How does paintComponent work

Question

This might be a very noob question  I m just starting to learn Java  I don t understand the operation of paintComponent method  I know if I want to draw something  I must override the paintComponent method   public void paintComponent Graphics g               But when is it called  I never see anything like  object paintComponent g   but still it is drawn when the program is running    And what is the Graphics parameter  Where is it from  Parameter must be supplied when the method is called  But as I said before  it seems like this method is never be explicitly called  So who provides this parameter  And why do we have to cast it to Graphics2D   public void paintComponent Graphics g                Graphics2D g2   Graphics2D  g

User · Answer

The internals of the GUI system call that method  and they pass in the Graphics parameter as a graphics context onto which you can draw

User · Answer

Two things you can do here    Read Painting in AWT and Swing Use a debugger and put a breakpoint in the paintComponent method  Then travel up the stacktrace and see how provides the Graphics parameter    Just for info  here is the stacktrace that I got from the example of code I posted at the end   Thread  AWT-EventQueue-0   Suspended  breakpoint at line 15 in TestPaint         TestPaint paintComponent Graphics  line  15      TestPaint JComponent  paint Graphics  line  1054         JPanel JComponent  paintChildren Graphics  line  887         JPanel JComponent  paint Graphics  line  1063        JLayeredPane JComponent  paintChildren Graphics  line  887       JLayeredPane JComponent  paint Graphics  line  1063      JLayeredPane paint Graphics  line  585       JRootPane JComponent  paintChildren Graphics  line  887      JRootPane JComponent  paintToOffscreen Graphics  int  int  int  int  int  int  line  5228        RepaintManager PaintManager paintDoubleBuffered JComponent  Image  Graphics  int  int  int  int  line  1482      RepaintManager PaintManager paint JComponent  JComponent  Graphics  int  int  int  int  line  1413       RepaintManager paint JComponent  JComponent  Graphics  int  int  int  int  line  1206        JRootPane JComponent  paint Graphics  line  1040         GraphicsCallback PaintCallback run Component  Graphics  line  39         GraphicsCallback PaintCallback SunGraphicsCallback  runOneComponent Component  Rectangle  Graphics  Shape  int  line  78         GraphicsCallback PaintCallback SunGraphicsCallback  runComponents Component    Graphics  int  line  115      JFrame Container  paint Graphics  line  1967         JFrame Window  paint Graphics  line  3867        RepaintManager paintDirtyRegions Map lt Component Rectangle gt   line  781         RepaintManager paintDirtyRegions   line  728         RepaintManager prePaintDirtyRegions   line  677      RepaintManager access 700 RepaintManager  line  59       RepaintManager ProcessingRunnable run   line  1621       InvocationEvent dispatch   line  251         EventQueue dispatchEventImpl AWTEvent  Object  line  705         EventQueue access 000 EventQueue  AWTEvent  Object  line  101        EventQueue 3 run   line  666         EventQueue 3 run   line  664         AccessController doPrivileged PrivilegedAction lt T gt   AccessControlContext  line  not available  native method          ProtectionDomain 1 doIntersectionPrivilege PrivilegedAction lt T gt   AccessControlContext  AccessControlContext  line  76         EventQueue dispatchEvent AWTEvent  line  675         EventDispatchThread pumpOneEventForFilters int  line  211        EventDispatchThread pumpEventsForFilter int  Conditional  EventFilter  line  128         EventDispatchThread pumpEventsForHierarchy int  Conditional  Component  line  117        EventDispatchThread pumpEvents int  Conditional  line  113       EventDispatchThread pumpEvents Conditional  line  105        EventDispatchThread run   line  90     The Graphics parameter comes from here   RepaintManager paintDirtyRegions Map  line  781    The snippet involved is the following   Graphics g   JComponent safelyGetGraphics                          dirtyComponent  dirtyComponent                      If the Graphics goes away  it means someone disposed of                    the window  don t do anything                  if  g    null                        g setClip rect x  rect y  rect width  rect height                       try                           dirtyComponent paint g      This will eventually call paintComponent                         finally                           g dispose                                              If you take a look at it  you will see that it retrieve the graphics from the JComponent itself  indirectly with javax swing JComponent safelyGetGraphics Component  Component   which itself takes it eventually from its first  Heavyweight parent   clipped to the component bounds  which it self takes it from its corresponding native resource   Regarding the fact that you have to cast the Graphics to a Graphics2D  it just happens that when working with the Window Toolkit  the Graphics actually extends Graphics2D  yet you could use other Graphics which do  not have to  extends Graphics2D  it does not happen very often but AWT Swing allows you to do that    import java awt Color  import java awt Graphics   import javax swing JFrame  import javax swing JPanel   class TestPaint extends JPanel        public TestPaint             setBackground Color WHITE               Override     public void paintComponent Graphics g            super paintComponent g           g drawOval 0  0  getWidth    getHeight                public static void main String   args            JFrame jFrame   new JFrame            jFrame setDefaultCloseOperation JFrame EXIT ON CLOSE           jFrame setSize 300  300           jFrame add new TestPaint             jFrame setVisible true

User · Answer

Calling object paintComponent g  is an error    Instead this method is called automatically when the panel is created  The paintComponent   method can also be called explicitly by the repaint   method defined in Component class   The effect of calling repaint   is that Swing automatically clears the graphic on the panel and executes the paintComponent method to redraw the graphics on this panel

User · Answer

The  very  short answer to your question is that paintComponent is called  when it needs to be    Sometimes it s easier to think of the Java Swing GUI system as a  black-box   where much of the internals are handled without too much visibility   There are a number of factors that determine when a component needs to be re-painted  ranging from moving  re-sizing  changing focus  being hidden by other frames  and so on and so forth   Many of these events are detected auto-magically  and paintComponent is called internally when it is determined that that operation is necessary     I ve worked with Swing for many years  and I don t think I ve ever called paintComponent directly  or even seen it called directly from something else   The closest I ve come is using the repaint   methods to programmatically trigger a repaint of certain components  which I assume calls the correct paintComponent methods downstream   In my experience  paintComponent is rarely directly overridden   I admit that there are custom rendering tasks that require such granularity  but Java Swing does offer a  fairly  robust set of JComponents and Layouts that can be used to do much of the heavy lifting without having to directly override paintComponent   I guess my point here is to make sure that you can t do something with native JComponents and Layouts before you go off trying to roll your own custom-rendered components

[java] How does paintComponent work?

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