swing - Why does Java scrollPane remove contents of my Panel when I scroll Up and Down? -

i have problem scrollpane in java.

i programming gui using java, have implemented panel using jpanel scroll bar.

to simplify idea better understanding, panel displays number of jlabels depending on number entered user.

so width , height of panel dynamically changed each time user enters different value.

jpanel inp=new jpanel();  inp.setlayout(null); inp.setpreferredsize(new dimension(1000, 1000));  scroll=new jscrollpane(inp,jscrollpane.vertical_scrollbar_always,jscrollpane.horizontal_scrollbar_always); scroll.setbounds(10, 20, 830, 440); lowp.add(scroll);  

as can see, created jpanel preferred size constants (for simplicity, although dynamic).

however, probelm when run code, vertical scrollbar appears, when scroll down, rest of jlabel not appears (its cut in graph).

what's weird when scroll up, graph returns initial state , ignore update applied.

what's strange horizontal scroll bar works fine , shows whats left of graph right side.

here photos show problem:

here image how looks when scroll down:

but when scroll down, rest of jlabel not appears (its cut in graph).

it job of layout manager determine preferred size of components added panel. scrolling works if value calculated correctly. don't use null layout. let layout manager job.

so width , height of panel dynamically changed each time user enters different value.

don't use setpreferredsize(...)!!!

the layout manager dynmacally determine preferred size add components. basic code should be:

panel.add(...); panel.revalidate(); panel.repaint();