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.

Answer 1

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]