2nd
In addition to the data type restriction, the Vector class has other restrictions that distinguish it from the Array class:
A Vector is a dense array. Unlike an Array, which may have values in indices 0 and 7 even if there are no values in positions 1 through 6, a Vector must have a value (or null) in each index.
A Vector can optionally be fixed-length, meaning the number of elements it contains can’t change.
Access to a Vector’s elements is bounds-checked. You can never read a value from an index greater than the final element (length - 1). You can never set a value with an index more than one beyond the current final index (in other words, you can only set a value at an existing index or at index [length]).
As a result of its restrictions, a Vector has three primary benefits over an Array instance whose elements are all instances of a single class:
Performance: array element access and iteration are much faster when using a Vector instance than they are when using an Array.
Type safety: in strict mode the compiler can identify data type errors. Examples of data type errors include assigning a value of the incorrect data type to a Vector or expecting the wrong data type when reading a value from a Vector. Note, however, that when using the push() method or unshift() method to add values to a Vector, the arguments’ data types are not checked at compile time. Instead, they are checked at run time.
Reliability: runtime range checking (or fixed-length checking) increases reliability significantly over Arrays.
