Cast List<Object[]> to Object[][] in Java

Question

How to turn List of arrays into two dimensional array in Java?

//Prepare the list
List<Object[]> conf = new LinkedList<Object[]>();
conf.add(new Object[]{ "FOO", "BAR"});
conf.add(new Object[]{ "FOO", "BAR"});

I tried:

Object[][] array = (Object[][]) conf.toArray(new Object[0]);

But it fails at ClassCastException:

java.lang.RuntimeException: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [[Ljava.lang.Object;

The issue is that Object[] is an Object so a List<Object[] becomes an Object[] because you passing in an Object[] to the method. — Boris the Spider
– Boris the Spider, Commented Apr 10, 2013 at 7:51
this is what you want stackoverflow.com/questions/5803619/… — BevynQ
– BevynQ, Commented Apr 10, 2013 at 7:52

NPE · Accepted Answer · 2013-04-10 07:51:18Z

10

You are missing a pair of square brackets:

    Object[][] array = conf.toArray(new Object[0][]);
                                                 ^^

Or, if you wish to save on one unnecessary memory allocation:

    Object[][] array = conf.toArray(new Object[conf.size()][]);

The cast to Object[][] is unnecessary once the argument to toArray() has the correct type.

answered Apr 10, 2013 at 7:51

NPE

503k114 gold badges970 silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

The cast to Object[][] is not necessary.