Java反射：（Type）field.get（object）-未经检查的转换

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.

Answer 1

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.

Answer 2

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").