单击按钮后，Swing自动重绘JPanel

Question

I'm having this weird issue with Swing. I am drawing few points and lines between them, directly on JPanel. I'm just calling:

g.drawLine(pointAX, pointAY, pointBX, pointBY);

Where g is Graphics object taken from panel.getGraphics()

And this works alright, gives me nice output. But then I'm trying to move it and I use ActionListener on my button, which does:

    panel.revalidate();
    panel.repaint();
    drawPanel();
    moveVector(moveX, moveY);

    drawPoints();
    drawConnections(connections);

The other methods: drawPanel just draws some lines, so it's easier to see:

private void drawPanel(){
    Graphics g = panel.getGraphics();
    zeroX = panel.getWidth() / 2;
    zeroY = panel.getHeight() / 2;

    g.setColor(Color.GRAY);

    for(int i = zeroX + 10; i < panel.getWidth(); i += 10){
        g.drawLine(i, 0, i, panel.getHeight());
    }

    for(int i = zeroX + 10; i > 0; i -= 10){
        g.drawLine(i, 0, i, panel.getHeight());
    }

    for(int j = zeroY - 10; j < panel.getHeight(); j += 10){
        g.drawLine(0, j, panel.getWidth(), j);
    }

    for(int j = zeroY - 10; j > 0; j -= 10){
        g.drawLine(0, j, panel.getWidth(), j);
    }

    g.setColor(Color.BLACK);
    g.drawLine(zeroX, 0, zeroX, panel.getHeight());
    g.drawLine(0, zeroY, panel.getWidth(), zeroY);

    panel.paintComponents(g);
}

drawPoints looks like this

private void drawPoints() {
    for(int i = 0; i < points.size(); i++){
        int x = Integer.parseInt(points.get(i)[1]);
        int y = Integer.parseInt(points.get(i)[2]);
        drawPoint(x, y);
    }
}

private void drawPoint(int x, int y) {
    Graphics g = panel.getGraphics();

    for (int i = -1; i < 2; i++) {
        g.drawLine(zeroX + x + i, zeroY + y - 1, zeroX + x + i, zeroY + y);
    }

    panel.paintComponents(g);
}

    g.setColor(Color.BLACK);
    g.drawLine(zeroX, 0, zeroX, panel.getHeight());
    g.drawLine(0, zeroY, panel.getWidth(), zeroY);

    panel.paintComponents(g);
}

and drawConnections :

private void drawConnections(ArrayList<String[]> lines) {
    for (String[] arr : lines) {
        String x = arr[1];
        String y = arr[2];

        drawConnection(x, y);
    }
}

private void drawConnection(String pointA, String pointB) {
    int pointAX = 0, pointAY = 0, pointBX = 0, pointBY = 0;
    Graphics g = panel.getGraphics();

    for (String[] arr : points) {
        if (arr[0].equals(pointA)) {
            pointAX = Integer.parseInt(arr[1]);
            pointAY = Integer.parseInt(arr[2]);
        } else if (arr[0].equals(pointB)) {
            pointBX = Integer.parseInt(arr[1]);
            pointBY = Integer.parseInt(arr[2]);
        }
    }

    g.drawLine(zeroX + pointAX, zeroY + pointAY, zeroX + pointBX, zeroY + pointBY);
    panel.paintComponents(g);
}

What I don't understand here is that everything looks OK. I did debug and it looks ok and at the end of listener call, when everything is painted (although it paints over the old one instead of clearing it) it suddenly clears everything and nothing is visible at all.

Answer 1

You need to implement all the drawing in paintComponent(Graphics) in your JPanel class (or methods that are called from it). Calling getGraphics on the panel instance and using the Graphics object is not always guaranteed to work. It's also strange that you call panel.repaint() and immediately afterwards try to do additional drawing using the graphics object from the panel, that additional drawing should just be done in paintComponent(Graphics) .

Using paintComponent(Graphics) will ensure your painting is done at the right time, and that the panel's graphics will be cleared (if you call the super method at least).

Answer 2

panel.repaint does not immediately repaint the panel, but it tells Swing that this panel should be repainted in the near future (see API docs: https://docs.oracle.com/javase/7/docs/api/javax/swing/JComponent.html#repaint(long,%20int,%20int,%20int,%20int) . So your custom painting will be cleared once the panel repaints itself.

Create a custom component by inheriting from JPanel and overwriting the method paintComponent with your custom painting code. Then, whenever the panel needs to be repainted, your code is called, and not only the user pressed a button.