传递字符串数组作为论证并没有给出预期的结果

Question

I m working on a program, where I retrieve string from db and store into a String array. Now I pass this array to method in another class. Say The string array length is 10. Now I will check for particular occurence of a char and I will do some work. Now my problem is I am able to store the value from db into string array. But what problem I am getting is giving null pointer exception. As the loop which reads the string array in another method reads the last index array.

The program:

This is where I store the Db values into array. These arrays are global declarations.

 while(rst.next())
        {
        allitems[i]=rst.getString("transitems");
        alltuv[i]=rst.getString("tuv");


        }

Now pass this array into a method of same class.

 Window2 n=new Window2();

    n.calculateTWU(allitems,alltuv);

Now I pass this array into another method of different class.

  CalculateTWU ctw=new CalculateTWU();
  String avalue=ctw.calctA(s1, s2,"A"); //s1 and s2 are the string arrays s1=allitems     and s2=alltuv.

The method in another class say Class B

 public String calctA(String[] a1,String[] a2,String indexstr)

{

for(int i=0;i<10;i++)
{


 int is= a1[i].indexOf(indexstr);



if(is==-1)
{


}
else
{
valadd=Integer.parseInt(a2[i])+valadd;

}

}
 return String.valueOf(valadd);

}

Stack Trace :

      MNOETTAOHX   // these are strings


  BOCOPVBLEV


  MIAISEUECV


  MGUXPXPEDX


  EQMWLRVKZG


  MNFBFAXQNT


  YZKDHMVSSA




java.lang.NullPointerException
    at datasets.CalculateTWU.calctA(CalculateTWU.java:27)
    at datasets.Window2.calculateTWU(Window2.java:205)
    at datasets.Window2.jButton1ActionPerformed(Window2.java:146)
    at datasets.Window2.access$000(Window2.java:20)
    at datasets.Window2$1.actionPerformed(Window2.java:68)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)

Answer 1

When you create an array, by default all it's items are null until you populate them. In your example, you have 9 strings, but are looping on 10 items in the array. The last one is null , and hence your NullPointerException . You can avoid this by explicitly checking:

public String calctA(String[] a1, String[] a2, String indexstr) {
    for(int i=0; i < a1.length; ++i)  {
        if (a1[i] != null) {
            int is= a1[i].indexOf(indexstr);
            // rest of your code...

Answer 2

I think the problem lies here:

  for(int i=0;i<10;i++) \\ What line is this in the class?

change to

  for(int i =0; i < a1.length;i++)

The problem with your stack trace is since the whole class isn't present it's hard to determine where exactly the NullPointerException is occurring.