按下按钮时绘制字符串，矩形或椭圆形的GUI

Question

For my assignment, I had to write a program that will either print some text, an oval, or a rectangle depending on which button is pressed at the top of the screen, however; when I press a button nothing happens, how would I fix this? This is my first GUI and I would appreciate any help! I'll end up needing the program to: start out with a rectangle, make whichever shape happens to be on screen stay in the center of the drawing area when the window gets resized, and my ovals and rectangles have to have half the width and height of the display area. I'm taking this one step at a time so I'll try to figure those out once I can actually get a shape on the screen, thanks :-).

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

public class firstGUI extends JFrame
    implements ActionListener
{
    private boolean showText = false;
    private boolean showRect = false;
    private boolean showOval = false;
    private JButton text;
    private JButton oval;
    private JButton rectangle;
    private JPanel buttonPanel;

    public firstGUI()
    {
        super("First GUI");
        setSize(512, 512);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(1,3));

        text = new JButton("Text");
        text.addActionListener(this);
        buttonPanel.add(text);

        oval = new JButton("Oval");
        oval.addActionListener(this);
        buttonPanel.add(oval);

        rectangle = new JButton("Rectangle");
        rectangle.addActionListener(this);
        buttonPanel.add(rectangle);

        //JComponent drawArea = new JComponent();
        drawStuff d = new drawStuff();

        Container contentPane = this.getContentPane();
        contentPane.add(buttonPanel, BorderLayout.NORTH);
        contentPane.add(d);
    }

    public void actionPerformed(ActionEvent event)
    {
        Object source = event.getSource();

        if (source == text)
        {
            showText = true;
        }
        else if (source == oval)
        {
            showOval = true;
        }
        else if (source == rectangle)
        {
            showRect = true;
        }
    }


    public void draw(Graphics g) 
    {
        if(showText)
        {
            g.drawString("Hello", 0, 0);
        }
        else if (showOval)
        {
            g.drawOval(0, 0, 100, 100);
        }
        else if (showRect)
        {
            g.drawRect(0, 0, 100, 100);
        }
    }

    public static void main(String [] args)
    {
        firstGUI myTest = new firstGUI();
        myTest.setVisible(true);
    }
}

class drawStuff extends JPanel
{
    public void paint(Graphics g)
    {
        super.paint(g);
    }
}

Answer 1

Try this. I added some repaint() s and helped you out with centering the objects being painted. I also changed the draw to paintComponent . This is what you should use when drawing on JComponents

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

public class firstGUI extends JFrame
     implements ActionListener {

     private boolean showText = false;
     private boolean showRect = true;
     private boolean showOval = false;
     private JButton text;
     private JButton oval;
     private JButton rectangle;
     private JPanel buttonPanel;
     private DrawStuff drawPanel = new DrawStuff();

     public firstGUI() {
         super("First GUI");
         setSize(512, 512);
         setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

         buttonPanel = new JPanel();
         buttonPanel.setLayout(new GridLayout(1, 3));

         text = new JButton("Text");
         text.addActionListener(this);
         buttonPanel.add(text);

         oval = new JButton("Oval");
         oval.addActionListener(this);
         buttonPanel.add(oval);

         rectangle = new JButton("Rectangle");
         rectangle.addActionListener(this);
         buttonPanel.add(rectangle);


         Container contentPane = this.getContentPane();
         contentPane.add(buttonPanel, BorderLayout.NORTH);
         add(drawPanel);
    }

    @Override
    public void actionPerformed(ActionEvent event) {
        Object source = event.getSource();

        if (source == text) {
            showText = true;
            repaint();
        } else if (source == oval) {
            showOval = true;
            repaint();
        } else if (source == rectangle) {
            showRect = true;
            repaint();
        }
    }

    public static void main(String[] args) {
        firstGUI myTest = new firstGUI();
        myTest.setVisible(true);
    }

    class DrawStuff extends JPanel {

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

            if (showText) {
                g.drawString("Hello", getHeight() / 2, getWidth() / 2);
                showText = false;
            } else if (showOval) {
                g.drawOval(getWidth() / 4, getHeight() / 4, getWidth() / 2, getHeight() / 2);
                showOval = false;
            } else if (showRect) {
                g.drawRect(getWidth() / 4, getHeight() / 4, getWidth() / 2, getHeight() / 2);
                showRect = false;
            }
        }
    }
}

Answer 2

You have all the building blocks, but after the button is pressed, the draw() method isn't called. Once you set your state (which item to draw) you need to re-draw()

Answer 3

1. Take a look at Performing Custom Painting .
2. Don't override paint but instead use paintComponent .
3. Take your draw method and move it to your drawStuff class. Call drawStuff from your paintComponent method
4. Set up some kind of flag (in drawStuff ) that determines what should be painted
5. When you handle the button event, change the state flag on the panel and repaint the panel....

This will require your frame to have a reference to drawStuff

You may also want to take a look at and use Code Conventions for the Java Programming Language