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Thread: Inside the Graphics class

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    Default Inside the Graphics class

    Hello Java Community

    I have a question concerning the inner workings of the graphics class. I know it is not necessary to know anything about that in order to draw stuff. However, I would like to learn more about abstract classes.
    What exactly happens when I pass Graphics g into the paintComponent method?

    So thats what I figured out from the code:

    - JPanel seems to use paintComponent() from JComponent which seems to override that method from its superclass Container (right?)
    - the paintComponent() from JComponent creates a Graphics variable and calls create()
    - I already lost track here.

    What's going on now? ok create() ist called but nothing is really changed. Shouldn't the create method be defined somewhere? How does the compiler know what to do? =)

    I would be glad if someone could explain to me what the compiler does step by step.


    Thanks a lot


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    Super Moderator Norm's Avatar
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    Default Re: Inside the Graphics class

    What exactly happens when I pass Graphics g into the paintComponent method?
    When you override a method from a super class you need to define its signature exactly as in the super class. The java program somewhere creates a Graphics class object and passes it to the paintComponent() method. Do you have code where you create a Graphics object and call the paintCOmponent() method with it?

    Are you having problems reading the Java SE source code?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Inside the Graphics class

    Quote Originally Posted by Garaen View Post
    - JPanel seems to use paintComponent() from JComponent ...
    I imagine that much of the mechanics of paintComponent are to be found in Container, but I haven't looked at the source to know exactly what does what.

    Edit: JPanel doesn't even have a paintComponent method much less a paint method.

    ... JComponent which seems to override that method from its superclass Container (right?)
    No. If you look at the Container API you'll see that it has no such method. The paintComponent method originates in JComponent.


    - the paintComponent() from JComponent creates a Graphics variable and calls create()
    I thought that the Graphics object came directly from the JVM.

    Edit: the Graphics used to paint any UI delegates is created in JComponent's paintComponent method by copying the Graphics object passed in by the JVM, and then disposes this Graphics object once the UI delegates have been painted.

    Have you had a look at one of my favorite articles on Swing graphics?: Painting in AWT and Swing. It's well worth a study.
    Last edited by curmudgeon; September 2nd, 2012 at 10:42 PM.

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