How to shuffle list except an element?

Question

I have a list integer with value elements : 0, 7, 2, 1, 6, 5.

I know that I can use method

Collections.shuffle(list);

to shuffle my list. But I do not want change value of 2nd position. It should be always 7.

How can I do that ?

Answer 1

You can shuffle the Collection and then restore the 7 to the 2nd position :

Collections.shuffle(list);
list.set(list.indexOf(7),list.get(2));
list.set(2,7);

Or shorter :

Collections.shuffle(list);
Collections.swap(list, 2, list.indexOf(7));

As others suggested, you can also remove the element whose location you wish to preserve prior to the shuffling, and add it later to the same location.

Both ways should take similar time for ArrayLists (linear at the worst case), since in my answer indexOf would take linear time, but removing and adding an element in the alternative solution (especially if the index is close to the start of the list) would take linear time for ArrayList, since all the elements following the removed/added index has to be pushed to a new index.

Answer 2

To easily prevent moving any number of elements simply

• Remove them from list,
• shuffle rest of elements,
• put them back at their original position (start from left to avoid problem with elements shifting to the right).