Whizlab mock: ClassCasting

[adam Lui]

import java.util.*;
class Collection1 {
public static void main(String[] args) {
Set<String> s = new HashSet<String>();
s.add("apple");
s.add("Apple");
s.add("1");
s.add("1.0");
String[] ss = (String[])s.toArray(); //ClassCastException-----line 1

for (String str: ss) 
System.out.print(str);
}
}

there is an exception at line 1, but Whizlab answer states this would run. Where of the code go wrong?

[Kelvin Chenhao Lim]

Here's why the exception occurs: Collection.toArray() returns an Object array, not a String (or whatever) array. This seems counter-intuitive now that generics have been added to the language, but this is one of the many unfortunate consequences of type erasure. Basically, toArray() does not have the runtime type information to instantiate a more specific array type.

To eliminate the exception, that line should be rewritten this way:

String[] ss = s.toArray(new String[0]);

This uses the overloaded version of toArray() that takes an array parameter. This version of toArray() uses the type of the parameter to instantiate (if necessary) a new array of the same type as the return value. Thus the above line will run without problems (and we can eliminate the type cast, thanks to generics).

[adam Lui]

thanks Kelvin!