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我正在尝试实现一个函数来计算多边形的面积。这段代码允许你画一个多边形，但是当涉及到显示区域时，我不知道该怎么做。我尝试了几种方法，但我仍然是编程的初学者，所以我很感激任何帮助。这是代码：

import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.*;
import javax.swing.*;
import java.awt.Color;
import java.awt.Polygon;

public class DrawPolygons extends JApplet implements ActionListener, MouseListener
{
private static final int NUMPOINTS = 500; //Up to 500 points can be chosen

private JButton finish;                   //Button to indicate user is done entering     points
private Polygon shape;                    //polygon object to be drawn
private boolean isDrawn;                  //boolean flag for when the user is finished drawing
private int count;                        //how many points the user has clicked
private Color color;                      //color of the polygon after user finalizes points
private int[] x;                          //x coordinates of each point user picks
private int[] y;                          //y coordinates of each point user picks
private float sum;
private double area;



public void init()                        //set up GUI
{
    setLayout(new FlowLayout());

    addMouseListener(this);               //adds MouseListener for mouse clicks

    isDrawn = false;                      //isDrawn is initially false
    count = 0;                            //count starts as 0
    x = new int[NUMPOINTS];               //allows for up to 500 points to be chosen
    y = new int[NUMPOINTS];               //allows for up to 500 points to be chosen

    finish = new JButton("Finalize points");    //creates finish button
    finish.addActionListener(this);             //adds finish button to ActionListener
    add(finish);                                //adds finish button to GUI

    color = Color.BLACK;                //color is intially black, and will remain 
                                        //  so if user cancels the color chooser

    //JOptionPane.showMessageDialog(null, "Click points that will make up the polygon. After each" +    
    //                                    "point is entered, press the Finalize Points button");
    shape = new Polygon();              //creates the Polygon shape



}



 public void paint(Graphics g)          //draws the Polylines, Polygons, and sets the color
{            
    super.paint(g); 

    g.drawPolyline(x, y, count);        //draws the polyline specified by user
                                        //   mouseclick
    g.setColor(color);                  //sets the color to the user chosen color

    if(isDrawn)                         //if finalize button is clicked
    {

        g.fillPolygon(x, y, count);     //finalizes polylines into


    }                                   //   polygon and fills shape
}

  public void actionPerformed(ActionEvent a)  //decides what to do when finalize button is pressed
{
    if(a.getSource() == finish)             //if the finalize button is pressed
    {
        isDrawn = true;                     //isDrawn is set to true, ending the users ability
                                            //  to add more points, and fills the polygon
        color = Color.red; 


        //JColorChooser.showDialog(this, "Choose a color", color); //color is set to users choice
        repaint();    

      JOptionPane.showMessageDialog(null, "The area is ");
    }
 }

 public void mouseClicked(MouseEvent e)        //save coordinates of clicks
 { 


    if(isDrawn == false && count < NUMPOINTS) //if the finalize button is not pressed
    {                                         //  the user can add additional points
        x[count] = e.getX();                  //adds the x point at the current mouse x coordinate
        y[count] = e.getY();                  //adds the y point at the current mouse y coordinate
        count++;                              //count increases with each mouse click
        repaint();

    }

    else if (e.isShiftDown()) {
             // Clear the applet. (This only requires a repaint.)
             // Also, set count to zero to start a new polygon.
          count = 0;
          isDrawn = false;
          repaint();
                    } 


    }

     private float getPolygonArea(int[] x, int[] y, int count)
    {
      float sum_but_no_result=0;

     for(int i=0;i<(count-1);i++)      // count is point number of polygon
      {
        sum_but_no_result+=x[i]*y[i+1] + y[i]*x[i+1];
        }
       sum_but_no_result+=x[count-1]*y[0] + y[count-1]*x[0];

       float sum = (float)Math.abs(sum_but_no_result) / 2.0f;
     return sum;
     }


//Empty Implementation provided here so we can implement MouseListener (needed because we must
//  provide concrete forms of all methods of an interface to implement it

  public void mousePressed(MouseEvent e){};
  public void mouseReleased(MouseEvent e){};
  public void mouseEntered(MouseEvent e){};
  public void mouseExited(MouseEvent e){};    

  }
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2 回答 2

1 
float x[N],y[N]; // point coordinates as x1,y1   x2,y2 ....
...
  ...
  ...
...
float sum_but_no_result=0;

for(int i=0;i<(N-1);i++) // N is point number of polygon
{
      sum_but_no_result+=x[i]*y[i+1] + y[i]*x[i+1];
}
sum_but_no_result+=x[N-1]*y[0] + y[N-1]*x[0];

float sum= (float)Math.abs(sum_but_no_result) / 2.0f;

http://www.mathopenref.com/coordpolygonarea.html

对于自相交的多边形，您可以添加一个相交坐标查找器和多边形加法器算法来递归地查找相交产生的所有子多边形。

于 2013-07-10T19:20:20.870 回答
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在 Kotlin 中，使用 java.awt.Polygon 上的扩展：

import java.awt.Polygon

fun Polygon.area(): Float {
    var sum = 0f
    for (i in 0 until npoints - 1) {
        sum += xpoints[i] * ypoints[i + 1] + ypoints[i] * xpoints[i + 1]
    }
    sum += xpoints[npoints - 1] * ypoints[0] + ypoints[npoints - 1] * xpoints[0]
    return sum / 2.0f
}
于 2018-07-18T09:01:16.477 回答