Java get() changes initialized Hashtable keys to Null

Question

I'm currently having a problem with the get() method used with hashtables.

My initialization code:

        Hashtable<Integer, pageEntry> pageTable = new Hashtable<Integer, pageEntry>();
        Hashtable<Integer, LinkedList<Integer>> lookAhead = new Hashtable<Integer, LinkedList<Integer>();

        //Initialize pageTable and co.
        for(int i = 0; i < 10; i++) {
            pageEntry p = new pageEntry();
            pageTable.put(i, p);
            lookAhead.put(i, new LinkedList<Integer>());
        }

when I use System.out.println(lookAhead); , the output is as follows:

{0=[]}
{1=[], 0=[]}
{2=[], 1=[], 0=[]}
{3=[], 2=[], 1=[], 0=[]}
{4=[], 3=[], 2=[], 1=[], 0=[]}

If I use System.out.println(lookAhead.get(0)) , the output is as follows

[]
[]
[]
[]
[]

However, if I use System.out.println(lookAhead.get(3)) , the output changes to

null
null
null
[]
[]

Is there some reason I'm overlooking as to why it changes my values to null?

Thanks for your time.

Answer 1

The get method returns null when the key is not present in the map.

In the first 3 iterations, the key is not present in the map. It's added on the 4th iteration, and the output shows a value associated with the key from the 4th iteration onwards.

Answer 2

It depends on where you accessed lookAhead.get(3)

When you use lookAhead.get(0) , the program is printing the empty linkedlists which were initialized to the index '0' in your HashTable.

You would have accessed the index location 3 ( lookAhead.get(3) ) without initializing at all. So, the program is printing null for non existent keys.