JPanel内未显示图形

Question

我的代码需要一些帮助； 我有一个程序可以显示欧姆定律图。 在显示保存按钮之前，该图已显示。 当我运行程序时，它将仅显示除图形之外的所有内容。 另外，在将电流和电压数组保存到.txt文件中时遇到问题。 请帮忙！

import java.awt.*;  
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.font.FontRenderContext;
import java.awt.font.LineMetrics;
import java.awt.geom.*;
import java.io.BufferedWriter;
import java.io.IOException;
import java.nio.charset.Charset;
import static java.nio.file.Files.newBufferedWriter;
import java.nio.file.StandardOpenOption;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.*;  

public class DrawGraph extends JPanel {  
double current[] = new double [999];
double voltage[] = new double [999];
final int TEXT = 20;   

@Override
public void paintComponent(Graphics g) {  
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D)g;
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    int w = 400;  
    int h = 400;  
    g2.setStroke(new BasicStroke(3));
    g2.drawLine(TEXT, TEXT, TEXT, h-TEXT);  
    g2.drawLine(TEXT, h-TEXT, w-TEXT, h-TEXT);  

    for(int x= 0; x<1000; x++ )
    {
        current[x]=x+1;
        voltage[x]=x+1;
    }

    g2.setPaint(Color.red);  
    g2.setStroke(new BasicStroke(2));
    g2.draw(new Line2D.Double(TEXT, h-TEXT, w-TEXT ,TEXT ));  

    // Draw labels.
    g2.setPaint(Color.black);   
    Font font = g2.getFont();

    FontRenderContext frc = g2.getFontRenderContext();
    LineMetrics lm = font.getLineMetrics("0", frc);
    float sheight = lm.getAscent() + lm.getDescent();
    // Ordinate label.
    g2.setFont(new Font("Century Gothic", Font.PLAIN, 15));
    String s = "Voltage V";
    float sY = TEXT + ((h - 2*TEXT) - s.length()*sheight)/2 + lm.getAscent();
    for(int r = 0; r < s.length(); r++) 
    {
          String letter = String.valueOf(s.charAt(r));
          float swidth = (float)font.getStringBounds(letter, frc).getWidth();
          float sX = (TEXT - swidth)/2;
          g2.drawString(letter, sX, sY);
          sY += sheight;
    }
    // Abcissa label.
    s = "Current A";
    sY = h - TEXT + (TEXT - sheight)/2 + lm.getAscent();
    float swidth = (float)font.getStringBounds(s, frc).getWidth();
    float sX = (w - swidth)/2;
    g2.drawString(s, sX, sY);

    //Gridlines
    int b=TEXT+(((w-TEXT)-TEXT)/10);
    g2.setPaint(Color.gray);
    for(int a=1; a<=10; a++)
    {
           b+=36;
           g2.setStroke(new BasicStroke(1));
           g2.drawLine(b, h-TEXT, b, TEXT);  
           g2.drawLine(TEXT, b, w-TEXT, b); 

    }

}  

private static void createAndShowGui() {
  JFrame frame = new JFrame("Ohm's Law");

  JPanel panel = new JPanel(new BorderLayout(3,1));

  JPanel titlepanel = new JPanel();
  titlepanel.setPreferredSize(new Dimension(400,50));
  JLabel title = new JLabel("OHM'S LAW");
  title.setFont(new Font("Century Gothic", Font.BOLD, 25));
  titlepanel.add(title);
  panel.add(titlepanel,BorderLayout.NORTH);

  JPanel graphpanel = new JPanel();
  graphpanel.setBackground(Color.white);
  graphpanel.setPreferredSize(new Dimension(420,420));
  DrawGraph drawgraph = new DrawGraph();
  graphpanel.add(drawgraph);
  panel.add(graphpanel,BorderLayout.CENTER);

  JPanel buttonpanel = new JPanel ();
  buttonpanel.setPreferredSize(new Dimension(400,50));
  JButton save = new JButton("SAVE");
  save.addActionListener(
    new ActionListener()
    {

        @Override
        public void actionPerformed (ActionEvent event)
        {

            JFrame parentFrame = new JFrame();

            JFileChooser fileChooser = new JFileChooser();
            fileChooser.setDialogTitle("Specify a file to save");   

            int userSelection = fileChooser.showSaveDialog(parentFrame);

            if (userSelection == JFileChooser.APPROVE_OPTION) 

            {
                java.io.File fileToSave = fileChooser.getSelectedFile();
                try 
                {                                    
                    fileToSave.createNewFile();
                    try (BufferedWriter writer = newBufferedWriter(fileToSave.toPath(), Charset.defaultCharset(), StandardOpenOption.WRITE)) 
                    {

                            writer.write("V=I\t R=1\r Voltage \t Current\n");

                            //writer.write("Material: " + material.getSelectedValue().toString()+"\r\nv = " + v + "\r\nE1 = " + e1 + "\r\nE2 = " + e2);
                    }
                } 
                catch (IOException ex) 
                {
                    Logger.getLogger(DrawGraph.class.getName()).log(Level.SEVERE, null, ex);
                }                                
                System.out.println("Save as file: " + fileToSave.getAbsolutePath());
            }
        }
    }
  );
  save.setFont(new Font("Century Gothic", Font.BOLD, 15));
  buttonpanel.add(save);
  panel.add(buttonpanel, BorderLayout.SOUTH);

  frame.add(panel);
  frame.getContentPane().setBackground(Color.GREEN);
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.pack();
  frame.setLocationByPlatform(true);
  frame.setVisible(true);
}

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

Answer 1

  JPanel graphpanel = new JPanel();
  graphpanel.setBackground(Color.white);
  graphpanel.setPreferredSize(new Dimension(420,420));
  DrawGraph drawgraph = new DrawGraph();
  graphpanel.add(drawgraph);
  panel.add(graphpanel,BorderLayout.CENTER);

您将DrawGraph组件添加到JPanel。 默认情况下，JPanel使用FlowLayout（）来preferred size添加到它的任何组件的preferred size 。 您的自定义DrawGraph组件的首选大小为0，因此无需绘制任何内容。

每个Swing组件都有责任确定自己的首选大小，因此您需要重写DrawGraph组件的getPreferredSize()方法以返回其首选大小，以便布局管理器可以执行其工作。

阅读有关定制绘画的Swing教程中的部分，以获取更多信息和工作示例。

另外，不要使用setPreferredSize（）。 布局管理器将根据添加到其中的组件确定面板的首选大小。

Answer 2

首先是第一件事。 创建一个JFrame派生的类，并在该类中分别插入您的buttonPanel （在BorderLayout.SOUTH上扩展JPanel和在DrawGraph上的BorederLayout.Center面板。 不要将butttonPanel添加到要绘制的窗口中。

我还建议从改变名称DrawGraph到GraphPanel你可以通过点击任何参考类和击球做alt+shift+r ，如果你使用Eclipse IDE中。

因此得出结论：

public class MainWindow extends JFrame(){


  public void createGUI(){
    add(graphPanel = new GraphPanel(), BorderLayout.CENTER);
    add(buttonPanel = new JPanel(), BorderLayout.SOUTH);

  }
}

并围绕此构建您的解决方案。