java - Java 图形上的 NullPointerException

Question

我正在尝试实现一种方法来绘制网格的特定部分。为此，我覆盖了 paintComponent 方法:

public class Frame extends JPanel {
...
@Override
public void paintComponent( Graphics g ) {

    super.paintComponent( g );

    g.clearRect(0, 0, getWidth(), getHeight());
    this.rectWidth = getWidth() / this.NUM_COLUMNS;
    this.rectHeight = getHeight() / this.NUM_ROWS;

    for (int i = 0; i < NUM_ROWS; i++) {

        for (int j = 0; j < NUM_COLUMNS; j++) {

            int x = i * this.rectWidth;
            int y = j * this.rectHeight;
            g.setColor( Color.WHITE );
            g.fillRect( x, y, this.rectWidth, this.rectHeight );

        }
    }

}
}

这很好，但是，我想在这样的函数调用上绘制特定部分:

public void specificPaint( int coordinateX, int coordinateY, Color color ){

    Graphics g = this.getGraphics();

    int x = coordinateX * this.rectWidth;
    int y = coordinateY * this.rectHeight;

    g.setColor( color );
    g.fillRect( x, y, this.rectWidth, this.rectWidth);

}

调用应该是这样的

// TESTING
    this.modelMap.specificPaint( 40,40,Color.RED );
    this.modelMap.specificPaint( 10,10,Color.RED );
    this.modelMap.specificPaint( 20,25,Color.BLUE );

我遇到 Graphics 对象的空指针错误，为什么我不能恢复并使用它？

有没有更好的方法？

Answer 1

从组件获取 Graphics 对象时切勿这样做:

Graphics g = this.getGraphics();

由此获得的 Graphics 对象将不会长期存在，这可能会导致 NPE(就像您得到的那样)或在重绘时不持久的图像。相反，在 paintComponent 方法中进行绘图。请注意，您可以调用 repaint(...) 并通过传入 Rectangle 参数指定要绘制的特定 Rectangle。

请注意，您可以在 BufferedImage 上调用 getGraphics() 并对其进行绘制，然后在您的 paintComponent 方法中绘制 BufferedImage 如果您想进行不移动的点绘制。

一个不相关的建议:避免调用您的类 Frame，因为如果您不小心，这可能会导致与 java.awt.Frame 类的名称冲突。

例如:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import javax.swing.*;

@SuppressWarnings("serial")
public class MyPanel extends JPanel {
    private static final int PREF_W = 400;
    private static final int PREF_H = PREF_W;
    private int cols;
    private int rows;
    private static final Color BG = Color.BLACK;
    private static final int GAP = 1;
    private BufferedImage img;
    private int rectWidth;
    private int rectHeight;

    public MyPanel(int rows, int cols) {
        this.cols = cols;
        this.rows = rows;
        img = createMyImage();
    }

    public void specificPaint(int coordinateX, int coordinateY, Color color) {
        Graphics g = img.getGraphics(); // get img's Graphics object
        int x = coordinateX * this.rectWidth + GAP;
        int y = coordinateY * this.rectHeight + GAP;
        g.setColor(color);
        g.fillRect(x, y, rectWidth - 2 * GAP, rectWidth - 2 * GAP);
        g.dispose();
        repaint();
    }

    private BufferedImage createMyImage() {
        img = new BufferedImage(PREF_W, PREF_H, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2 = img.createGraphics();
        g2.setBackground(BG);
        g2.clearRect(0, 0, img.getWidth(), img.getHeight());
        this.rectWidth = img.getWidth() / cols;
        this.rectHeight = img.getHeight() / rows;

        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                int x = i * this.rectWidth + GAP;
                int y = j * this.rectHeight + GAP;
                g2.setColor(Color.WHITE);
                g2.fillRect(x, y, this.rectWidth - 2 * GAP, this.rectHeight - 2 * GAP);
            }
        }
        g2.dispose();
        return img;
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (img != null) {
            g.drawImage(img, 0, 0, this);
        }

        // if you need to draw changing non-static images, do it here
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet() || img == null) {
            return super.getPreferredSize();
        }
        int w = img.getWidth();
        int h = img.getHeight();
        return new Dimension(w, h);
    }

    private static void createAndShowGui() {
        MyPanel modelMap = new MyPanel(50, 50);
        JFrame frame = new JFrame("MyPanel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(modelMap);
        frame.pack();
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);

        modelMap.specificPaint( 40,40,Color.RED );
        modelMap.specificPaint( 10,10,Color.RED );
        modelMap.specificPaint( 20,25,Color.BLUE );
    }

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