Question

I’ve been trying to figure out what is wrong with my code here. The idea is to create a small Paint program and to have red, green, blue, and clear buttons. I have everything that I can think of for it to work, but can’t figure out what is wrong with the code. The Program opens, and immediately closes.

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

public class Paint{

    public static void main(String[] args){
            gui g = new gui();
            g.setVisible(true);

    }

}

 public class gui extends JComponent implements ActionListener{
    JButton red, green, blue, clear;
    Image image;
    Graphics2D draw;
    int x, y, prevX, prevY;

    gui(){
            JFrame frame = new JFrame("Paint");
            Container content = frame.getContentPane();
            content.setLayout(new BorderLayout());
            setDoubleBuffered(false);

            JPanel panel = new JPanel();
            content.add(panel, BorderLayout.SOUTH);
            panel.setPreferredSize(new Dimension(32, 68));
            panel.setMinimumSize(new Dimension(32, 68));
            panel.setMaximumSize(new Dimension(32, 68));


            red = new JButton("Red");
            green = new JButton("Green");
            blue = new JButton("Blue");
            clear = new JButton("Clear");

            red.setPreferredSize(new Dimension(50, 16));
            green.setPreferredSize(new Dimension(50,16));
            blue.setPreferredSize(new Dimension(50, 16));

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

            red.addActionListener(this);
            green.addActionListener(this);
            blue.addActionListener(this);
            clear.addActionListener(this);

            frame.setSize(500, 500);


            addMouseListener(new MouseAdapter(){
                    public void mousePressed(MouseEvent e){
                            prevX = e.getX();
                            prevY = e.getY();
                    }

            });

            addMouseMotionListener(new MouseMotionAdapter(){
                    public void mouseDragged(MouseEvent e){
                            x = e.getX();
                            y = e.getY();
                            draw.drawLine(prevX, prevY, x, y);
                            repaint();
                            prevX = x;
                            prevY = y;

                    }


            });

    }


    public void paintComponent(Graphics g){

           if(image==null){
                    image = createImage(getSize().width, getSize().height);

                    draw = (Graphics2D)image.getGraphics();

                    draw.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

                    draw.setPaint(Color.white);
                    draw.fillRect(0, 0, getSize().width, getSize().height);
                    draw.setPaint(Color.black);
                    repaint();
           }

            g.drawImage(image, 0, 0, null);

    }


    public void actionPerformed(ActionEvent e) {
        if( e.getSource()==red){
            draw.setPaint(Color.red);
            repaint();
        }
        if( e.getSource()==green){
            draw.setPaint(Color.green);
            repaint();
        }
        if( e.getSource()==blue){
            draw.setPaint(Color.blue);
            repaint();
        }
        if( e.getSource()==clear){
            draw.setPaint(Color.white);
            draw.fillRect(0, 0, getSize().width, getSize().height);
            draw.setPaint(Color.black);
            repaint();
        }


    }


 }

Answer 1

your have to use the visibility properties with frame and panel as well like

frame.setVisible(true);

line g.setVisible(true); is not working for you as you have extended your class jcomponent and your are using frame and not setting its property to set it visible.

Same problem will occure with your panel so your have to set its property as well i-e

panel.setVisible(true);

Here is your full code that is working after adding these properties

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Paint{

    public static void main(String[] args){
            gui g = new gui();
            g.setVisible(true);

    }

}

 class gui extends JComponent implements ActionListener{
    JButton red, green, blue, clear;
    Image image;
    Graphics2D draw;
    int x, y, prevX, prevY;

    gui(){
            JFrame frame = new JFrame("Paint");
            Container content = frame.getContentPane();
            content.setLayout(new BorderLayout());
            setDoubleBuffered(false);

            JPanel panel = new JPanel();
            content.add(panel, BorderLayout.SOUTH);
            panel.setPreferredSize(new Dimension(32, 68));
            panel.setMinimumSize(new Dimension(32, 68));
            panel.setMaximumSize(new Dimension(32, 68));


            red = new JButton("Red");
            green = new JButton("Green");
            blue = new JButton("Blue");
            clear = new JButton("Clear");

            red.setPreferredSize(new Dimension(50, 16));
            green.setPreferredSize(new Dimension(50,16));
            blue.setPreferredSize(new Dimension(50, 16));

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

            panel.setVisible(true);

            red.addActionListener(this);
            green.addActionListener(this);
            blue.addActionListener(this);
            clear.addActionListener(this);

            frame.setSize(500, 500);

            frame.setVisible(true)            ;


            addMouseListener(new MouseAdapter(){
                    public void mousePressed(MouseEvent e){
                            prevX = e.getX();
                            prevY = e.getY();
                    }

            });

            addMouseMotionListener(new MouseMotionAdapter(){
                    public void mouseDragged(MouseEvent e){
                            x = e.getX();
                            y = e.getY();
                            draw.drawLine(prevX, prevY, x, y);
                            repaint();
                            prevX = x;
                            prevY = y;

                    }


            });

    }


    public void paintComponent(Graphics g){

           if(image==null){
                    image = createImage(getSize().width, getSize().height);

                    draw = (Graphics2D)image.getGraphics();

                    draw.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

                    draw.setPaint(Color.white);
                    draw.fillRect(0, 0, getSize().width, getSize().height);
                    draw.setPaint(Color.black);
                    repaint();
           }

            g.drawImage(image, 0, 0, null);

    }


    public void actionPerformed(ActionEvent e) {
        if( e.getSource()==red){
            draw.setPaint(Color.red);
            repaint();
        }
        if( e.getSource()==green){
            draw.setPaint(Color.green);
            repaint();
        }
        if( e.getSource()==blue){
            draw.setPaint(Color.blue);
            repaint();
        }
        if( e.getSource()==clear){
            draw.setPaint(Color.white);
            draw.fillRect(0, 0, getSize().width, getSize().height);
            draw.setPaint(Color.black);
            repaint();
        }


    }


 }