How to turn an ArrayList with Object[]'s into an Object[][] array?

Question

I have an ArrayList with Object[]'s and I need to convert the ArrayList into an Object[][] array in order to put the data in a JTable. How can I do this?

I have tried:

(Object[][]) arraylist.toArray();

but that gave:

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [[Ljava.lang.Object;

Sergey Kalinichenko · Accepted Answer · 2013-06-16T17:18:18.873

5

You can use toArray, like this:

Object[][] array2d = (Object[][])arraylist.toArray(new Object[arraylist.size()][]);

This is a non-generic version; generic toArray(T[] a) lets you avoid the cast.

edited Jun 16 '13 at 17:18

answered Jun 16 '13 at 16:52

Sergey Kalinichenko

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That gives an `Object[]` instead of an `Object[][]` according to Netbeans – jobukkit Jun 16 '13 at 17:16
@com.BOY It looks like you've got a non-generic list then - you need to add a cast. – Sergey Kalinichenko Jun 16 '13 at 17:17

score 2 · Answer 2 · answered Jun 16 '13 at 16:53

2

Try this, an explicit solution that takes into account arrays with variable lengths:

ArrayList<Object[]> input = new ArrayList<>();
Object[][] output = new Object[input.size()][];

for (int i = 0; i < input.size(); i++) {
    Object[] array = input.get(i);
    output[i] = new Object[array.length];
    System.arraycopy(array, 0, output[i], 0, array.length);
}

answered Jun 16 '13 at 16:53

Óscar López

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Instead of the second `for` loop, you can use `System.arraycopy(array, 0, output[i], 0, array.length);` – BackSlash Jun 16 '13 at 16:56
@BackSlash yes, that's nicer. Edited! – Óscar López Jun 16 '13 at 16:59

score 0 · Answer 3 · answered Jun 16 '13 at 17:17

0

    List<Object[]> list = new ArrayList();
    list.add(new Object[]{1,2,3});

    Object[][] objArray = new Object[list.size()][];
    for (int i=0;i<objArray.length;i++)
        objArray[i] = list.get(i).clone();

answered Jun 16 '13 at 17:17

Artem Kuchin

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How to turn an ArrayList with Object[]'s into an Object[][] array?

3 Answers3