java - JUnit testing lines drawn by user -

i'm having problem writing test code , don't have clue how it.

this code draws line when 1 of key pressed length of 1. when holding shift down create line length of 5. need test draw line correct length parameter each case.

class sketchpanel extends jpanel {     private point2d last;  private final arraylist<line2d> lines;  private static final int small_increment = 1;  private static final int large_increment = 5;  public sketchpanel() {      super();     last = new point2d.double(100, 100);     lines = new arraylist<>();     keyhandler listener;     listener = new keyhandler();     addkeylistener(listener);     setfocusable(true);       }  public void add(final int dexx, final int dexy) {       point2d end;     end = new point2d.double(last.getx() + dexx, last.gety() + dexy);     line2d line;     line = new line2d.double(last, end);     lines.add(line);     repaint();     last = end; }  @override public void paintcomponent(final graphics graph) {       super.paintcomponent(graph);     graphics2d graph2 = (graphics2d) graph;     lines.foreach((s) -> graph2.draw(s)); }  private class keyhandler implements keylistener {        @override     public void keypressed(final keyevent event) {           final int keycode = event.getkeycode();          int dex = 0;         if (event.isshiftdown()) {             dex = large_increment;                        } else {             dex = small_increment;         }         switch (keycode) {             case keyevent.vk_left:                 add(-dex, 0);                 break;             case keyevent.vk_right:                 add(dex, 0);                 break;             case keyevent.vk_up:                 add(0, -dex);                 break;             case keyevent.vk_down:                 add(0, dex);                 break;             default:                 break;         }     }      @override     public void keyreleased(final keyevent event) {}      @override     public void keytyped(final keyevent event) {           char keychar = event.getkeychar();         int dex;         if (character.isuppercase(keychar)) {               dex = large_increment;             keychar = character.tolowercase(keychar);         } else {             dex = small_increment;         }          switch (keychar) {             case 'h':                 add(-dex, 0);                 break;             case 'l':                 add(dex, 0);                 break;             case 'k':                 add(0, -dex);                 break;             case 'j':                 add(0, dex);                 break;             default:                 break;         }     } }