在ArrayLists的ArrayList中使用[] java

Question

I'm creating a hashtable that is a ArrayList<ArrayList<Integer>> Table = new ArrayList<ArrayList<Integer>>(size); I want to check if the slot with hash code n is null, using if(Table [n] == null) but ths is wrong. What's wrong with it?

Answer 1

You cannot access the elements of an ArrayList with the [...] syntax. That is only available for arrays. Use

if(Table.get(n) == null)

instead.

As an unrelated note, make it a habit to follow the Java naming conventions. Variable names should start with a lower case letter.

Answer 2

Fundamentally a hashtable works as follows to insert an item (give or take):

int hash = obj.hashCode();
table[hash % table.length].push(obj);

You must have a definite length for the table to work correctly and be able to access all positions in the array at any given time. Sure an ArrayList can mimic this behavior, but it just makes it slower and under the hood its just a plain old array anyway.