No type conversions are necessary to return the result of the ternary operator as a Collection or an Object .

The java compiler finds the most specific type that matches both parts. It gets tricky with primitive types, but only void is flatly forbidden. See the Java Language Specification on the conditional operator .

Answer 2

Starting point

This code by itself is not the whole picture – it contains a "return" statement, but we do not see the method signature, and thus do not see the return type.

boolean condition = true;
return condition ? Collections.singleton("a field") : Collections.EMPTY_LIST;

Method: return type Object

It would be totally fine to mix object types with a method return type of Object. For example, here's a method returning either a HashMap or an Integer. This is allowed because the method return type is Object, and both of those return types are Objects.

private static Object returnObject() {
    boolean condition = true;
    return condition ? new HashMap<>() : Integer.valueOf(1);
}

You could put the original code in a method with return type Object, and the compiler would be fine with it.

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Actual object types

The type of Collections.singleton("a field") is Set<String> , and Set has the following definition, showing it extends Collection:

public interface Set<E> extends Collection<E>

The type of Collections.EMPTY_LIST is List which has this definition, showing it extends Collection, too:

public interface List<E> extends Collection<E>

Both the set and the list share a common parent: Collection.

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Method: return type Collection

If you put the original lines of code along with a method signature having return type "Collection", that would be allowed by the compiler since both possible return values are Collections.

private static Collection returnCollection() {
    boolean condition = true;
    return condition ? Collections.singleton("a field") : Collections.EMPTY_LIST;
}

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Third option: Iterable

As shown above, the method can have a return type of either Collection or Object. Additionally, you could specify the return type as Iterable , since Iterable is the parent of Collection .