Why Does `List.toArray()` Throw a `ClassCastException` in Android When Casting to `String[]`?

ClassCastException on (String[])List.toArray() in Android

When attempting to cast an ArrayList to a String[] using the toArray() method, Android apps commonly encounter a ClassCastException. For instance, the following code triggers this error:

<code class="java">final String[] v1 = i18nCategory.translation.get(id);
final ArrayList<String> v2 = new ArrayList<>(Arrays.asList(v1));
String[] v3 = (String[]) v2.toArray();</code>

This happens regardless of the content of v2 (even an empty array).

Explanation

This error occurs because toArray() returns an Object[], not a String[]. Generics are only available at compile time, so the Java Virtual Machine (JVM) cannot determine which array type to create. Therefore, it defaults to Object[], which cannot be cast to String[].

Solution

To resolve this issue, explicitly specify the array type using the toArray(T[] a) method and provide the desired array type as a parameter. For example, this code will correctly create a String[]:

<code class="java">String[] v3 = v2.toArray(new String[v2.size()]);</code>

This method ensures that the returned array will be of the correct type and size.

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