Collection.toArray() method, detail about it

Question

Why would not it work?

List<String> lista = new ArrayList<>();
        lista.add("Lol");
        lista.add("ball");
        String [] array = (String[])lista.toArray();

It throws a RunTimeException (ClassCastException), I am aware that there is another method for the purpose of returning the object contained in the List, however what is happening behind the scenes? I mean I am casting an array of Objects which actually is an array of Strings to an Array of Strings. So it should compile, but it does not. Thanks in advance.

score 4 · Answer 1 · answered Aug 28 '13 at 08:13

4

That version of toArray() returns Object[]. You can't cast an Object array into a String array even if all the objects in it are Strings.

You can use the lista.toArray(new String[lista.size()]); version to get the actual type correctly.

answered Aug 28 '13 at 08:13

Kayaman

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I am aware of it as I specified in the question. However I would like to know why. since using generics It's clear that it's an array of strings – Rollerball Aug 28 '13 at 08:21

rocketboy · Accepted Answer · 2013-08-28T08:17:47.960

2

List.toArray() returns an Object[], because of type erasure. At runtime your list does not know if it has String objects. From there you can see where that error is coming from.

You cannot type cast an Object[] into a String[]

edited Aug 28 '13 at 08:17

answered Aug 28 '13 at 08:12

rocketboy

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score 0 · Answer 3 · edited May 23 '17 at 12:13

0

Array of objects is not array of Strings and can't be cast to one.

Check this.

edited May 23 '17 at 12:13

Community

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answered Aug 28 '13 at 08:13

BobTheBuilder

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score 0 · Answer 4 · answered Aug 28 '13 at 08:16

use toArray(T[] a) instead.

Ie.

List<String> lista = new ArrayList<String>();
    lista.add("Lol");
    lista.add("ball");
    String [] array = lista.toArray(new string[1]);

This insures that toArray returns an array of type String[]

As others have noted, toArray() returns an array of type Object[], and the cast from Object[] to String[] is illegal.

score 0 · Answer 5 · answered Aug 28 '13 at 08:17

0

List lista = new ArrayList<>(); ---> List lista = new ArrayList();

answered Aug 28 '13 at 08:17

sprite

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score -2 · Answer 6 · answered Aug 28 '13 at 08:32

-2

There are two toArray() versions.You can use another one!

answered Aug 28 '13 at 08:32

Nerd Dragon

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Collection.toArray() method, detail about it

6 Answers6