大家看下!
1. import java.awt.*;
2. import javax.swing.*;
3. class HTNode
4. {
5.int weight;
6.int parent;
7.int lchild;
8.int rchild;
9. }
10. public class HuffmanTree extends JApplet
11. {
12. public int s1,s2;
13. int n=(int)(2+9*Math.random());//随机生成结点的个数
14.int m=2*n-1;
15.public void paint(Graphics g)
16.{
17.Graphics2D g2=(Graphics2D)g;
18.HTNode[] HT=new HTNode[m+1];
19.int i;
20.int w[]=new int[n];//存取每个结点的权值
21.for(i=0;i <n;i++)
22.w[i]=(int)(1+20*Math.random());
23.for(i=1;i <=n;i++)
24.{
25.HT[i].weight=w[i-1];
26. HT[i].parent=0;
27. HT[i].lchild=0;
28. HT[i].rchild=0;
29.}
30.for(i=n+1;i <=m;i++)
31.{
32.HT[i].weight=0;
33. HT[i].parent=0;
34. HT[i].lchild=0;
35. HT[i].rchild=0;
36.}
37.for(i=n+1;i <=m;i++)
38.{
39.Select(HT,i-1);
40. HT[s1].parent=i;
41. HT[s2].parent=i;
42. HT[i].lchild=s1;
43. HT[i].rchild=s2;
44. HT[i].weight=HT[s1].weight+ HT[s2].weight;
45.}
46.for(i=1;i <=n;i++)
47.g2.drawString(Integer.toString(HT[i].weight),20*i,100);
48.g2.dispose();
49.}
50.public void Select(HTNode HT[],int n)
51.{
52.int i,j;
53.for(i=1;i <= n;i++) if(HT[i].parent==0){s1=i;break;}
54.for(j=i+1;j <=n;j++)
55.if(HT[j].parent==0){s2=j;break;}
56.for(i=1;i <=n;i++)
57.if((HT[s1].weight> HT[i].weight)&&(HT[i].parent==0)&&(s2!=i)) s1=i;
58.for(j=1;j <= n;j++)
59.if((HT[s2].weight> HT[j].weight)&&(HT[j].parent==0)&&(s1!=j)) s2=j;
60.}
61. }

为什么上面程序报下面的错:
Exception in thread "AWT-EventQueue-1 " java.lang.NullPointerException
at HuffmanTree.paint(HuffmanTree.java:25)

[解决办法]
HTNode[] HT=new HTNode[m+1];
你这样初始化数组之后还要对数组的每一个元素进行初始化