如何在java中向应用程序顶部绘制一个矩形

Question

How can I draw a rectangle towards the top of the application in java ? Normally the drawRect method draws towards the bottom I tried to use a negative number but this would not work

Graphics g = p.getGraphics();
g.fillRect(50, 200,50,100);

Answer 1

In rectangles, the X and Y coordinates represent the top left corner. The length and width then draw away from the defining point. Your example draws a rectangle with the top left corner at 50,200 and with a width of 50 and a hight of 100, both away from those points in a positive direction. If you wanted a rectangle with 50,200 representing the lower left corner, simply subtract the height from that y coordinate (200), and use that as the starting y:

Graphics g = p.getGraphics();
g.fillRect(50, 100, 50, 100);

To address your examples, try something like this (I'll just use rectangle objects rather than actually filling the graphics):

int baseline = 200;
Rectangle rect1 = new Rectangle(50, baseline - 100, 50, 100);
Rectangle rect2 = new Rectangle(150, baseline - 50, 50, 50);
Rectangle rect3 = new Rectangle(250, baseline - 80, 50, 80);

After filling rectangles with these dimensions on the graphics object, you'll have three rectangles with a width of 50 each, spaced 50 apart, with the bottom all on the y=200 line.

Answer 2

Java's Graphics class assumes that the origin (0, 0) is the top-left corner of the frame, that is, that (1, 1) is down and to the right of (0, 0) . This is contrary to mathematics, in which the origin in a standard Cartesian plane is the lower-left corner, and (1, 1) is above and to the right of (0, 0) .

Also, Java doesn't allow you to use negative values for widths and heights. This results in special logic that typically treats the rectangle differently than a normal, positive-dimension rectangle.

To get what you want, first reverse your thinking about the y-coordinate in Java's Graphics coordinate system. Positive y is down, not up (though positive x is still right, like standard Cartesian plots).

That being said, the fields for drawRect and fillRect are:

1. The x-coordinate of the top-left corner of the rectangle
2. The y-coordinate of the top-left corner of the rectangle
3. The positive width of the rectangle
4. The positive height of the rectangle

Zoe's answer shows a good example of how to get what you want, I just thought you might like a more thorough explanation of why "the drawRect method draws towards the bottom."

Answer 3

Run it.. Drag and drop the mouse in any direction on the applet window..and see what's happening... Hope you will get some idea from it....

//Simulation of the desktop screen

package raj_java;
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;

public class DesktopDemo extends Applet implements MouseListener, MouseMotionListener {

    int x1, y1, x2, y2;
    Image img;

    public void init() {
        setSize(1200, 700);
        setVisible(true);
        img = getImage(getCodeBase(), "Nature.jpg");
        addMouseListener(this);
        addMouseMotionListener(this);
    }

    public void mouseEntered(MouseEvent me) {
        //blank
    }

    public void mouseExited(MouseEvent me) {
        //blank
    }

    public void mouseClicked(MouseEvent me) {
        //blank
    }

    public void mouseReleased(MouseEvent me) {
        Graphics g = getGraphics();
        g.drawImage(img, 0, 0, 1200, 700, this);
    }

    public void mouseMoved(MouseEvent me) {
        //blank
    }

    public void mousePressed(MouseEvent me) {
        x1 = me.getX();
        y1 = me.getY();
    }

    public void mouseDragged(MouseEvent me) {
        x2 = me.getX();
        y2 = me.getY();
        repaint();
    }

    public void paint(Graphics g) {
        g.drawImage(img, 0, 0, 1200, 700, this);
        g.setColor(new Color(10, 99, 126));
        g.drawLine(x1, y1, x2, y1);
        g.drawLine(x2, y1, x2, y2);
        g.drawLine(x2, y2, x1, y2);
        g.drawLine(x1, y2, x1, y1);
        g.setColor(new Color(193, 214, 220, 70));
        int width = Math.abs(x2 - x1);
        int height = Math.abs(y2 - y1);
        if(x2 < x1) {
            g.fillRect(x2, y1, width, height);
        }else if(y2 < y1) {
            g.fillRect(x1, y2, width, height);
        }else {
            g.fillRect(x1, y1, width, height);
        }
    }

}