#998 False positive ClassCastExceptionWithToArray with generics

[Anonymous]

pmd reports "This usage of the Collection.toArray() method will throw a ClassCastException." for the first code line of this "method":

  public static <E> E[] toArrayWrapped(Collection<E> c) {
    E[] retVal = (E[]) c.toArray();
    return retVal;
  }

But this is not true. It would probably be true if E were a concrete type, but for generic types with erasure Object it is not, and in fact such a cast seems to be the shortest way of getting an array of type E.

[Andreas Dangel]

• Description has changed:

Diff:

--- old
+++ new
@@ -1,8 +1,11 @@
 pmd reports "This usage of the Collection.toArray() method will throw a ClassCastException." for the first code line of this "method":

+~~~~~
+:::java
   public static <E> E[] toArrayWrapped(Collection<E> c) {
     E[] retVal = (E[]) c.toArray();
     return retVal;
   }
+~~~~~

 But this is not true. It would probably be true if E were a concrete type, but for generic types with erasure Object it is not, and in fact such a cast seems to be the shortest way of getting an array of type E.

• status: open --> wont-fix
• assigned_to: Andreas Dangel
• module: --> PMD
• milestone: --> PMD-5.0.3
• priority: 5 --> 4-Minor
• type: --> Bug
• affects_version: -->

[Andreas Dangel]

Yes, you won't get a class cast exception on this line.
However, this utility method is still questionable for me.

Consider this test class:

import java.util.ArrayList;
import java.util.Collection;
import org.junit.Test;

public cass ToArrayTest {
    @Test
    public void testWrap() {
        Collection<String> c = new ArrayList<String>();
        c.add("String1");
        c.add("String2");
        Object o = BasicRulesTest.<String> toArrayWrapped(c);
        System.out.println(o);            // [Ljava.lang.Object;@199836ed
        System.out.println(o.getClass()); // class [Ljava.lang.Object;

        // ClassCastException here: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
        String[] array = BasicRulesTest.<String> toArrayWrapped(c);
    }

    public static <E> E[] toArrayWrapped(Collection<E> c) {
        E[] retVal = (E[])c.toArray();
        return retVal;
    }
}

As you see, you'll get your class cast exception later on, as your method "toArrayWrapped" won't always return the correct array type.
Btw. - it works, if you use Arrays.asList("foo", "bar") because the collection implementation internally uses then a String[] array. ArrayList however uses a Object[] array to store the data.

The correct way to convert a collection to an array is still using Collection.toArray(T[] a) see javadoc.

That's why I'm closing this bug as won't fix. Feel free to reopen it, if you see a different solution.