大家帮看下!
1. package datastructure;
2. import java.awt.*;
3. import java.awt.geom.*;
4. import java.awt.image.*;
5. import java.io.*;
6. import javax.imageio.*;
7. import javax.servlet.*;
8. import javax.servlet.http.*;
9. class HTNode
10.{
11.int weight;
12.int parent;
13.int lchild;
14.int rchild;
15.}
16. public class HuffmanTree
17.{
18.private int pictureWidth=500;
19. private int pictureHeight=500;
20. public int s1,s2;
21. int n=(int)(2+9*Math.random());//随机生成结点的个数
22.int m=2*n-1;
23.public void draw(HttpServletResponse response)
24.{
25.response.reset();
26.response.setContentType( "image/png ");
27.BufferedImage image=new BufferedImage(pictureWidth,pictureHeight,BufferedImage.TYPE_INT_RGB);
28.Graphics2D g2d=image.createGraphics();
29.g2d.setPaint(Color.WHITE);
30.g2d.fillRect(0,0,pictureWidth, pictureHeight);
31.g2d.setPaint(Color.RED);
32.g2d.setFont(new Font( "TimesRoman ",Font.BOLD,24));
33.g2d.drawString( "赫夫曼树和赫夫曼编码演示 ",100,20);
34.HTNode HT[]=new HTNode[m+1];
35. HTNode HC[]=new HTNode[n];
36.HuffmanCoding(HT,HC,g2d);
37.g2d.dispose();
38. ServletOutputStream sos=null;
39. try
40. {
41. sos=response.getOutputStream();
42. ImageIO.write(image, "PNG ",sos);
43. sos.close();
44. }
45. catch (IOException ex){
46. }
47.}
48.public void HuffmanCoding(HTNode HT[],HTNode HC[],Graphics2D g2d)
49.{
50.int i;
51.int w[]=new int[n];//存取每个结点的权值
52.for(i=0;i <n;i++)
53.w[i]=(int)(1+20*Math.random());
54.for(i=1;i <=n;i++)
55.{
56.HT[i].weight=w[i-1];
HT[i].parent=0;
HT[i].lchild=0;
HT[i].rchild=0;
}
for(i=n+1;i <=m;i++)
{
HT[i].weight=0;
HT[i].parent=0;
HT[i].lchild=0;
HT[i].rchild=0;
}
for(i=n+1;i <=m;i++)
{
Select(HT,i-1);
HT[s1].parent=i;
HT[s2].parent=i;
HT[i].lchild=s1;
HT[i].rchild=s2;
HT[i].weight=HT[s1].weight+ HT[s2].weight;
}
for(i=1;i <=n;i++)
g2d.drawString(Integer.toString(HT[i].weight),20*i,100);
}
public void Select(HTNode HT[],int n)
{
int i,j;
for(i=1;i <= n;i++) if(HT[i].parent==0){s1=i;break;}
for(j=i+1;j <=n;j++)
if(HT[j].parent==0){s2=j;break;}
for(i=1;i <=n;i++)
if((HT[s1].weight> HT[i].weight)&&(HT[i].parent==0)&&(s2!=i)) s1=i;
for(j=1;j <= n;j++)
if((HT[s2].weight> HT[j].weight)&&(HT[j].parent==0)&&(s1!=j)) s2=j;
}
}
为什么上面的程序老是报下面的错:
java.lang.NullPointerException
datastructure.HuffmanTree.HuffmanCoding(HuffmanTree.java:56)
datastructure.HuffmanTree.draw(HuffmanTree.java:36)
org.apache.jsp.WebRoot.huffmanTree_jsp._jspService(huffmanTree_jsp.java:61)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
[解决办法]
HuffmanTree.java:56)56行空指针异常
