hello, can y tell me why??

[sofun]

the key program:
ArrayList al = new ArrayList();
al.add("1");
al.add("2");
al.add("3");
al.add("4");
//this is wrong,why??
String[] str = (String[]) al.toArray();
//but this is crrect.
Object[] obj = al.toArray();
for (int i = 0; i < obj.length; i++) {
String str = (String) obj[i];
}

[Jim Yingst]

The toArray() method returns an Object[]. It may contain Strings inside, but the array itself is fundamentally an Object[] not a String[], and cannot ever be cast to String[]. (String[] is a type of Object[]; and Object[] is not a type of String[].) If you want to create a String[] array (or other array type more specific than Object[]) then you must use toArray(Object[]) rather than toArray(). This allows you to create a specific type of array beforehand, and then load it in one step:

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

or alternately:

    String[] str = al.toArray(new String[0]);

The second option is a bit strange - if you create an array which is the right type but too small for the size of your collection, toArray(Object[]) will merely use the type of the array provided in the argument, and will create a new array of the same type but different size. I prefer the first method as it seems more straightforward (and doesn't create an extra object which is discarded) -but either way works.
[ November 05, 2002: Message edited by: Jim Yingst ]

[sofun]

ok,I know!
thanks!
[ November 05, 2002: Message edited by: sofun ]