repaint（）不調用PaintComponent以使用Graphics2D

Question

我現在花了幾天時間試圖讓Graphics2D類在我的代碼中工作。 我以這樣的方式構造它：注冊了click事件時，完成了對重新繪制的調用，但是，僅當它到達調用repaint（）的階段時，才產生nullpointer異常。

調試時，它都按預期方式工作，而不是從paintComponent方法中調用，但是，當嘗試使用paintComponent和repaint（）正確調用代碼以允許Graphics2D類顯示到每個點的線時，它都無法正常工作。

我已經將我的代碼部分包括在內，但我上班時遇到了困難。 任何幫助將不勝感激。 先感謝您。

下面是包含我的mouseListener的GUI類。

public class GUI extends JPanel implements MouseListener {

private JLabel label;

    public BufferedImage getImg() {
    return img;
    }

    public void mouseClicked(java.awt.event.MouseEvent e) {
    // TODO Auto-generated method stub
    label = new JLabel();

    //set point equal to the location of the mouse click on the image label
    Point b = e.getPoint();

    //place we are going to print the dots
    segmentation.x = b.x; //gets the x coordinate 
    segmentation.y = b.y; //gets the y coordinate

    System.out.println("x = " + segmentation.x);
    System.out.println("y = " + segmentation.y);        

    //set the global img in the segmentation class equal to that of the one in the current tab
    segmentation.setImg(tabbedPane.getSelectedIndex(), getImg());
    segmentation.paintUpdate();
    label = segmentation.getLabel();

    //if i run this line of code the existing label I already have with simply vanish because of the paintComponent method not being called upon properly.
    //tabbedPane.setComponentAt(tabbedPane.getSelectedIndex(), label);
}

這是Segmentation類，其中包含我無法正確調用的paintComponent方法。

public class Segmentation extends JLabel {

public int[] xpoints = new int[50];
public int[] ypoints = new int[50];
private int npoints = 0;
public int x;
public int y;
GeneralPath polyline;
Graphics2D g2;
JLabel label;
BufferedImage img;
ImageIcon icon;

public void paintUpdate() {
    repaint();
}

public void setImg(int tabNum, BufferedImage img) {
    this.img = img;
}

public GeneralPath createPath() {

    //      if (npoints > 0) {
    polyline.moveTo(xpoints[0], ypoints[0]);

    for(int i = 1; i < xpoints.length; i++) {
        //add the position of the point to the respective x and y arrays
        polyline.lineTo(xpoints[i], ypoints[i]);        
    }
    //      }
    return polyline;
}

public void paintComponent(Graphics g) {
    super.paintComponent(g);

    System.out.println("we're in the paint component method");

    //set up for the new jlabel
    label = new JLabel();
    label.setIcon(new javax.swing.ImageIcon(img));
    label.setHorizontalAlignment(JLabel.LEFT);
    label.setVerticalAlignment(JLabel.TOP);

    //add the position of the point to the respective x and y arrays
    xpoints[npoints] = x;
    ypoints[npoints] = y;

    if (npoints == 0) {
        JOptionPane.showMessageDialog(null, "Your first point has been added successfully");
    }
    else {
        JOptionPane.showMessageDialog(null, "Your " + npoints + " rd/th" + " point has been added successfully");
    }

    polyline = createPath();

    // Draws the buffered image to the screen.
    g2.drawImage(img, 0, 0, this);

    g2.draw(polyline);

    npoints++;
}

Answer 1

您的Segmentation類有兩個JLabel，一個是該類本身的對象，它具有paintComponent重寫，並且可能從未使用過：

public class Segmentation extends JLabel {

另一個JLabel稱為label，它由類中的composition持有，該類實際上沒有使用方法重寫，並且似乎在“遮蓋”該類的實例：

JLabel label

如果要調用paintComponent，則需要使用實際上覆蓋該方法的label對象。

同樣，您似乎在paintComponent(...)方法（??）中創建組件。 永遠不要這樣做，也不要在此方法中包含程序邏輯。

編輯：
要繪制圖像，通常會覆蓋paintComponent並使用其Graphics / Graphics2D對象進行繪制。 例如：

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Line2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

import javax.imageio.ImageIO;
import javax.swing.*;

public class DrawOnLabel extends JLabel {
   public static final String GIRAFFE_IMG_URL = "http://upload.wikimedia.org/wikipedia/commons/thumb" +
        "/9/9e/Giraffe_Mikumi_National_Park.jpg/474px-Giraffe_Mikumi_National_Park.jpg";
   private static final Color LINES_COLOR = Color.red;
   private static final Color CURRENT_LINE_COLOR = new Color(255, 200, 200);
   private List<Line2D> lineList = new ArrayList<Line2D>();
   private Line2D currentLine = null;

   public DrawOnLabel() throws IOException {
      URL giraffeUrl = new URL(GIRAFFE_IMG_URL);
      BufferedImage img = ImageIO.read(giraffeUrl);
      ImageIcon icon = new ImageIcon(img);
      setIcon(icon);

      MyMouseAdapter myMouseAdapter = new MyMouseAdapter();
      addMouseListener(myMouseAdapter);
      addMouseMotionListener(myMouseAdapter);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);

      g2.setColor(LINES_COLOR);
      for (Line2D line : lineList) {
         g2.draw(line);
      }
      if (currentLine != null) {
         g2.setColor(CURRENT_LINE_COLOR);
         g2.draw(currentLine);
      }
   }

   private class MyMouseAdapter extends MouseAdapter {
      Point p1 = null;

      @Override
      public void mousePressed(MouseEvent e) {
         p1 = e.getPoint();
      }

      @Override
      public void mouseReleased(MouseEvent e) {
         if (currentLine != null) {
            currentLine = new Line2D.Double(p1, e.getPoint());
            lineList.add(currentLine);
            currentLine = null;
            p1 = null;
            repaint();
         }
      }

      @Override
      public void mouseDragged(MouseEvent e) {
         if (p1 != null) {
            currentLine = new Line2D.Double(p1, e.getPoint());
            repaint();
         }

      }
   }

   private static void createAndShowGui() {
      DrawOnLabel mainPanel = null;
      try {
         mainPanel = new DrawOnLabel();
      } catch (IOException e) {
         e.printStackTrace();
         System.exit(-1);
      }

      JFrame frame = new JFrame("DrawOnLabel");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.setResizable(false);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

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