为什么我不能在 java 中的 Frame 上绘制任何东西?

As a general piece of advice, I'd discourage you from creating a frame within the constructor of another component, this will make the component pretty much unusable again (if you wanted to re-use it on another container for example)

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class C2F extends JComponent {

    public C2F() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                TestPane center = new TestPane();
                JPanel top = new JPanel();
                JPanel east = new JPanel();
                JPanel south = new JPanel();
                //give specific location
                JLabel L1 = new JLabel("Please input Celcius or Fahrenheit to Convert");
                top.add(L1);

                JLabel l1 = new JLabel("Cel -> Fah");
                south.add(l1);

                JTextField T1 = new JTextField(12);
                south.add(T1);

                JButton B1 = new JButton("Convert");
                south.add(B1);

                JLabel l2 = new JLabel("Fah -> Cel");
                south.add(l2);

                JTextField T2 = new JTextField(12);
                south.add(T2);

                JButton B2 = new JButton("Convert");
                south.add(B2);
                //to create buttons and labels to give an answer
                B1.addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent e) {

                        double input1 = Double.parseDouble(T1.getText());
                        double output1 = input1 * (9 / 5) + 32;
                        T2.setText("" + output1);
                        center.setOutput1(output1);
                    }
                });

                B2.addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent e) {

                        double input2 = Double.parseDouble(T2.getText());
                        double output2 = (input2 - 32) / 9 * 5;
                        T1.setText("" + output2);
                        center.setOutput2(output2);
                    }
                });
                //making events
                frame.getContentPane().add(top, BorderLayout.NORTH);
                frame.getContentPane().add(center, BorderLayout.CENTER);
                frame.getContentPane().add(south, BorderLayout.SOUTH);
                frame.getContentPane().add(east, BorderLayout.EAST);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private double output1, output2;

        public TestPane() {
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 600);
        }

        public void setOutput1(double output1) {
            this.output1 = output1;
            repaint();
        }

        public void setOutput2(double output2) {
            this.output2 = output2;
            repaint();
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g;

            g2d.setColor(Color.BLACK);
            g2d.drawOval(350, 500, 12, 12);

            g2d.setColor(Color.RED);
            g2d.fillRect(350, 0, 10, (int) output1);
            g2d.fillOval(350, 0, 10, 10);

            g2d.setColor(Color.BLUE);
            g2d.fillRect(350, 0, 10, (int) output2);
            g2d.fillOval(350, 0, 10, 10);
            g2d.dispose();
        }

    }

    public static void main(String[] args) {//to run the program
        new C2F();
    }
}