If your toolbar is inside an AppBarLayout which is probably inside your CoordinatorLayout then something like this should work.
AppBarLayout appBarLayout = (AppBarLayout)findViewById(R.id.appBar);
appBarLayout.setExpanded(true, true);
Or to collapse it
AppBarLayout appBarLayout = (AppBarLayout)findViewById(R.id.appBar);
appBarLayout.setExpanded(false, true);
Here is the definition
setExpanded(boolean expanded, boolean animate)
Take note that this method is available from v23 of the support library, here is some documentation for reference, the key thing to note is "As with AppBarLayout's scrolling, this method relies on this layout being a direct child of a CoordinatorLayout." Hope this helps!