Because the Java API is inconsistent. For lists, you use list.size().

But to answer your question, for arrays, length is an attribute. For strings,
length is a method.

Eric

String implements the CharSequence interface, which is a readable
sequence of char values. It has a method (length) that returns the
length of the character sequence (in this case, the length of the
String). String is not an array, but it is a CharSequence, so you use
length(). But you can also use just the attribute length if you create
a char[] from the original String with the method toCharArray(). And
for lists, stacks, vectors, ... you use size() because they implement
the interface Collection (which has the method size()).