单击鼠标后在JPanel上绘制圆圈

Question

I want to draw circle only after mouse gets click. As paintComponent method called itself, so first circle draw without click.

public class DrawPanel extends JPanel implements MouseListener {
private static final long serialVersionUID = 1L;
int x, y;

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

public void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    g2.setColor(Color.red);
    g2.fillOval(x, y, 20, 20);
}

@Override
public void mouseClicked(MouseEvent e) {
    x = e.getX();
    y = e.getY();
    repaint();
}

}

Answer 1

There are a few issues with your code:

1. You never call super.paintComponent();
2. You only have one x and y
3. Note how when you resize the frame, some circles will disappear and it overall behaves in a strange way.
4. I would store all the Point s where the user has clicked in an ArrayList and then loop through that list inside the paintComponent method. This way you can call super.paintComponent(); without the circles disappearing.

Changed, working code:

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class DrawPanel extends JPanel {
    private static final long serialVersionUID = 1L;
    private ArrayList<Point> points;

    public DrawPanel() {
        points = new ArrayList<Point>();
        setBackground(Color.WHITE);
        addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                points.add(new Point(e.getX(), e.getY()));
                repaint();
            }
        });
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setColor(Color.red);
        for (Point point : points) {
            g2.fillOval(point.x, point.y, 20, 20);
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new DrawPanel());
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setSize(400, 400);
                frame.setVisible(true);
            }
        });
    }

}

Answer 2

You should have some initial state, so you know not to draw when it is set.

This can be done with an easy boolean variable, that you set to true when the user has pressed on the screen.

public class DrawPanel extends JPanel implements MouseListener {
private static final long serialVersionUID = 1L;
int x, y;
boolean mustDraw = false;

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

public void paintComponent(Graphics g) {
    if(!mustDraw) return;
    Graphics2D g2 = (Graphics2D) g;
    g2.setColor(Color.red);
    g2.fillOval(x, y, 20, 20);
}

@Override
public void mouseClicked(MouseEvent e) {
    x = e.getX();
    y = e.getY();
    mustDraw = true;
    repaint();
}

}

Answer 3

You should place your circle inside its own class. That class will hold information on its location, radius, and its color. You can abstract your shape and have a list of shapes to draw on the panel. This will make it easy to implement a triangle, square, hexagon, etc. later.

You can add more methods and attributes to your shape objects later and only have to change their internal implementation of THEIR OWN paintComponent(g) method. This makes the DrawPanel depend on how each Shape does its own drawing.

---

App.java

import java.awt.Dimension;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class App {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame f = new JFrame("Circle Click Application");
                DrawPanel p = new DrawPanel(10f);

                p.setPreferredSize(new Dimension(300, 200));;

                f.setContentPane(p);
                f.pack();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.setLocationRelativeTo(null);
                f.setVisible(true);
            }
        });
    }
}

DrawPanel.java

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.util.ArrayList;
import java.util.List;

import javax.swing.JPanel;

public class DrawPanel extends JPanel implements MouseListener {
    private static final long serialVersionUID = -6817035652787391530L;

    private List<Shape> shapes;
    protected float radius;

    private float sat = 0.7f;
    private float bri = 0.8f;

    public DrawPanel(float radius) {
        this.shapes = new ArrayList<Shape>();
        this.radius = radius;

        setBackground(Color.WHITE);
        addMouseListener(this);
    }

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

        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        for (Shape shape : shapes) {
            shape.paintComponent(g);
        }
    }

    @Override
    public void mouseClicked(MouseEvent e) {
        shapes.add(new Circle(e.getX(), e.getY(), radius, ColorUtils.randHue(sat, bri)));

        repaint();
    }

    @Override
    public void mousePressed(MouseEvent e) {
        // TODO Auto-generated method stub
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        // TODO Auto-generated method stub
    }

    @Override
    public void mouseEntered(MouseEvent e) {
        // TODO Auto-generated method stub
    }

    @Override
    public void mouseExited(MouseEvent e) {
        // TODO Auto-generated method stub
    }
}

Shape.java

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;

public interface Shape {
    Point getOrigin();
    void setOrigin(Point origin);

    Color getColor();
    void setColor(Color color);

    void paintComponent(Graphics g);
}

Circle.java

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;

public class Circle implements Shape {
    private Point origin;
    private float radius;
    private Color color;

    public Circle() {
        this(0, 0, 0.5f, Color.BLACK);
    }

    public Circle(int x, int y, float radius, Color color) {
        this(new Point(x, y), radius, color);
    }

    public Circle(Point origin, float radius, Color color) {
        this.origin = origin;
        this.radius = radius;
        this.color = color;
    }

    @Override
    public Point getOrigin() {
        return origin;
    }

    @Override
    public void setOrigin(Point origin) {
        this.origin = origin;
    }

    public float getRadius() {
        return radius;
    }

    public void setRadius(float radius) {
        this.radius = radius;
    }

    @Override
    public Color getColor() {
        return color;
    }

    @Override
    public void setColor(Color color) {
        this.color = color;
    }

    @Override
    public void paintComponent(Graphics g) {
        int diameter = (int) (this.radius * 2);
        int x = (int) (origin.x - this.radius);
        int y = (int) (origin.y - this.radius);

        g.setColor(this.color);
        g.fillOval(x, y, diameter, diameter);
    }
}

ColorUtils.java

import java.awt.Color;
import java.util.Random;

public class ColorUtils {
    private static final Random RAND;

    static {
        RAND = new Random(System.currentTimeMillis());
    }

    public static Color randHue(float saturation, float brightness) {
        return Color.getHSBColor(RAND.nextFloat(), saturation, brightness);
    }
}

---

Extension

You can easily add a class such as a Triangle just by implementing the Shape interface.

DrawPanel#mouseClicked

@Override
public void mouseClicked(MouseEvent e) {
    long time = System.currentTimeMillis();
    boolean isEven = time % 2 == 0;

    if (isEven) {
        shapes.add(new Circle(e.getX(), e.getY(), radius, ColorUtils.randHue(sat, bri)));
    } else {
        shapes.add(new Triangle(e.getX(), e.getY(), radius * 2, ColorUtils.randHue(sat, bri)));
    }

    repaint();
}

Triangle.java

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Polygon;
import java.awt.geom.Point2D;

public class Triangle implements Shape {
    private Point origin;
    private float side;
    private Color color;

    private Polygon poly;

    public Triangle(int x, int y, float side, Color color) {
        this(new Point(x, y), side, color);
    }

    public Triangle(Point origin, float side, Color color) {
        this.origin = origin;
        this.side = side;
        this.color = color;

        recalculate();
    }

    protected void recalculate() {
        this.poly = createPolygon((float) origin.getX(), (float) origin.getY(), side, false);
    }

    protected Polygon createPolygon(float x, float y, float side, boolean invert) {
        float xOff = side / 2f;
        float yOff = (float) (xOff * Math.sqrt(3));

        float r1 = 1f / 3f;
        float r2 = 2f / 3f;

        if (invert) {
            yOff *= -1;
        }

        return createPolygon(new Point2D.Float[] {
            new Point2D.Float(x,        y - (yOff * r2)), // Top
            new Point2D.Float(x - xOff, y + (yOff * r1)), // Left
            new Point2D.Float(x + xOff, y + (yOff * r1))  // Right
        });
    }

    protected Polygon createPolygon(Point2D.Float[] points) {
        int nPoints = points.length + 1;
        int xCoords[] = new int[nPoints];
        int yCoords[] = new int[nPoints];

        for (int i = 0; i < nPoints; i++) {
            xCoords[i] = (int) points[i % points.length].x;
            yCoords[i] = (int) points[i % points.length].y;
        }

        return new Polygon(xCoords, yCoords, nPoints);
    }

    @Override
    public Point getOrigin() {
        return origin;
    }

    @Override
    public void setOrigin(Point origin) {
        this.origin = origin;

        recalculate();
    }

    public float getSide() {
        return side;
    }

    public void setSide(float side) {
        this.side = side;

        recalculate();
    }

    @Override
    public Color getColor() {
        return color;
    }

    @Override
    public void setColor(Color color) {
        this.color = color;
    }

    @Override
    public void paintComponent(Graphics g) {
        g.setColor(this.color);
        g.fillPolygon(poly);
    }
}