JPanel内未显示图形

Question

I need some help with my code; I have a program that will show the graph of Ohm's Law. The graph was showing before I put a save button. When i run the program, it will only show the everything except for the graph. Also, I have problems in saving the current and voltage array into a .txt file. Please help!

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);

You add your DrawGraph component to a JPanel. By default a JPanel uses a FlowLayout() which respects the preferred size of any component added to it. Your custom DrawGraph component has a preferred size of 0, so there is nothing to paint.

Every Swing component is responsible for determining its own preferred size so you need to override the getPreferredSize() method of your DrawGraph components to return its preferred size so the layout manager can do its job.

Read the section from the Swing tutorial on Custom Painting for more information and working examples.

Also, don't use setPreferredSize(). The layout manager will determine the preferred size of the panel based on the components added to it.

Answer 2

First things first. Make a JFrame derived class and in that class insert separately your buttonPanel which extends JPanel on the BorderLayout.SOUTH and your DrawGraph panel on the BorederLayout.Center . Don't add butttonPanel to the window you are drawing in.

I also suggest to change name from DrawGraph to GraphPanel you can do it just by clicking on any reference to the class and hitting alt+shift+r if you use Eclipse IDE.

So to conclude:

public class MainWindow extends JFrame(){


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

  }
}

and build your solution around that.