Question regarding Java's LinkedList class

I have a question regarding the LinkedList class in Java. I have a scenario wherein i need to add or set an index based on whether the index exists in the linkedlist or not. A pseudo-code of what i want to achieve is --

if index a exists within the linkedlist ll 
     ll.set(a,"arbit")
else
      ll.add(a,"arbit")

I did go through the Javadocs 开发者_运维知识库for the LinkedList class but did not come across anything relevant.

Any ideas ?

Thanks p1ng

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What about using a Map for this:

Map<Integer, String> map = new HashMap<Integer, String>();

// ...

int a = 5;

map.put(a, "arbit");

Even if a already exists, put will just replace the old String.

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Searching in linked list is not very efficient (O(n)). Have you considering using different data structure - e.g. HashMap which would give you O(1) access time?

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If you need sequential access as well as keyed access you might want to try a LinkedHashMap, available as from 1.4.2
http://download.oracle.com/javase/1.4.2/docs/api/java/util/LinkedHashMap.html

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Map<Integer, String> is definitely a good (the best?) way to go here.

Here's an option for keeping with LinkedList if that's for some bizarre reason a requirement. It has horrible runtime performance and disallows null, since null now becomes an indicator that an index isn't occupied.

String toInsert = "arbit";
int a = 5;

//grow the list to allow index a
while ( a >= ll.size() ) {
   ll.add(null);
}

//set index a to the new value
ll.set(a, toInsert);

If you're going to take this gross road, you might be better off with an ArrayList.

Why is it so bad? Say you had only one element at index 100,000. This implementation would require 100,000 entries in the list pointing to null. This results in horrible runtime performance and memory usage.

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LinkedList cannot have holes inside, so you can't have list [1,2,3,4] and then ll.add(10,10), so I think there's something wrong with your example. Use either Map or search for some other sparse array

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It looks like you're trying to use a as a key, and don't state whether you have items at index i < a. If you run your code when ll.size() <= a then you'll end up with a NullPointerException.

And if you add an item at index a the previous item at a will now be at a+1. In this case it would be best to remove item at a first (if it exists) then add item "arbit" into a. Of course, the condition above re: ll.size() <=a still applies here.

If the order of the results is important, a different approach could use a HashMap<Integer,String> to create your dataset, then extract the keys using HashMap<?,?>.getKeySet() then sort them in their natural order (they're numeric after all) then extract the values from the map while iterating over the keySet. Nasty, but does what you want... Or create your own OrderedMap class, that does the same...

Could you expand on why you need to use a LinkedList? Is ordering of the results important?