Java 个图形出现 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 );

我遇到了 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 class Frame，因为如果您不小心，这可能会导致与 java.awt.Frame class 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();
            }
        });
    }
}

Answer 2

改变

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

到

public void specificPaint(int coordinateX, int coordinateY, Color color, Graphics g)

然后在您的 paintComponent 方法中调用 specificPaint 并使用您正在绘制的 Graphics 对象

请参阅 Hovercraft 的回答以了解有关为什么使用 this.getGraphics() 不起作用的更多详细信息

Java 个图形出现 NullPointerException

NullPointerException on Java graphics

java

swing

awt