How to change ViewPager's page?

I'm not sure that I fully understand the question, but from the title of your question, I'm guessing that what you're looking for is pager.setCurrentItem( num ). That allows you to programatically switch to another page within the ViewPager.

I'd need to see a stack trace from logcat to be more specific if this is not the problem.

slide to right

viewPager.arrowScroll(View.FOCUS_RIGHT);

slide to left

viewPager.arrowScroll(View.FOCUS_LEFT);

Without checking your code, I think what you are describing is that your pages are out of sync and you have stale data.

You say you are changing the number of pages, then crashing because you are accessing the old set of pages. This sounds to me like you are not calling pageAdapter.notifyDataSetChanged() after changing your data.

When your viewPager is showing page 3 of a set of 10 pages, and you change to a set with only 5, then call notifyDataSetChanged(), what you'll find is you are now viewing page 3 of the new set. If you were previously viewing page 8 of the old set, after putting in the new set and calling notifyDataSetChanged() you will find you are now viewing the last page of the new set without crashing.

If you simply change your current page, you may just be masking the problem.