Drawing with paintComponent in Applet Upon Event

问题

I'm trying to create an applet that will produce as many ovals as the number specified within a textbox. The textbox appears, but upon hitting enter, my paintComponent does not draw. Thank you in advance.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import net.miginfocom.layout.*;
import net.miginfocom.swing.MigLayout;
import java.awt.geom.*;

public class OvalDrawer extends JApplet
{
private JLabel numberL;
private JTextField numberTF;

private NumHandler numHandler;

public static final int WIDTH = 500;
public static final int HEIGHT = 500;

//Create Layout
public void init()
{
    setLayout(new MigLayout("wrap 2"));
    numberL = new JLabel("Enter number of ovals to draw:");
    numberTF = new JTextField(7);

    add(numberL);
    add(numberTF);

    numHandler = new NumHandler();
    numberTF.addActionListener(numHandler);

    setSize(500, 500);
}

//Event Handler
public class NumHandler implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        repaint();
    }

}

//Draw Ovals
public void paintComponent (Graphics g)
{
    super.paintComponents(g);
    int number;
    int x = 10;
    int y = 30;
    int width = 20;
    int height = 10;

    number = Integer.parseInt(numberTF.getText());

    for (int i = 0; i < number; i++)
    {
        g.drawOval(x, y, width, height);

        x += 5;
        y += 5;
        width += 5;
        height += 5;
    }
}
}

回答1:

A JApplet class does not have a paintComponent method to override. Note that your compiler won't let you call the actual super method (you think you may be doing this, but you're actually calling super.paintComponents(...), a completely different method).

A bad solution is to override the JApplet's paint method, but I strongly advise you not to do this. Instead you should draw in the paintComponent method of a JPanel and then have the JApplet display that JPanel. Also, you'll want to get into the habit of using the @Override annotation to be sure that you're actually overriding methods you think are.

回答2:

/*  * <Applet code=PressButton2 width=600 height=600>  * </Applet> 
*/

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


class MyPanel extends JPanel {
    static String s="n";

    public void paintComponent(Graphics g)  { 
        super.paintComponent(g);

        if(s.equals("g"))
            setBackground(Color.green);
        if(s.equals("b"))
            setBackground(Color.blue);
        if(s.equals("c"))
            setBackground(Color.white);
     }
}

public class PressButton2 extends JApplet {
    MyPanel panel;
    MyPanel screen;
    String s="n";

    JButton green, clear, blue;

    public void init() {
        Container container = getContentPane();
        panel = new MyPanel();
        screen = new MyPanel();
        panel.setLayout(new GridLayout(1, 3));

        green = new JButton("Green");
        blue = new JButton("Blue");
        clear = new JButton("Clear");
        green.addActionListener(new ActionEventHandler1());
        blue.addActionListener(new ActionEventHandler1());
        clear.addActionListener(new ActionEventHandler1());

        panel.add(green);
        panel.add(blue);
        panel.add(clear);

        container.add(panel, BorderLayout.SOUTH);
        container.add(screen);
    }

class ActionEventHandler1 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        String temp = e.getActionCommand(); 

        if (temp.equals("Green")) {
            MyPanel.s = "g";

            screen.repaint();
        }
        if (temp.equals("Blue")) {
            MyPanel.s = "b";
            screen.repaint();
        }
        if (temp.equals("Clear")) {
            MyPanel.s = "c";
            screen.repaint();
        }
    }
}

来源：https://stackoverflow.com/questions/13707942/drawing-with-paintcomponent-in-applet-upon-event