Java - using paintComponent for graphics, calling functions from within?

I'm really confused with the program flow for how the paintComponent function works within my JPanel. Ideally I'd like to have access to the Graphics object to draw stuff from other functions based on my program flow. I'm thinking along the lines of the following:

private Graphics myG;

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    myG = g; //I want a graphics object that I can just draw with. 
             //Should I just initialize to myG = new Graphics(); or something?
    //private Graphics myG = new Graphics(); does not seem to work        
  }

//this is what's getting called, where I want to call other functions that 
//can use myG to 开发者_JAVA技巧draw graphics as I please
public void startPanelView() {      

    myG.drawRect(350, 20, 400, 300); //doesn't work

    otherFunctionForGraphics(); //which will also use myG.  
}

I hope I've made myself clear here. I just want to be able to do use the Graphics class as I please. Currently I can only do stuff like g.drawRect(...) inside of the paintComponent() function. This could work for my purposes but I'd like more flexibility.

Thanks.

EDIT - Alright, I understand that I should not try and reference a Graphics object outside. But how should I go about separating application logic from the paintComponent function? Right now this class is looking a little messy because I have the following:

public void paintComponent(Graphics g) {

    if (flag1 == true) {
        //do some graphics stuff, that I would prefer to be in another function
    }

    if (flag2 == true) {
        //do some other graphics stuff, again, which I would prefer 
        //to be in another function
    }

    //... etc. More of these cases.
}

And basically the paintComponent function is getting stupidly long and complicated for me, so I would like to break it up in whatever ways possible.

Call repaint from your startPanelView method on the panel object. You should never act on the graphics object outside of the paint methods, nor should you maintain a reference to the graphics object for use outside of the paint method.

The others are right that you cannot keep the Graphics object as a member variable, but if the problem is that your paintComponent method is getting too long, you can still pass the Graphics Swing gives you as an argument to other methods:

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.drawRect(350, 20, 400, 300);
    doSomeStuff(g);
    if (flag1) {
        doMoreStuff(g);
    } else {
        doDifferentStuff(g);
    }
}

I'd like to have access to the Graphics object to draw stuff from other functions based on my program flow. I'm thinking along the lines of the following:

No, you shouldn't do that in a standard Swing application as the Graphics object obtained will not persist and your drawings will disappear whenever the JVM or the operating system decide that a repaint is necessary. Consider creating Lists or other collections of objects to be drawn (such as from classes that implement Shape) and iterating through these collections in the paintComponent method. Another technique is to draw on a BufferedImage and then display it in your paintComponent method.

basic JComponent for Painting/Custom Painting/2D Graphics in Swing or for Image/ImageIcon too, is JLabel,

don't call another viod(s) or class(es) from 2D Graphics or CustomPainting, some valuable examples are here or here, search on this forum for excelent suggestions about Custom Painting and 2D Graphics