求助：关于类数组的问题

classname[] arr=new classname[3];或classname arr[]=new classname[3];
应该都是可以的呀。

classname[] arr=new classname[3];或classname arr[]=new classname[3];你只是定义了一个reference arr，并没有对数组元素（classname对象）初始化
当你试图使用数组元素时，自然就抛出异常
java.lang.NullPointerException

哦,知道了,你的数组里的数据没有初始化..
可能是吧.
classname[] arr ={new classname(), new classname(), new classname()};
这样应该可以了

classname[] arr = (new classname[] {
            classname_instant1,
            classname_instant2,
            classname_instant3,
        });

代码如下：
----文件ace\allvar.java
package ace;
public class allvar{
  int num;
  public void num(int n){num=n;}
  public int num(){return num;}
  String str;
  public void str(String s){str=s;}
  public String str(){return str;}
  public allvar(){}
}
--------文件test.jsp
<%@ page contentType="text/html;charset=Gb2312"%>
<%@ page import="java.lang.*,java.util.*"%>
<%@ page import="java.sql.*,oracle.jdbc.pool.*" %>
<%@ page import="ace.*" %>
<html>
<head>
  <META http-equiv=CONTENT-TYPE content="TEXT/HTML; CHARSET=GB2312">
</HEAD>
<body>
<%
allvar k[]=new allvar[3];
k[0].num(6);//错误在此发生：java.lang.NullPointerException
out.println(k[0].num());
%>
</body>
</html>
--------------------------
这个问题折腾了我很长时间，请高手指教！

allvar k[]=new allvar[3];->allvar k[]={new classname(), new classname(), new classname()};

faint!
allvar k[]={new allvar(), new allvar(), new allvar()};

allvar k[]={new allvar(), new allvar(), new allvar()};
用这种方法就没问题了，可数组数量若不是3个，而是1000个该怎么办？

allvar k[]=new allvar[1000];
for(int i=0;i<k.length;i++)
k[i]=new allvar();

我原来作的东西：
bean中如下：
    /**************************年报返回结果**************************/
    public String[][] getYearReport(Statement Stmt, String nf, String UnitID) throws SQLException
    {
       ResultSet RS=null;
   String sqlStr=null,StrTmp=null;
   int cnt=0;String MontData[][];
   //取得数组的一维下标，并填充固定资产分类、分类编码和年折旧率
       try
       {
         sqlStr="select count(distinct ID) as cnt From E_Category";
         RS=Stmt.executeQuery(sqlStr);
         RS.next();
         cnt=RS.getInt("cnt");
          RS.close();
       }
       catch(Exception e){System.out.println("Depreciate.java-->getYearReport():" + e.toString());}
   MontData = new String [cnt+1][17];// //最后一行为累计值，初始化数组
   try
   {
    sqlStr="select * From E_Category";
         RS=Stmt.executeQuery(sqlStr);
RS.beforeFirst();
for (int i=0;i<cnt;i++)
{
RS.next();
MontData[i][0]=RS.getString("Name");
MontData[i][1]=RS.getString("ID");
MontData[i][2]=RS.getString("Percentage2");
}
   }
   catch (Exception e)
   {
   System.out.println("Depreciate.java-->getYearReport():" + e.toString());
   }
   float fData[][]=new float[cnt+1][14];
  try{
   //查询表Depreciate
   for (int i=0;i<cnt;i++)
{
   if ((MontData[i][1].indexOf('*'))!=-1)
   {
   StrTmp=MontData[i][1].substring(0,MontData[i][1].indexOf('*'));
   }else{
   StrTmp=MontData[i][1];
   }
   sqlStr="select FLID,NZhJL,avg(JTZhJYZh1+JTZhJYZh2+JTZhJYZh3+JTZhJYZh4+JTZhJYZh5+JTZhJYZh6+JTZhJYZh7+JTZhJYZh8+JTZhJYZh9+JTZhJYZh10+JTZhJYZh11+JTZhJYZh12) as JTZhJYZh";
   sqlStr=sqlStr+",sum(ZhJE1+ZhJE2+ZhJE3+ZhJE4+ZhJE5+ZhJE6+ZhJE7+ZhJE8+ZhJE9+ZhJE10+ZhJE11+ZhJE12) as ZhJE";
   sqlStr=sqlStr+",sum(ZhJE1) as ZhJE1,sum(ZhJE2) as ZhJE2,sum(ZhJE3) as ZhJE3,sum(ZhJE4) as ZhJE4,sum(ZhJE5) as ZhJE5,sum(ZhJE6) as ZhJE6,sum(ZhJE7) as ZhJE7,sum(ZhJE8) as ZhJE8,sum(ZhJE9) as ZhJE9,sum(ZhJE10) as ZhJE10,sum(ZhJE11) as ZhJE11,sum(ZhJE12) as ZhJE12";
   sqlStr=sqlStr+" from Depreciate where NF='"+nf+"' and DW='" +UnitID+ "' and FLID like '"+StrTmp+"%' order by FLID";
   RS=Stmt.executeQuery(sqlStr);
   RS.next();
   fData[i][0]=RS.getFloat("JTZhJYZh");
   fData[i][1]=RS.getFloat("ZhJE");
   fData[i][2]=RS.getFloat("ZhJE1");
   fData[i][3]=RS.getFloat("ZhJE2");
   fData[i][4]=RS.getFloat("ZhJE3");
   fData[i][5]=RS.getFloat("ZhJE4");
   fData[i][6]=RS.getFloat("ZhJE5");
   fData[i][7]=RS.getFloat("ZhJE6");
   fData[i][8]=RS.getFloat("ZhJE7");
   fData[i][9]=RS.getFloat("ZhJE8");
   fData[i][10]=RS.getFloat("ZhJE9");
   fData[i][11]=RS.getFloat("ZhJE10");
   fData[i][12]=RS.getFloat("ZhJE11");
   fData[i][13]=RS.getFloat("ZhJE12");
   }
  RS.close();
      }
       catch(Exception e){System.out.println("Depreciate.java-->getYearReport():" + e.toString());}
//计算“合计”
MontData[cnt][0]="合        计";
MontData[cnt][1]="";MontData[cnt][2]="";// 
for (int j=0;j<14;j++ )
   for (int i=0;i<cnt+1 ;i++ )
   {fData[cnt][j]+=fData[i][j];}
for (int i=0;i<cnt+1 ;i++ )
   for (int j=3;j<=16;j++ )
   {
    MontData[i][j]=String.valueOf(fData[i][j-3]);
   }       return MontData;
    }jsp中：
String Data[][] = Dep.getYearReport(Stmt, Year,UnitID);
int RowCount = Data.length;
for (int i=0; i<=RowCount-1; i++)
{System.out.println("Data["+i+"][0]:"+Data[i][0]);｝

调试易

求助：关于类数组的问题

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