sum of non numeric elements in an ArrayList

[Vijay Tyagi]

why is it that the sum of all elements in the ArrayList list (code below) is 10 ?
there are non numeric elements in the arraylist

import java.util.*;

class TestList

{

public static void main(String[] args)
{

ArrayList list=new ArrayList();
list.add("Dog");
list.add(345678);
list.add(35456.74444);
list.add(true);
list.add('a');

System.out.println("list is  "+list);

double sum=0.0;
for(int i=0;i<list.size();i++)
{
sum=sum+i;
}
System.out.println("sum is  "+sum);
}
}

[Henry Wong]

You are taking a "sum" of the count from zero to the size of the list.... ie 0 + 1 + 2 + 3 + 4 ==> 10.


Henry

[Vijay Tyagi]

when I change it to

sum=sum+list.get(i);

I get error

TestList.java:22: operator + cannot be applied to double,java.lang.Object
sum=sum+list.get(i);

[Hunter McMillen]

The compiler is telling you that you can't add objects of different types.

"Dog" + 345678 causes an error.

Hunter

[Joanne Neal]

You're trying to add together a double and an Object, which you can't do in Java.

[Rob Spoor]

Hunter McMillen wrote:"Dog" + 345678 causes an error.

Actually that would work, if the right references are used. Remember that + is used for string concatenations. However, in this case all references are Object references, and + definitely does not work for objects.

Vijay, print out the classes of the elements. Then find out if the numeric values have a common super class (they do, and it's not Object). If so, find a method that returns something you can add together (hint: it ends with "Value"). You can use the instanceof operator and casting to give you an instance of that common super class.

[Hunter McMillen]

Rob Prime wrote:

Hunter McMillen wrote:"Dog" + 345678 causes an error.

Actually that would work, if the right references are used. Remember that + is used for string concatenations. However, in this case all references are Object references, and + definitely does not work for objects.

Sorry I should have said: in this case, this will cause an error.

Hunter