java loops

[yeshello54]

Hello. So i have a question regarding incrementing a value in a loop.

I have a data table in my swing program. the data table has 5 columns. In the last column is a integer. I want to be able to grab all the prices in a table of n size. as of right now i just grab the first column's price by doing the following..

Object colIndex = dataTable.getValueAt(0,4);

how would i go about constructing a loop that adds up every price in that table and stores it in a variable so that i can use it for other things. Thanks any help would be great.

[JosAH]

@yeshello54
I assume that getValueAt( ... ) method takes a row number and a column number parameter. If so getValueAt(i, 4) takes the value of the fifth column (the integer value) from the i-th row. All you have to do is make variable i loop over all values 0 ... n-1 (all 'n' values) and add the corresponding column values in those rows. Something like this:

int sum= 0;
for (int i= 0 i < n; i++)
   sum+= dataTable.getValueAt(i, 4);
 

That's all there is to it ...

kind regards,

Jos

[yeshello54]

thank you. But i get an error message when trying to compile. Do you know what this means?? here is my code

int sum= 0;
         for (int i= 0; i < Contact.NUM_FIELDS; i++)
          sum += dataTable.getValueAt(i, 4);

Prog3.java:368: operator + cannot be applied to int,java.lang.Object
sum += dataTable.getValueAt(i, 4);

[JosAH]

@yeshello54

That method returns an Object type; you can't add Objects and ints. You have to explicitly cast it to an Integer, like this:

sum+= (Integer)dataTable.getValueAt(i, 4);
 

kind regards,

Jos

[yeshello54]

sweet thank you. I guess i have more questions on casting. so once i get the value it is stored in an int. I try to use the setText() on one of my textfields. but this functions parameter is only a string. so when i try to convert the int to a string i get a error that says string cant be cast to int..am i doing this wrong??

String total = Integer.toString(sum);

[JosAH]

@yeshello54
That code snippet seems ok to me; set the text of that field to the 'total' value.

kind regards,

Jos

[yeshello54]

i still get an error of "java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer"

this is my code

int sum= 0;
 
         for (int i= 0; i < Contact.NUM_FIELDS; i++)
              sum += (Integer)dataTable.getValueAt(i, 4);
            String total = Integer.toString(sum);
            total1.setText(total);

[JosAH]

On which line is that Exception thrown?

kind regards,

Jos

[yeshello54]

line 469 which is

sum += (Integer)dataTable.getValueAt(i, 4);

[JosAH]

@yeshello54
Ah, does that getValueAt( ... ) method return a String? We (I) assumed it returned an Integer. You have to convert that String to an int by using the static method Integer.parseInt( ... ) then and forget about the cast to an Integer type, i.e. first cast it to a String and then apply that method.

kind regards,

Jos

[yeshello54]

I think im still a little confused on what to do

[JosAH]

@yeshello54
If that getValueAt( ... ) method returns a String (check that) you have to convert it to an int before you can add the value to your sum variable:

sum += Integer.parseInt((String)dataTable.getValueAt(i, 4)); 
 

You have to convert that sum value back to a String again if you want to store it in your textField:

textField.setText(""+sum);
 

kind regards,

Jos

[yeshello54]

im must be tired..so i was not paying attention but my getValueAt() returns an object not a string. so does that make things different??

[JosAH]

@yeshello54
The String class extends from the Object class so a String is an Object; that's how object oriented programming works. If that method returns a String (and I'm about sure it does, given the error messages) but tells you it is returning an Object it isn't lying to you but you have to cast it back to the String type and that's exacly what my first code snippet does.

kind regards,

Jos

[yeshello54]

ok i got the error to stop..now when it displays the total it just gives a 0.
the values i had were 50 and 40..

int rowIndex = dataTable.getSelectedRow(); 
         int sum= 0;
            for (int i= 0; i < rowIndex; i++)
 
 
            sum += Integer.parseInt((String)dataTable.getValueAt(i, 4));
            String total = Integer.toString(sum);
            total1.setText(total);
 

[JosAH]

@yeshello54
System.out.println( ... ) is a very good debugger; print everything you don't trust and the funny bug pops up all by itself.

kind regards,

Jos

[yeshello54]

ok ..i posted print statements to see what was going on. i did a print statement after

sum += Integer.parseInt((String)dataTable.getValueAt(i, 4));
System.out.println(sum);

and it prints out 0.

so is the sum+= not working right??

[yeshello54]

actually i just figured it out..i fixed it with the following:

for(int i=0; i < dataTable.getRowCount(); i++) {
        int entry = Integer.parseInt( (String) dataTable.getValueAt(i,4));
        sum += entry;
        String total = Integer.toString(sum);
    total1.setText(total);

Thanks for your help!

[JosAH]

@yeshello54
I don't understand why that would help, i.e. the parseInt( ... ) method returns an int that can be directly added to the 'sum' variable, no need for a temporary variable. btw, your indentation stinks. That 'setText( ... )' method shouldn't be in the body of that loop.

kind regards,

Jos