size()
is a method specified in java.util.Collection
, which is then inherited by every data structure in the standard library. length
is a field on any array (arrays are objects, you just don't see the class normally), and length()
is a method on java.lang.String
, which is just a thin wrapper on a char[]
anyway.
Perhaps by design, Strings are immutable, and all of the top-level Collection subclasses are mutable. So where you see "length" you know that's constant, and where you see "size" it isn't.