It really depends what you need. In array you will put the entry in specified index. Arrays also has to have definite size, you cannot increase the size of the array on the fly.
Hashtables are using keys and values. You can increase the size of the hashtable. You have a lot more flexibility with accessing entries of hashtable by key or by entry itself, you can remove the entry (which you cannot do in array). Check
java API for more references.