java: (String[])List.toArray() gives ClassCastException

问题

The following code (run in android) always gives me a ClassCastException in the 3rd line:

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

It happens also when v2 is Object[0] and also when there are Strings in it. Any Idea why?

回答1:

This is because when you use

 toArray() 

it returns an Object[], which can't be cast to a String[] (even tho the contents are Strings) This is because the toArray method only gets a

List 

and not

List<String>

as generics are a source code only thing, and not available at runtime and so it can't determine what type of array to create.

use

toArray(new String[v2.size()]);

which allocates the right kind of array (String[] and of the right size)

回答2:

You are using the wrong toArray()

Remember that Java's generics are mostly syntactic sugar. An ArrayList doesn't actually know that all its elements are Strings.

To fix your problem, call toArray(T[]). In your case,

String[] v3 = v2.toArray(new String[v2.size()]);

Note that the genericized form toArray(T[]) returns T[], so the result does not need to be explicitly cast.

回答3:

String[] str = new String[list.size()];
str = (String[]) list.toArray(str);

Use like this.

来源：https://stackoverflow.com/questions/5690351/java-stringlist-toarray-gives-classcastexception