Java Paint方法不绘画？

Question

I'm working on a simple little swing component, and I'm tearing out my hair trying to figure out why my paint method isn't working.

The idea behind this component is that it's a small JPanel with a label. The background (behind the label) is supposed to be white, with a colored rectangle on the left-hand side indicating the ratio of two measurements: "actual" and "expected".

If you had a bunch of these components aligned vertically, they'd form a bar chart, composed of horizontal bars.

This kind of thing should be super-simple.

Anyhow, here's the code:

package com.mycompany.view;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Rectangle;

import javax.swing.JLabel;
import javax.swing.JPanel;

public class BarGraphPanel extends JPanel {

   private static final Color BACKGROUND = Color.WHITE;
   private static final Color FOREGROUND = Color.BLACK;

   private static final Color BORDER_COLOR = new Color(229, 172, 0);
   private static final Color BAR_GRAPH_COLOR = new Color(255, 255, 165);

   private int actual = 0;
   private int expected = 1;

   private JLabel label;

   public BarGraphPanel() {
      super();
      label = new JLabel();
      label.setOpaque(false);
      label.setForeground(FOREGROUND);
      super.add(label);
      super.setOpaque(true);
   }

   public void setActualAndExpected(int actual, int expected) {
      this.actual = actual;
      this.expected = expected;
   }

   @Override
   public void paint(Graphics g) {

      double proportion = (expected == 0) ? 0 : ((double) actual) / expected;
      Rectangle bounds = super.getBounds();

      g.setColor(BACKGROUND);
      g.fillRect(bounds.x, bounds.y, bounds.width, bounds.height);

      g.setColor(BAR_GRAPH_COLOR);
      g.fillRect(bounds.x, bounds.y, (int) (bounds.width * proportion), bounds.height);

      g.setColor(BORDER_COLOR);
      g.drawRect(bounds.x, bounds.y, bounds.width, bounds.height);

      label.setText(String.format("%s of %s (%.1f%%)", actual, expected, proportion * 100));
      super.paint(g);
      g.dispose();
   }

}

And here's the simple test harness:

package com.mycompany.view;

import java.awt.Dimension;
import java.awt.GridLayout;

import javax.swing.JFrame;
import javax.swing.UIManager;

public class MyFrame extends JFrame {

   public MyFrame() {
      super();
      super.setLayout(new GridLayout(3, 1));
      super.setPreferredSize(new Dimension(300, 200));

      BarGraphPanel a = new BarGraphPanel();
      BarGraphPanel b = new BarGraphPanel();
      BarGraphPanel c = new BarGraphPanel();

      a.setActualAndExpected(75, 100);
      b.setActualAndExpected(85, 200);
      c.setActualAndExpected(20, 300);

      super.add(a);
      super.add(b);
      super.add(c);
   }

   public static void main(String[] args) {
      javax.swing.SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGUI();
         }
      });
   }

   public static void createAndShowGUI() {

      try {
         UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
      } catch (Throwable t) { }

      MyFrame frame = new MyFrame();
      frame.pack();
      frame.setVisible(true);
   }

}

The test harness creates a simple frame and then adds three of these controls.

The labels are all rendered correctly, which indicates to me that the paint() method is actually being called, but the rectangles aren't being drawn to the Graphics object.

What am I doing wrong?

And why does Swing programming suck so much?

---

Here's my final code. Thanks, everyone, for your help!

public void paintComponent(Graphics g) {

   double proportion = (expected == 0) ? 0 : ((double) actual) / expected;

   Rectangle bounds = super.getBounds();

   g.setColor(BACKGROUND);
   g.fillRect(0, 0, bounds.width, bounds.height);

   g.setColor(BAR_GRAPH_COLOR);
   g.fillRect(0, 0, (int) (bounds.width * proportion), bounds.height);

   g.setColor(BORDER_COLOR);
   g.drawRect(0, 0, bounds.width - 1, bounds.height - 1);

   FontMetrics metrics = g.getFontMetrics();
   String label = String.format("%s of %s (%.1f%%)", actual, expected, proportion * 100);
   Rectangle2D textBounds = metrics.getStringBounds(label, g);

   g.setColor(FOREGROUND);
   g.drawString(label, 5, (int) ((bounds.height + textBounds.getHeight()) / 2));
}

Answer 1

I think you've almost answered your own question in the comments, along with David's answer. Change paint(Graphics g) to paintComponent(Graphics g) and remove the last two lines of the method, and you should be fine.

EDIT: Oddly, this only works for the first bar of the three. More testing in progress...

By the way, you have an off-by-one error in the border-painting code. It should be:

g.setColor(BORDER_COLOR);
g.drawRect(bounds.x, bounds.y, bounds.width - 1, bounds.height - 1);

EDIT2: OK, got it. Your full paintComponent method should be as follows:

@Override
public void paintComponent(Graphics g) {
    double proportion = (expected == 0) ? 0 : ((double) actual) / expected;
    Rectangle bounds = super.getBounds();
    g.setColor(BACKGROUND);
    g.fillRect(0, 0, bounds.width, bounds.height);
    g.setColor(BAR_GRAPH_COLOR);
    g.fillRect(0, 0, (int) (bounds.width * proportion), bounds.height);
    g.setColor(BORDER_COLOR);
    g.drawRect(0, 0, bounds.width-1, bounds.height-1);
    label.setText(String.format("%s of %s (%.1f%%)", actual, expected, proportion * 100));
}

Note that the coordinates given to g.fillRect() and g.drawRect() are relative to the component, so they must start at (0,0).

And no, I can't help you with your last question, though I feel your pain. :)

Answer 2

In your JPanel, you called super.setOpaque(true). The JPanel is going to completely fill in the background when you call super.paint() and overwrite your retangles.

Answer 3

不确定这是否是你的问题的根源，但在Swing你应该覆盖paintComponent(Graphics2D)而不是......

Answer 4

If anything, I think you should be calling super.paint(g); at the TOP of your method, not at the very bottom. It's possible the superclass is drawing on top of your stuff.