Java Reflection: (Type) field.get(object) - unchecked cast

Question

Following code retrieves the value of a field using the Reflection API. As you can see in the provided image, this generates an unchecked cast warning. The warning can be suppressed using @SuppressWarnings("unchecked"). I was wondering if there is an alternative to this though?

Update: KeyType is a generic. So KeyType.class.cast(object); wouldn't work due to type erasure.

private K getId(Field idField, V o) {
    K id = null;
    try {
        idField.setAccessible(true);
        id = (K) idField.get(o);
    } catch (IllegalAccessException ignored) {
        /* This never occurs since we have set the field accessible */
    }
    return id;
}

Solution: Seems like the SuppressWarnings annotation IS the way to go here.. thanks for your time guys.

KeyType is a generic. Due to type erasure this won't work. Any other ideas? — Jasper Catthoor, Nov 02 '15 at 15:49
That is kind of important information, which should be in question... Please edit you post and include this info there. — Pshemo, Nov 02 '15 at 15:51
Also to avoid such confusion when reading code later we generally use one letter names for generic types like `K` for key type, `V` for value type and so on. — Pshemo, Nov 02 '15 at 15:52

laune · Accepted Answer · 2015-11-02T16:01:56.220

The Class method cast doesn't require the @SupressWarnigns even though it still can throw ClassCastException.

KeyType keyType = KeyType.class.cast( idField.get(o) );

You can - since at that location you should know the generic paramter(s) - proceed like this:

private static class ListInteger extends ArrayList<Integer>{}

Object obj = new ArrayList<Integer>();
ListInteger test = ListInteger.class.cast(obj);

Once you have an object of class KeyType you can, of course,

KeyType keyTypeX = ...; // not null

KeyType keyType = keyTypeX.getClass().cast( obj );

There are alternatives, although the @SuppressWarnings isn't so bad - try to limit it to an declaration+assignment, don't put it on the method.

Sorry, I should have mentioned that KeyType is a generic. Due to type erasure your solution won't work. I'm considering passing the KeyType's class type to the constructor of this class though. — Jasper Catthoor, Nov 02 '15 at 15:52

score 1 · Answer 2 · answered Nov 02 '15 at 15:55

The signature of get method of Field is

public Object get(Object obj) { 
 ...
}

Since it is not generic, the return type is Object, you cannot enforce any error to appear at compile-time.

If you are sure that the type of the value is ValueType all the time, then add documentation to the method, saying the type will be always ValueType and use @SuppressWarnings("unchecked").

Java Reflection: (Type) field.get(object) - unchecked cast

2 Answers2