Java: sort List from index to index

Question

For arrays there is a special function for sorting a part of the array from index to index:

Arrays.sort(Object[] a, int fromIndex, int toIndex)

For List< T>

there is also a function for sorting

Collections.sort(List<T> list)

Unluckily there is no variant accepting a fromIndex and toIndex parameter.

I know that I could solve this problem by either applying

Convert the List into an array and apply Arrays.sort, then convert it back to a List
Copying the list entries indexed by fromIndex to toIndex to a new List (by using list.subList(fromIndex, toIndex)), sort it and overwrite the old list entries

But I hope there is a prettier way to do that.

Hey man, there's no shame in converting to an Array, enjoying a few extra functions then converting back ;) — CFL_Jeff
– CFL_Jeff, Commented Mar 9, 2012 at 23:47

Adam · Accepted Answer · 2012-03-09 23:47:47Z

17

Just use .subList() to get a "backed" view onto the main list, then call sort. The sublist is "write-through" so changes are reflected in the original.

List<Integer> foo = Arrays.asList(5,3,1,6,2,1);
Collections.sort(foo.subList(0, 3)); // sort first 3 elements 
System.out.println(foo);
Collections.sort(foo.subList(3, 6)); // sort last 3 elements
System.out.println(foo);

Output

[1, 3, 5, 6, 2, 1]
[1, 3, 5, 1, 2, 6]

edited Mar 9, 2012 at 23:47

answered Mar 9, 2012 at 23:42

Adam

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Guillaume Polet · Accepted Answer · 2012-03-09 23:46:51Z

3

You can use subList() on your original list, then sort the sublist and it will reflect on your original list without having to write back.

answered Mar 9, 2012 at 23:46

Guillaume Polet

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brazilianldsjaguar · Accepted Answer · 2012-03-09 23:48:43Z

2

Looking at the Oracle documentation, Collections and List simply do not contain this functionality, like the Arrays do. If I had to choose between your two suggestions, I'd implement the second one, using the List.subList(fromIndex, toIndex)).

Here's the docs: http://docs.oracle.com/javase/7/docs/api/java/util/List.html
http://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html

answered Mar 9, 2012 at 23:48

brazilianldsjaguar

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Ryan · Accepted Answer · 2012-03-09 23:49:01Z

Copying the list entries indexed by fromIndex to toIndex to a new List (by using list.subList(fromIndex, toIndex)), sort it and overwrite the old list entries

No, there is no object copy when you call list.subList. The function subList creates a view backed by the original list. Only reference copies; no actual object copies.

Any operations (sorting) on the view will be reflected on the original list.

 public static void main(String[] args) throws Exception {
    List<Integer> list = Arrays.asList(1, 9, 8 ,7, 2, 3, 4);

    // [9, 8 ,7] => [7, 8, 9]
    sortList(list, 1, 4);

    System.out.println(list);       // [1, 7, 8, 9, 2, 3, 4]
  }

  public static <T extends Comparable<T>> void sortList(
      List<T> list, int fromIndex, int toIndex) {
    Collections.sort(list.subList(fromIndex, toIndex));
  }

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