Pass an array of objects instead of an indeterminate number of objects

Question

I have a function that gets as parameter an indefinite number of ImageButtons.

private void addButtons(ImageButton... ib) {
// ...
}

So this is perfect if I want to call it for example this way:

addButtons(button1, button2, button3);

But now it happens to me that I have to use as a parameter an unknown number of objects, for example an array. Inside addButtons the ImageButton parameters are used as an array, so I tried this:

ArrayList<ImageButton> ibs = new ArrayList<ImageButton>();
// feed the ibs ArrayList
addButtons((ImageButton[])ibs.toArray());

And I get a ClassCastException.

Why?

Alexis C. · Accepted Answer · 2013-11-04 16:48:15Z

5

Actually the method toArray() returns an Object[]

You have to use the overloaded method toArray(T[])

You should use :

addButtons(ibs.toArray(new ImageButton[ibs.size()]));

edited Nov 4, 2013 at 16:48

answered Nov 4, 2013 at 16:43

Alexis C.

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8 Comments

rolfl Over a year ago

You mean: ibs.toArray(new ImageButton[ibs.size()]))

Beppi's Over a year ago

ZouZou on Android I cannot find toArray(int x), there's not such a function.

Alexis C. Over a year ago

@Beppi's That ? developer.android.com/reference/java/util/…

Beppi's Over a year ago

Thank you, it works!!! I use it. I still don't understand why, anyway: it clearly states that it's not necessary to specify the array: "Returns an array containing all elements contained in this ArrayList. If the specified array is large enough to hold the elements, the specified array is used, otherwise an array of the same type is created."

rolfl Over a year ago

@Beppi's toArray() returns Object[] but toArray(new ImageButton[...]) returns ImageButton[], and addButtons expects ImageButton[]

|

Sotirios Delimanolis · Accepted Answer · 2013-11-04 16:43:52Z

1

Use the overloaded toArray() method where you can specify the array type.

addButtons(ibs.toArray(new ImageButton[0]));

You can specify the size if you want to. If you don't, the method will simply create a bigger array before adding the elements.

answered Nov 4, 2013 at 16:43

Sotirios Delimanolis

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1 Comment

Beppi's Over a year ago

That's smart. I don't need to specify the size.

SLaks · Accepted Answer · 2013-11-04 16:44:13Z

1

Due to type erasure, a generic collection cannot create a strongly typed array unless you help it out:

addButtons(ibs.toArray(new ImageButton[ibs.size()]));

Unless you pass it an array of the correct type, it can only return an Object[].

answered Nov 4, 2013 at 16:44

SLaks

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3 Comments

rolfl Over a year ago

Just being pedantic.... the Array-type has nothing to do withgenerics and erasure. This was true before that: docs.oracle.com/javase/1.4.2/docs/api/java/util/…

SLaks Over a year ago

@rolfl: Yes, but that's more expected. You would expect a generic collection to be able to do this (like C# can)

rolfl Over a year ago

That is true (it is counter-intuitive that it does not work without the assistance - with erasure the 'intuitive' information is gone though...)

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