Question

Can someone tell me what is wrong with the following code?

    Set<String> cmds = *a method call that returns a Set<String>*
    String[] cmdarr = (String[]) cmds.toArray();
    int i;
    for(i=0; i<cmdarr.length;i++){
        System.out.println(cmdarr[i]);

It gave the following error:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
at a.jim.Test.main(Test.java:79)

Thanks in advance!

Answer 1

The Setitself doesn’t have any type information about its elements available at runtime; the type parameter <String> is erased during compilation. So, the toArray() method always creates an array of type Object[].

You can pass an array to the method and have the contents of the set copied into it:

String[] cmdarr = cmd.toArray(new String[cmd.size()]);

The the array you pass in is too small, a new one with the same component type will be created using reflection, but that is not as efficient as passing in one of the correct size.