Odd generics behaviour of List.toArray(T[])

Question

I came across something very basic but extremely bewildering today. I needed to convert a list to an array. The list contained String instances. Perfect example of using List.toArray(T[]), since I wanted a String[] instance. It would not work, however, without explicitly casting the result to String[].

As a test scenario, I used the following code:

import java.util.Arrays;
import java.util.List;

public class MainClass {
    public static void main(String args[]) {
        List l = Arrays.asList("a", "b", "c");
        String stuff[] = l.toArray(new String[0]);
        System.err.println(Arrays.asList(stuff));
    }
}

which does not compile. It's nearly an exact copy of the example in the javadoc, yet the compiler says the following:

MainClass.java:7: incompatible types
found   : java.lang.Object[]
required: java.lang.String[]
    String stuff[] = l.toArray(new String[0]);
                          ^

If I add a cast to String[] it will compile AND run perfectly. But that is not what I expect when I looked at the signature of the toArray method:

<T> T[] toArray(T[] a)

This tells me that I shouldn't have to cast. What is going on?

Edit:

Curiously, if I change the list declaration to:

List<?> l = Arrays.asList("a", "b", "c");

it also works. Or List<Object>. So it doesn't have to be a List<String> as was suggested. I am beginning to think that using the raw List type also changes how generic methods inside that class work.

Second edit:

I think I get it now. What Tom Hawtin wrote in a comment below seems to be the best explanation. If you use a generic type in the raw way, all generics information from that instance will be erased by the compiler.

dfa dfa · Accepted Answer · 2009-07-02T11:18:42

you forgot to specify the type parameter for your List:

List<String> l = Arrays.asList("a", "b", "c");

in this case you can write safety:

String[] a = l.toArray(new String[0]);

without any cast.

Odd generics behaviour of List.toArray(T[])

5 Answers