Why does DefaultListModel.toArray() throw a ClassCastException?

Question

I am trying to copy a DefaultListModel contents into an array. The following line causes the exception

testArray = (cGenIndicator[]) indObjList.toArray();

void testCasting() {
    DefaultListModel<cGenIndicator> indObjList;
    indObjList = new DefaultListModel<cGenIndicator>();
    indObjList.addElement(new cGenIndicator(null, null));

    cGenIndicator[] testArray;
    try {
        // This line causses exception saying
        // [Ljava.lang.Object; cannot be cast to [LIndicator.cGenIndicator;
        testArray = (cGenIndicator[]) indObjList.toArray();
    } catch(Exception e) {
        test++;
    }

    test++;
}

Mureinik · Answer 1 · 2018-10-01T11:57:39.877

7

toArray, without an argument, will return an Object[], that can't be cast to a cGenIndicator[]. Instead, you can use the overloaded method that gets an array to fill as an argument:

testArray = indObjList.toArray(new cGenIndicator[indObjList.size()]);

EDIT:

DefaultListModel does not have this overloaded method, Mia Kulpa. One way to convert an Object[] to a cGenIndicator is with streams:

testArray = Arrays.stream(indObjList.toArray())
                  .map(cGenIndicator.class::cast)
                  .toArray(cGenIndicator[]::new)

edited Oct 01 '18 at 11:57

answered Oct 01 '18 at 11:48

Mureinik

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it is a `DefaultListModel`, not an instance of a `List` – jmuet Oct 01 '18 at 11:50

score 2 · Accepted Answer · edited Oct 01 '18 at 20:13

2

DefaultListModel.toArray returns Object[], and Object[] can not be casted to cGenIndicator[] directly.

You can achieve it this way:

Object[] objectArray = defaultListModel.toArray();
int length = objectArray.length;

cGenIndicator[] testArray = new cGenIndicator[length];
System.arraycopy(objects, 0, testArray, 0, length);

edited Oct 01 '18 at 20:13

Kamil Gosciminski

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answered Oct 01 '18 at 11:54

xingbin

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Stephen C · Answer 3 · 2018-10-01T11:59:57.457

The DefaultModel::toArray() method produces an Object[] not a cGenIndicator[].

If this was a List, you would use toArray(new cGenIndicator[0]) instead. For more information, refer to the javadoc or to https://stackoverflow.com/a/5374346/139985. Note that the latter talks about String[] but the principle is exactly the same.

For a DefaultModelList you do not have this option:

The simple solution is to get rid of the type cast, and change to type of testArray to Object[].
You could also explicitly copy the elements of the Object[] to a more appropriately typed array. Whether this is worthwhile will depend on how you are going to use the array.

^{And please fix your class / interface names to conform to the Java style rules. cGenIndicator should be CGenIndicator ... or something more readable.}

Why does DefaultListModel.toArray() throw a ClassCastException?

3 Answers3