[java] UnnecessaryCast false positive with unchecked cast

[maikelsteneker]

Affects PMD Version: 6.26.0

Description:

The rule UnnecessaryCast tells me that I'm doing a cast that is not necessary, but omitting it results in a compiler error.

This refers to the cast of (T[])list.toArray() from the example below. The toArray() methods returns an Object[], not a T[]. There is an alternative method available that takes an array as argument and uses the type of that for its return value, but in a case like this, where T is a type parameter instead of a concrete type for which an array can be initialized, that alternative method can't be used either.

Code Sample demonstrating the issue:

    /** Returns all elements from the given iterator in a new array */
    @SuppressWarnings("unchecked")
    public static <T> T[] asArray(Iterator<T> elements) {
        final List<T> list = new ArrayList<T>();
        while( elements.hasNext() ) list.add( elements.next() );
 
        T[] result = toArrayC( (Class<T>)null, list );
        if( result == null ) result = (T[])list.toArray();  // <----- this cast is really necessary
        return result;
    }

Running PMD through: CLI

[tweimer]

That cast (T[])list.toArray() will even cause a ClassCastException at runtime if T is not an Object.

See the PMD rule ClassCastExceptionWithToArray for an example.

[oowekyala]

That cast (T[])list.toArray() will even cause a ClassCastException at runtime if T is not an Object.

For reference, the cast is unchecked because T is erased to Object. So (T[]) is erased to (Object[]) and is a noop. But a caller might see a ClassCastException if they store the result in a reified array type that is not Object[], eg String[]:

Iterator<String> iter = Arrays.asList("a").iterator();
String[] array = asArray(iter); // boom

[adangel]

This has been fixed with PMD 7.0.0-rc1.