将整数添加到ArrayList <Integer>

Question

I have an ArrayList of LinkedList s (an array of linked lists). The LinkedList s contains integers ( Integer ).

private List<LinkedList> buckets;
buckets = new ArrayList<LinkedList>();
for (int i = 0; i < 10; i++) {
    LinkedList<Integer> temp = new LinkedList<Integer>();
    buckets.add(temp);
}

I later want to remove the items from the linked list (in the order they were added) and add them to an array list. When I try this:

ArrayList<Integer> sorted = new ArrayList<Integer>(unsorted.size());
for (int i = 0; i < buckets.size(); i++) {
    for (int j = 0; j < buckets.get(i).size(); j++) {
        sorted.add(buckets.get(j).removeLast());
        // sorted.add((Integer)buckets.get(j).removeLast());
    }
}

I get an error saying:

add(java.lang.Integer) in ArrayList cannot be applied to (java.lang.Object)

But when I cast it to an Integer (the commented out line), the array is full of null values . Anyone see what I am doing wrong?

Here is where I am adding items to bucket :

for (int i = 0; i < unsorted.size(); i++) {
    int digit = (unsorted.get(i) / position) % 10;
    buckets.get(digit).add(unsorted.get(i));
}

Note that sorted is an ArrayList<Integer> . When I trace it in debug mode, I can see that the LinkedLists have Integer objects with the correct values.

Screenshot of buckets contents:

Working Example:

class Ideone {
    private static List<LinkedList<Integer>> buckets;
    public static void main (String[] args) throws Exception {
        ArrayList<Integer> arr = new ArrayList<Integer>();
        arr.add(6);
        arr.add(8);
        arr.add(1);
        arr.add(3);
        arr.add(9);
        System.out.println(arr);
        arr = sort(arr);
        System.out.println(arr);
    }
    public static ArrayList<Integer> sort(ArrayList<Integer> unsorted) {
        buckets = new ArrayList<LinkedList<Integer>>();
        for (int i = 0; i < 10; i++) {
            LinkedList<Integer> temp = new LinkedList<Integer>();
            buckets.add(temp);
        }
        ArrayList<Integer> sorted = new ArrayList<Integer>(unsorted.size());
        for (int i = 0; i < unsorted.size(); i++) {
            int digit = unsorted.get(i) % 10;
            buckets.get(digit).add(unsorted.get(i));
        }
        for (int i = 0; i < buckets.size(); i++) {
            for (int j = 0; j < buckets.get(i).size(); j++) {
                sorted.add(buckets.get(j).poll());
                // sorted.add((Integer)buckets.get(j).removeLast());
            }
        }
        return sorted;
    }
}

Answer 1

You are using the raw form of LinkedList here:

private List<LinkedList> buckets;

Because of this, removeLast will return Object , not Integer . Try

private List<LinkedList<Integer>> buckets;

and

buckets = new ArrayList<LinkedList<Integer>>();

Casting the return of removeLast to Integer was the pre-generics way of getting this to work. However, you never inserted any items into each LinkedList , so removeLast returns null . If you want something returned, first insert something into each LinkedList that gets inserted into buckets .

Casting to Integer would still work, but supplying Integer as the type argument to LinkedList is preferred, especially since you are using generics by supplying LinkedList as the type parameter to List already.

Answer 2

In your nested loop,

  for (int i = 0; i < buckets.size(); i++) {
     for (int j = 0; j < buckets.get(i).size(); j++) {

        // ***** here *****
        sorted.add(buckets.get(j).poll());

     }
  }

You look to be polling the wrong List.

Try changing

sorted.add(buckets.get(j).poll());

to:

sorted.add(buckets.get(i).poll());

Perhaps a cleaner more intuitive way to code this would be something like:

  for (int i = 0; i < buckets.size(); i++) {
     LinkedList<Integer> innerList = buckets.get(i);
     for (int j = 0; j < innerList.size(); j++) {
        sorted.add(innerList.poll());
     }
  }

Although this may not work if the innerList has multiple items. Why not instead remove items safely with an iterator?

  for (int i = 0; i < buckets.size(); i++) {
     LinkedList<Integer> innerList = buckets.get(i);
     for (Iterator<Integer> iterator = innerList.iterator(); iterator.hasNext();) {
        sorted.add(iterator.next());
        iterator.remove(); // this guy is optional
     }
  }

Either that or simply use get(j)

  for (int i = 0; i < buckets.size(); i++) {
     LinkedList<Integer> innerList = buckets.get(i);
     for (int j = 0; j < innerList.size(); j++) {
        sorted.add(innerList.get(j));
     }
  }

Although this isn't efficient use of a LinkedList

Answer 3

The item that you inserted into the ArrayList "sorted" is the item you took from the link list LinkedList.

But you never actually add any item to it. You simply just created a LinkedList and added it to your bucket list.

You need to add something into the temp list.

for (int i = 0; i < 10; i++) {
    LinkedList<Integer> temp = new LinkedList<Integer>();
    // Add something to the temp LinkedList
    buckets.add(temp);
}