Hi Guys , I am rather new to Java and I am trying to extend the ArrayList object to that when an ArrayList of Integers exists it has a function to return the Average value of the Integers within the ArrayList. Below is the code I have so far:

import java.util.*;*
*import java.lang.*;
public class MathArrayList<Integer> extends ArrayList<Integer> {
 
double average;
 
public double Average() {
CalculateAverage();
 
return average;
}
 
private void CalculateAverage() {
double total = 0;
 
for(int i = 0; i < size(); i++) { 
 
total += get(i).intValue();
}
 
average = total / size();
}
}
 

When I attempt to compile this code I get: Cannot find symbol - Line 18 - ".intValue()" and I cannot work out why! I have created a smaller java project to test that I am using the method '.intValue()' correctly and it is converting and Integer to an int fine! Why cannot it not find a symbol?

Any help/comments if much appricated!

Thanks, Stewart

 

 

get returns an Object not the parameterized type. Use an iterator instead.

 

 

One approach:

// Method names should not start with capital letters
private void calculateAverage() {
   double total = 0;
   for( final Integer value : this ) {
      total += value; // You can autounbox if you're confident the List will never contain nulls.
   }
   average = total / size();
} 

My inclination would be to implement this as an external function however:

public static double calculateCollectionAverage(final Collection<Integer> collection) {
   double total = 0;
   for( final Integer value : collection ) {
      total += value;
   }
   return (total / collection.size());
}

You should note that none of these approaches is threadsafe though.

Edited by: dcminter on 03-Nov-2009 15:00

 

 

Eh? ArrayList<E>'s get method is:
E get(int index);
So if E is Integer, get returns Integer, right?

@OP: I don't believe that's the actual code you're trying to compile.

If it is, there are some other issues with it.

#1: There's no reason to extend ArrayList for this. You aren't creating a special kind of ArrayList - you should just use an ArrayList and create a method that operates on that ArrayList to return the average. Favor composition over inheritance.

#2: Your calculate average method is not thread safe. You're calculating a member variable and then returning it, leaving room for another thread to alter that member variable in-between and get the wrong result. It should just calculate and return the average by using a local method variable. No need for the member variable there.

 

 

It does?

// from java.util.List
 E get(int index);
 
// from java.util.ArrayList
public E get(int index) {
	RangeCheck(index);
 
	return elementData[index];
}

I wonder if you're thinking of Map.get() which does indeed accept Object rather than the parameterized type.

 

 

+1

 

 

Yes. God knows what I was thinking... insufficient caffeine.

 

 

Since no one else has said it, I would like to point out that the code

public class MathArrayList<Integer> extends ArrayList<Integer> { ... }

defines a placeholder type named "Integer" that has absolutely no relation to java.lang.Integer

 

 

I strongly advise against that. A List's job is to maintain its elements in the order they're added. Adding responsibilities like computing the average is not good OO.

 

 

Thanks for all the responses guys, I have tried using your code dcminter and it gives me the same error I was getting before I tried using the '.intValue' method. That is that the '+' operator cannot be applied to Integer. I have researched this before and am sure this is because an object Integer is returned and not an int. I wanted to created what I called a MathArrayList as i wanted to add more functions such as StandardDev. extra to the list of methods so i could use this new ArrayList within many of the project I have to create. The import java.lang.*; is only in there as I was trying to work out why the method could not be found. I still haven't found a direct solution to my problem and still need an object which can hold 'int's.


After reading all of the comment I have decided to try and complete this project in a different way. Thanks very much for all the help !

Edited by: stew123dai on Nov 3, 2009 10:41 AM

 

 

In that case you're running an old version of Java (or of the JDK at least). Generics and auto(un)boxing weren't introduced until Java 5. Upgrade.