Java 图形 JApplet

Question

我目前正在开发一个针对学校的计划。我们必须制作一个Applet，我选择了JApplet。出于某种原因，我用来显示特定字符​​串的面板将不会显示。这里可能是什么问题？请指出我正确的方向。另外，我看了一些教程，有些人建议我有“开始”、“停止”和“破坏”方法，有些没有。这些方法是否对我的 JPanel 不显示图形有影响？

谢谢

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;

public class Shape extends JApplet {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    // making the radiobuttons for the shape choices 
    JRadioButton squareButton = new JRadioButton("Square",true);
    JRadioButton ovalButton = new JRadioButton("Oval",false);
    JRadioButton rectangleButton = new JRadioButton("Rectangle",false);
    JRadioButton triangleButton = new JRadioButton("Triangle",false);

    // making radiobuttons for the color choices
    JRadioButton redButton = new JRadioButton("Red",true);
    JRadioButton blueButton = new JRadioButton("Blue",false);
    JRadioButton greenButton = new JRadioButton("Green",false);
    JRadioButton yellowButton = new JRadioButton("Yellow",false);

    // making buttons draw and animate
    JButton drawButton = new JButton("Draw!");
    JButton animateButton = new JButton("Animate!");

    // making JTextFields for length and width
    JTextField lengthField = new JTextField("Enter a length",15);
    JTextField widthField = new JTextField("Enter a width",15);

    // making JPanel, in which the radiobuttons will go01
    JPanel shapePanel = new JPanel();
    JPanel colorPanel = new JPanel();
    JPanel buttonPanel = new JPanel();
    JPanel textPanel = new JPanel();
    drawPanel dPanel;

    ButtonGroup shapeGroup = new ButtonGroup();
    ButtonGroup colorGroup = new ButtonGroup();

    // variables that will dictates the shape, size and color
    int length = 200;
    int width = 200;
    Color color = Color.RED;
    String shape = "square";

    public void init() {
        setLayout(new FlowLayout()); // setting layout for the applet
        setSize(680,480);

        // setting the layout for the shapePanel - gridlayout 2 rows, 2 cols and space of 5
        shapePanel.setLayout(new GridLayout(2,2,5,5));

        // adding a border to the shapePanel to indicate to the user what it does "titledBorder"
        shapePanel.setBorder(BorderFactory.createTitledBorder("Choose a shape"));

        // setting layout for the color panel - gridlayout 2 rows, 2 cols and space of 5
        colorPanel.setLayout(new GridLayout(2,2,5,5));

        // adding a border to the  colorPanel to indicate to the user what it does "titledBorder"
        colorPanel.setBorder(BorderFactory.createTitledBorder("Choose a color"));

        // setting the layout for the buttonPanel - gridlayout 1 row, 2 cols and space of 5
        buttonPanel.setLayout(new GridLayout(1,2,5,5));

        // adding a color border
        buttonPanel.setBorder(BorderFactory.createLineBorder(Color.red, 2));

        // setting the layout of the textField - gridlayout 1 row, 2 cols and space of 5
        textPanel.setLayout(new GridLayout(1,2,5,5));

        // adding some attributes for lengthField and widthField
        lengthField.setFont(new Font("Arial",Font.PLAIN,12));
        lengthField.setForeground(new Color(150,150,150));

        widthField.setFont(new Font("Arial",Font.PLAIN,12));
        widthField.setForeground(new Color(150,150,150));

        // using shapegroup to organize the JRadioButtons
        shapeGroup.add(squareButton);
        shapeGroup.add(ovalButton);
        shapeGroup.add(rectangleButton);
        shapeGroup.add(triangleButton);

        // using colorgroup to organize the color radiobuttons
        colorGroup.add(redButton);
        colorGroup.add(blueButton);
        colorGroup.add(greenButton);
        colorGroup.add(yellowButton);

        // add the shape buttons to the panel so they appear in a square form 
        shapePanel.add(squareButton);
        shapePanel.add(ovalButton);
        shapePanel.add(rectangleButton);
        shapePanel.add(triangleButton);

        // adding color buttons to the color panel
        colorPanel.add(redButton);
        colorPanel.add(blueButton);
        colorPanel.add(greenButton);
        colorPanel.add(yellowButton);

        // adding jbuttons
        buttonPanel.add(drawButton);
        buttonPanel.add(animateButton);

        // adding textfields to the textPanel
        textPanel.add(lengthField);
        textPanel.add(widthField);

        dPanel  = new drawPanel();

        // adding panels to the applet
        add(shapePanel);
        add(colorPanel);
        add(buttonPanel);
        add(textPanel);
        add(dPanel);

        // adding focus listener to lengthField and widthField
        lengthField.addFocusListener(new FocusListener() {

            public void focusGained(FocusEvent e) {
                lengthField.setText("");
                lengthField.setForeground(Color.black);
            }

            public void focusLost(FocusEvent e) {}

        });

        widthField.addFocusListener(new FocusListener() {

            public void focusGained(FocusEvent e) {
                widthField.setText("");
                widthField.setForeground(Color.black);
            }

            public void focusLost(FocusEvent e) {}

        });

        drawButton.addActionListener(new drawListener());

    }

    // when the person presses paint, this will be executed to paint the specific shape, color with the width and length
    class drawListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {

            int mylength = 5; 
            int mywidth = 5;

            try {
                mylength = Integer.parseInt(lengthField.getText());;
                mywidth = Integer.parseInt(widthField.getText());;
            }catch(Exception ex) {
                JOptionPane.showMessageDialog(null,""+ex,"",JOptionPane.ERROR_MESSAGE);
            }

            if((mylength > 200 || mylength < 5)) {
                JOptionPane.showMessageDialog(null, "Please make sure the number is above 5 and less than 200", 
                        "Invalid length message", JOptionPane.ERROR_MESSAGE);
            }else if((mywidth > 200 || mywidth < 5)) {
                JOptionPane.showMessageDialog(null, "Please make sure the number is above 5 and less than 200", 
                        "Invalid width message", JOptionPane.ERROR_MESSAGE);
            }else {
                length = mylength;
                width = mywidth;

                // checking which color button is selected
                if(redButton.isSelected()) {
                    color = Color.RED;
                }else if(blueButton.isSelected()) {
                    color = Color.BLUE;
                }else if(greenButton.isSelected()) {
                    color = Color.GREEN;
                }else if(yellowButton.isSelected()) {
                    color = Color.YELLOW;
                }

                // checking which shape has been selected
                if(rectangleButton.isSelected()) {
                    shape = "rectangle";
                }else if(triangleButton.isSelected()) {
                    shape = "triangle";
                }else if(ovalButton.isSelected()) {
                    shape = "oval";
                }else if(squareButton.isSelected()) {
                    shape = "square";
                }

                //System.out.printf("%3d %3d %s %s \n",length,width,shape,color);

            }

        }
    }

    // This will be used to do the painting
    class drawPanel extends JPanel {
        private static final long serialVersionUID = 1L;

        //Paint Method
        public void paintComponent(Graphics g){
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;
            g2.setColor(Color.black);

            g2.drawString("My awesome string", 200, 200);
        }

    }

}

Answer 1

没有组件的JPanel 的默认大小为 0x0。试试这个来源：

// <applet code=Shape width=640 height=480></applet>
import java.awt.*;

import java.awt.event.*;
import javax.swing.*;

public class Shape extends JApplet {

    /**
     *
     */
    private static final long serialVersionUID = 1L;

    // making the radiobuttons for the shape choices
    JRadioButton squareButton = new JRadioButton("Square",true);
    JRadioButton ovalButton = new JRadioButton("Oval",false);
    JRadioButton rectangleButton = new JRadioButton("Rectangle",false);
    JRadioButton triangleButton = new JRadioButton("Triangle",false);

    // making radiobuttons for the color choices
    JRadioButton redButton = new JRadioButton("Red",true);
    JRadioButton blueButton = new JRadioButton("Blue",false);
    JRadioButton greenButton = new JRadioButton("Green",false);
    JRadioButton yellowButton = new JRadioButton("Yellow",false);

    // making buttons draw and animate
    JButton drawButton = new JButton("Draw!");
    JButton animateButton = new JButton("Animate!");

    // making JTextFields for length and width
    JTextField lengthField = new JTextField("Enter a length",15);
    JTextField widthField = new JTextField("Enter a width",15);

    // making JPanel, in which the radiobuttons will go01
    JPanel shapePanel = new JPanel();
    JPanel colorPanel = new JPanel();
    JPanel buttonPanel = new JPanel();
    JPanel textPanel = new JPanel();
    drawPanel dPanel;

    ButtonGroup shapeGroup = new ButtonGroup();
    ButtonGroup colorGroup = new ButtonGroup();

    // variables that will dictates the shape, size and color
    int length = 200;
    int width = 200;
    Color color = Color.RED;
    String shape = "square";

    public void init() {
        setLayout(new FlowLayout()); // setting layout for the applet
        // This is set by HTML!
        //setSize(680,480);

        // setting the layout for the shapePanel - gridlayout 2 rows, 2 cols and space of 5
        shapePanel.setLayout(new GridLayout(2,2,5,5));

        // adding a border to the shapePanel to indicate to the user what it does "titledBorder"
        shapePanel.setBorder(BorderFactory.createTitledBorder("Choose a shape"));

        // setting layout for the color panel - gridlayout 2 rows, 2 cols and space of 5
        colorPanel.setLayout(new GridLayout(2,2,5,5));

        // adding a border to the  colorPanel to indicate to the user what it does "titledBorder"
        colorPanel.setBorder(BorderFactory.createTitledBorder("Choose a color"));

        // setting the layout for the buttonPanel - gridlayout 1 row, 2 cols and space of 5
        buttonPanel.setLayout(new GridLayout(1,2,5,5));

        // adding a color border
        buttonPanel.setBorder(BorderFactory.createLineBorder(Color.red, 2));

        // setting the layout of the textField - gridlayout 1 row, 2 cols and space of 5
        textPanel.setLayout(new GridLayout(1,2,5,5));

        // adding some attributes for lengthField and widthField
        lengthField.setFont(new Font("Arial",Font.PLAIN,12));
        lengthField.setForeground(new Color(150,150,150));

        widthField.setFont(new Font("Arial",Font.PLAIN,12));
        widthField.setForeground(new Color(150,150,150));

        // using shapegroup to organize the JRadioButtons
        shapeGroup.add(squareButton);
        shapeGroup.add(ovalButton);
        shapeGroup.add(rectangleButton);
        shapeGroup.add(triangleButton);

        // using colorgroup to organize the color radiobuttons
        colorGroup.add(redButton);
        colorGroup.add(blueButton);
        colorGroup.add(greenButton);
        colorGroup.add(yellowButton);

        // add the shape buttons to the panel so they appear in a square form
        shapePanel.add(squareButton);
        shapePanel.add(ovalButton);
        shapePanel.add(rectangleButton);
        shapePanel.add(triangleButton);

        // adding color buttons to the color panel
        colorPanel.add(redButton);
        colorPanel.add(blueButton);
        colorPanel.add(greenButton);
        colorPanel.add(yellowButton);

        // adding jbuttons
        buttonPanel.add(drawButton);
        buttonPanel.add(animateButton);

        // adding textfields to the textPanel
        textPanel.add(lengthField);
        textPanel.add(widthField);

        dPanel  = new drawPanel();
        dPanel.setPreferredSize(new Dimension(500,300));

        // adding panels to the applet
        add(shapePanel);
        add(colorPanel);
        add(buttonPanel);
        add(textPanel);
        add(dPanel);

        // adding focus listener to lengthField and widthField
        lengthField.addFocusListener(new FocusListener() {

            public void focusGained(FocusEvent e) {
                lengthField.setText("");
                lengthField.setForeground(Color.black);
            }

            public void focusLost(FocusEvent e) {}

        });

        widthField.addFocusListener(new FocusListener() {

            public void focusGained(FocusEvent e) {
                widthField.setText("");
                widthField.setForeground(Color.black);
            }

            public void focusLost(FocusEvent e) {}

        });

        drawButton.addActionListener(new drawListener());

    }

    // when the person presses paint, this will be executed to paint the specific shape, color with the width and length
    class drawListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {

            int mylength = 5;
            int mywidth = 5;

            try {
                mylength = Integer.parseInt(lengthField.getText());;
                mywidth = Integer.parseInt(widthField.getText());;
            }catch(Exception ex) {
                JOptionPane.showMessageDialog(null,""+ex,"",JOptionPane.ERROR_MESSAGE);
            }

            if((mylength > 200 || mylength < 5)) {
                JOptionPane.showMessageDialog(null, "Please make sure the number is above 5 and less than 200",
                        "Invalid length message", JOptionPane.ERROR_MESSAGE);
            }else if((mywidth > 200 || mywidth < 5)) {
                JOptionPane.showMessageDialog(null, "Please make sure the number is above 5 and less than 200",
                        "Invalid width message", JOptionPane.ERROR_MESSAGE);
            }else {
                length = mylength;
                width = mywidth;

                // checking which color button is selected
                if(redButton.isSelected()) {
                    color = Color.RED;
                }else if(blueButton.isSelected()) {
                    color = Color.BLUE;
                }else if(greenButton.isSelected()) {
                    color = Color.GREEN;
                }else if(yellowButton.isSelected()) {
                    color = Color.YELLOW;
                }

                // checking which shape has been selected
                if(rectangleButton.isSelected()) {
                    shape = "rectangle";
                }else if(triangleButton.isSelected()) {
                    shape = "triangle";
                }else if(ovalButton.isSelected()) {
                    shape = "oval";
                }else if(squareButton.isSelected()) {
                    shape = "square";
                }

                //System.out.printf("%3d %3d %s %s \n",length,width,shape,color);

            }

        }
    }

    // This will be used to do the painting
    class drawPanel extends JPanel {
        private static final long serialVersionUID = 1L;

        //Paint Method
        public void paintComponent(Graphics g){
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;
            g2.setColor(Color.black);

            g2.drawString("My awesome string", 200, 200);
        }

    }

}