### Question:
JXPanel extends JPanel implements AlphaPaintable, BackgroundPaintable, Scrollable { public final void setScrollableWidthHint(ScrollableSizeHint hint) { Contract.asNotNull(hint, "hint cannot be null"); ScrollableSizeHint oldValue = getScrollableWidthHint(); if (oldValue == hint) return; this.scrollableWidthHint = hint; revalidate(); firePropertyChange("scrollableWidthHint", oldValue, getScrollableWidthHint()); } JXPanel();  JXPanel(boolean isDoubleBuffered);  JXPanel(LayoutManager layout);  JXPanel(LayoutManager layout, boolean isDoubleBuffered); @Override void setOpaque(boolean opaque); @Override boolean isOpaque(); @Override float getAlpha(); @Override void setAlpha(float alpha); @Override float getEffectiveAlpha(); @Override boolean isInheritAlpha(); @Override void setInheritAlpha(boolean val); final void setScrollableWidthHint(ScrollableSizeHint hint); final void setScrollableHeightHint(ScrollableSizeHint hint); @Override boolean getScrollableTracksViewportHeight(); @Override boolean getScrollableTracksViewportWidth(); @Override Dimension getPreferredScrollableViewportSize(); @Override int getScrollableBlockIncrement(Rectangle visibleRect, int orientation, int direction); @Override int getScrollableUnitIncrement(Rectangle visibleRect, int orientation, int direction); void setScrollableTracksViewportHeight(boolean scrollableTracksViewportHeight); void setScrollableTracksViewportWidth(boolean scrollableTracksViewportWidth); @Override void setBackground(Color bg); @Override void setBackgroundPainter(Painter p); @Override Painter getBackgroundPainter(); @Override boolean isPaintBorderInsets(); @Override void setPaintBorderInsets(boolean paintBorderInsets); @Override void paint(Graphics g);  }### Answer:
@Test (expected = NullPointerException.class) public void testScrollableWidthTrackNull() { new JXPanel().setScrollableWidthHint(null); }