在 Java 中操作数组列表的部分

Question

I have a matrix of W of large row and column dimension. Each of the row represents feature values(filled with double value).And the matrix is constructed as:

  Hashmap<Integer,Arraylist<Double>> W = new Hashmap<Integer,Arraylist<Double>>();

While making computation, I need to take certain portion of each of the rows and update them in matrix. I looked for subList method in Arraylist . But the problem is it returns only list but I am in need of arraylist. Because many of the methods that I have already implemented take <Arraylist> as argument. So what can be the solution for this case?

Example

 w1 = [1,3 ,4,6,9,1,34,5,21,5,2,5,1]
 w11 = [1,3,4]
 w11 = w11 +1 = [2,4,5]
 This changes w1 to = [2,4 ,5,6,9,1,34,5,21,5,2,5,1]

Answer 1

I looked for subList method in Arraylist.But the problem is it returns only list but i am in need of arraylist

That is not a problem at all. In fact, you should change your code to use List wherever possible.

A List is an interface which concrete types such as the ArrayList implement. The below is perfectly valid:

List<String> list = new ArrayList<String>();
list.add("hello");
list.add("world");

I recommend changing your W to this:

Hashmap<Integer, List<Double>> W = new Hashmap<Integer, List<Double>>();

Answer 2

You could subclass ArrayList to provide a view of a slice of another ArrayList . Like this:

class ArrayListSlice<E> extends ArrayList<E> {
  private ArrayList<E> backing_list;
  private int start_idx;
  private int len;
  ArrayListSlice(ArrayList<E> backing_list, int start_idx, int len) {
    this.backing_list = backing_list;
    this.start_idx = start_idx;
    this.len = len;
  }
  public E get(int index) {
    if (index < 0 || idx >= len) throw new IndexOutOfBoundsException();
    return backing_list.get(start_idx + index);
  }
  ... set, others? ...
}

Then you can do w11 = new ArrayListSlice<Double>(w1, 0, 3) . Any operations on w11 will appear in w1 , assuming you implement set correctly.

You will probably need to implement most the methods of ArrayList to make this work. Some may work if they just rely on others, but that's hard to tell from the spec.