从数组到JPanel绘制点

Question

I am trying to write a simple GUI application that allows the user to click on a panel, BlackPanel in this situation, to paint a dot. than save those dots to a file. And the ability to load dots from file and display them.

I have successfully made all the functions and methods needed to save and load an ArrayList<Point> from file and have made sure that the points loaded contain the coordinates that are needed to be re-painted

My problem is that I am unable to create a method that paints all the dots from an array

My main class is a JFrame which has two JPanel s added to it:

• MainPanel which holds some stuff
• BlackPanel which is used to paint on.

My Class has an ArrayList - points as a class member.

All Point objects are made using java.awt.Point .

    private void paintPoint(Graphics g, Point p) {
        g.setColor(Color.white);
        g.fillOval(p.x, p.y, 5, 5);
    } // this one is used to create a single dot called by MouseClicked event - works

    private void paintPoints(Graphics g, ArrayList<Point> points) {
        g.setColor(Color.white);
        for (Point point : points) {
            g.fillOval(point.x, point.y, 5, 5);
        }
    } // this one is called by LoadPointsDialog() which in turn is called by a button action

    private void loadPointsDialog() {
        FileDialog fd = new FileDialog(this, "Open XML file", FileDialog.LOAD);
        fd.setDirectory("C:\\");
        fd.setFile("*.xml");
        fd.setFilenameFilter((File dir, String name) -> name.endsWith(".xml"));
        fd.setVisible(true);
        String folder = fd.getDirectory();
        String fileName = fd.getFile();
        ArrayList<Point> aux;
        try { // irrelevent for this question. works.
            aux = XMLio.read(folder+fileName); 
        } catch (IOException e) {
            System.err.println("Error! Failed reading from file");
            return;
        } 
        ItemClear.doClick(); // clears points class member and calls BlackPanel.updateUI();
        points.addAll(aux); // works 100% i checked to see if the points exist.
        paintPoints(PanelBlack.getGraphics(), points);
    }

private void ItemOpenActionPerformed(java.awt.event.ActionEvent evt) {                                         
    loadPointsDialog();
}   

private void PanelBlackMouseClicked(java.awt.event.MouseEvent evt) {                                        
    Point p = evt.getPoint();
    paintPoint(PanelBlack.getGraphics(), p);
    savePoint(p); // points.add(p);
}                                       

private void ItemClearActionPerformed(java.awt.event.ActionEvent evt) {                                          
    PanelBlack.updateUI();
    points.clear();
}  

i am using NetBeans designer tool to create this JFrame.

EDIT: Solution by @Berger

I have created a new nested class :

public class PaintPanel extends JPanel {

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.white);
            for (Point point : points) {
                g.fillOval(point.x, point.y, 5, 5);
            }
        }
    }

and defined PanelBlack to be an object of this class.

Then I called repaint() after loading the new point array - works like magic.

Answer 1

You have to override the paintComponent(Graphics) method of PanelBlack .

getGraphics() is not reliable to do your paintings, paintComponent(Graphics) is where you are supposed to customize the paintings.

Painting in Swing