添加通用类型的元素 <T> 到LinkedList的arrayList

Question

I want to make a hash table class in java where I store the key,value pairs in an ArrayList of Linked List's. I do this by declaring

 ArrayList<LinkedList<T>> storage = new ArrayList();

I then want to create a linkList object that I can use to then create a new linked list inside of each index of the arrayList. I do this by declaring:

  LinkedList<T> list = new LinkedList<T>();

Then I have my add function set up to add elements to the first index of the LinkedList that is inside the Hashed key index of the arrayList as such:

public void add(K key, T value){    
int arrayListIndex = (key.hashCode()) % this.initialCapacity;
    System.out.println(arrayListIndex); //This tells us where we access the Array List;


    if (hashBrown.get(arrayListIndex) == null){

        hashBrown.add(arrayListIndex, list);
        hashBrown.get(arrayListIndex).addFirst(value);
    }
}

Everytime I run this code I receive an error where my index is 7 and my size is 0. This causes an error:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 7, Size: 0
    at java.util.ArrayList.rangeCheck(ArrayList.java:571)
    at java.util.ArrayList.get(ArrayList.java:349)
    at FastHashtable.add(FastHashtable.java:72)
    at FastHashtable.main(FastHashtable.java:145)

I am unable to track down where this index out of bounds error is coming from can anyone offer advice. I am fairly new at working with ArrayLists which makes me think my original declaration of the arrayList is incorrect.

Answer 1

You are confusing an ArrayList s capacity with its size . From the Oracle Java documentation:

Each ArrayList instance has a capacity . The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList , its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

Instead, you should be looking at creating a plain array (eg Object[] a = new Object[maxSize] ) in which you can actually assign objects (linked lists in this case) at arbitrary index values. If you only want to store linked lists, create a LinkedList<T>[] array.

Answer 2

You cannot add to a list at index 7 if there are 0 elements in it. You can only add at the end (with the add method without index) or at a position that is no larger than the current list's size() .

When the list is empty and you add something at index 7, what do you expect the list to contain at the first position then, and what at index 6? (I once created a subclass of list to fill everything up to the index with null when the addition index is larger than the list size, but this behaviour is not universal enough to be part of List 's semantics.

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[edit in response to comment here and to praseodym's answer]

You could simply fill the (array) list with nulls replace those with a linked list when the respective position is first accessed. (Be sure to use set and not add , though, which is probably what you want above as well.) Alternatively, you could create an array of the desired size (that will be full of null s by default) and "wrap" it into a (non-resizable) list via Arrays.asList . You have to ignore an "unchecked conversion" warning then, however (not that you could avoid warnings when using an array). Also, I suggest you read " Program to an interface, not an implementation ".