为什么我不能在 java 中的 Frame 上绘制任何东西？

Question

编码在这里。 我无法在框架内创建任何矩形或圆形。 该项目的目标是创建转换摄氏度 2 华氏度和华氏度 2 摄氏度。

所以我想要的是，请教我如何在框架内绘制矩形或椭圆形。

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Frame;
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;

public class C2F  extends JComponent{

private double input1, output1;
private double input2, output2;
JPanel center = new JPanel();
JPanel top = new JPanel();
JPanel east = new JPanel();
JPanel south = new JPanel();
//for giving input & output

C2F(){

JFrame frame = new JFrame();
frame.setTitle("C2F");
frame.setSize(700,500);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());

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.setVisible(true);
CC2F();

}

public void CC2F(){
//making frame

//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){

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

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

input2 = Double.parseDouble(T2.getText());
output2 = (input2 - 32)/9*5;
T1.setText(""+output2);
}
});
//making events

//placing the buttons and labels
output1 = 0;
output2 = 0;
//initialize the value

}
public void paintComponent(Graphics g) {
//error spots. it compiles well. But this is not what I want. 
 super.paintComponent(g);   
Graphics2D gg = (Graphics2D) g;

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

gg.setColor(Color.RED);
gg.fillRect(350, 500, 10,(int) output1);
gg.fillOval(350, 500, 10, 10);

gg.setColor(Color.RED);
gg.fillRect(350, 500, 10,(int) output2);
gg.fillOval(350, 500, 10, 10);

//to draw stuffs
}

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

Answer 1

1. 您永远不会真正将C2F 添加到任何能够绘制它的东西上，因此您的paint 方法将永远不会被调用。
2. 您应该覆盖paintComponent 而不是paint，因为您已经破坏了组件的油漆链，这可能会导致无数有趣的油漆故障问题。约定还建议您在执行任何自定义绘制之前也应调用super.paintComponent（覆盖paintComponent）

请参阅Painting in AWT and Swing 和Performing Custom Painting 了解更多详情

作为一般建议，我不鼓励您在另一个组件的构造函数中创建框架，这将使该组件再次几乎无法使用（例如，如果您想在另一个容器上重用它）

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();
    }
}