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

[Ramesh Pramuditha Rathnayake]

Use Integer (package java.lang) class to parse int from a String[Integer.parseInt(...)].

This will return an exception if the argument cannot be converted into int data type.

[samuel lenk]

Hi.

If you only want to allow integers in your array list then with generics you can create your list this way. That brings the advantage that java will only let integers by added to the list and you can be sure sum will be calculated correctly. See the example:

public class ClassName {
	
	public static void main(String[] args) {
		
		ArrayList<Integer> list = new ArrayList<Integer>();
		list.add(13);
		list.add(34);
		list.add(2);
		
		System.out.println("list is " + list);
		
		double sum = 0.;
		
		for (Integer integer : list) {
			sum += integer;
		}
		
		System.out.println("sum is " + sum);
		
	}

}