Riassunti

Come realizzare una chat come la seguente?

Per quanto riguarda il file TopPanel.java:

package board;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File; import java.io.IOException;
import javax.imageio.ImageIO; import javax.swing.JButton;
import javax.swing.JColorChooser; import javax.swing.JFileChooser;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.filechooser.FileFilter;
import javax.swing.filechooser.FileNameExtensionFilter;

public class TopPanel extends JPanel {

    private static final long serialVersionUID = 8039202002735442212L;

    private JButton backgroundButton = new JButton("Change background");
    private JButton colorButton = new JButton("Change color");
    private JButton cleanButton = new JButton("Clean");

    public TopPanel(Board board) {
        setPreferredSize(new Dimension(600, 40));
        this.setLayout(new GridLayout(1, 3, 1, 1));
        this.add(backgroundButton); this.add(colorButton);
        this.add(cleanButton); addListeners(board);
    }

    private void addListeners(Board board) {
        backgroundButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                FileFilter imageFilter = new FileNameExtensionFilter("Image files", ImageIO.getReaderFileSuffixes());
                JFileChooser f = new JFileChooser();
                f.setFileFilter(imageFilter);
                int res = f.showOpenDialog(null);
                if(res == JFileChooser.APPROVE_OPTION) {
                    File file = f.getSelectedFile();
                    try {
                        BufferedImage img = ImageIO.read(file);
                        if(img == null)
                            throw new Exception("Error: cannot open the image");
                        board.changeBackground(img);
                    } catch (IOException e1) {
                        JOptionPane.showMessageDialog(null, "Error: cannot open the image");
                    } catch (Exception e1) {
                        JOptionPane.showMessageDialog(null, e1.getMessage());
                    }
                }
            }
        });

        colorButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                Color color = JColorChooser.showDialog(null, "Choose a new color", board.getCurrentColor());
                if(color != null) {
                    board.setCurrentColor(color);
                }
            }
        });

        cleanButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                board.clean();
            }
        });
    }
}

Per quanto riguarda il file Board.java:

package board;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.util.ArrayList;
import java.util.List;

import javax.swing.JPanel;

public class Board extends JPanel implements MouseListener, MouseMotionListener {

    private static final long serialVersionUID = 5379358483855227909L;
    private List<Point> points = new ArrayList<Point>();
    private Color currentColor = Color.BLACK;
    private Image background = null;

    public Board() {
        addMouseListener(this);
        addMouseMotionListener(this);
        setBackground(Color.WHITE);
    }

    public void changeBackground(Image img) {
        if(img != null) {
            background = img;
            background = background.getScaledInstance(getSize().width, getSize().height, Image.SCALE_SMOOTH);
            repaint();
        }
    }

    public void componentResized() {
        changeBackground(background);
    }

    //NO paintComponent***S***
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if(background != null)
            g.drawImage(background, 0, 0, null);
        else
            setBackground(Color.WHITE);
        for(Point p : points) {
            g.setColor(p.color);
            g.fillOval(p.x, p.y, 10, 10);
        }
    }

    private void handleMouseEvent(MouseEvent e) {
        Point p = new Point(e.getX(), e.getY(), currentColor);
        points.add(p);
        repaint();
    }

    public Color getCurrentColor() {
        return currentColor;
    }

    public void setCurrentColor(Color currentColor) {
        this.currentColor = currentColor;
    }

    public void clean() {
        points.clear();
        background = null;
        repaint();
    }

    @Override
    public void mousePressed(MouseEvent e) {
        handleMouseEvent(e);
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        handleMouseEvent(e);
    }

    public void mouseReleased(MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited(MouseEvent e) {}
    public void mouseClicked(MouseEvent e) {}
    public void mouseMoved(MouseEvent e) {}
}

Per quanto riguarda il file Point.java:

package board;

import java.awt.Color;

public class Point {
    int x;
    int y;
    Color color;

    public Point(int x, int y, Color color) {
        super();
        this.x = x;
        this.y = y;
        this.color = color;
    }
}

Per quanto riguarda il file Main.java:

package board;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;

import javax.swing.JFrame;

public class Main {

    public static void main(String[] args) {
        JFrame f = new JFrame("Board");
        f.setSize(600, 600);
        Board board = new Board();
        TopPanel top = new TopPanel(board);
        f.add(top, BorderLayout.NORTH);
        f.add(board, BorderLayout.CENTER);

        f.addComponentListener(new ComponentAdapter() {

            @Override
            public void componentResized(ComponentEvent e) {
                board.componentResized();
            }
        });

        f.setMinimumSize(new Dimension(600, 600));
        f.setVisible(true);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}