Generics Help

[authorityaction]

I am working on an assignment to write a generic class called MyStatisticsCla ss with type parameter T where T is a numeric object. with a method called standardDeviati on that takes a ArrayList of type T and returns a double. I am getting the following error when I try to compile:

[code=java]MyStatisticsCla ss.java:27: standardDeviati on(java.util.Ar rayList<T>) in MyStatisticsCla ss<T> cannot be applied to (java.util.Arra yList<java.lang .Double>)
Double result = standardDeviati on(numbers);[/code]

Here are some tid bits of the code I have:

[code=java] public static void main(String[] args)
{
ArrayList<Doubl e> numbers = new ArrayList<Doubl e>();
//some random numbers are assigned to numbers here
Double result = standardDeviati on(numbers);//this is causing the error
}

public double standardDeviati on(ArrayList<T> a)
{
double average = calcAverage(a);
double top = 0.0;
//calculate the standard deviation
for (int i = 0; i < a.size(); i++)
{
top = top + Math.pow(a.get( i).doubleValue( ) - average, 2);
}
return Math.sqrt(top / average);
}[/code]

Anyone know what I am doing wrong? I can't seem to figure it out! Thanks for any help.

[BigDaddyLH]

Your method standardDeviati on should be static, right? Shouldn't that solve your problems?

[authorityaction]

Your method standardDeviati on should be static, right? Shouldn't that solve your problems?

If I change it to static I get the following error:

[code=java]MyStatisticsCla ss.java:37: non-static class T cannot be referenced from a static context
public static double standardDeviati on(ArrayList<T> a)[/code]

[BigDaddyLH]

Your static method should be generic. I don't know about the class:

[CODE=Java]import java.util.*;

public class Example {
public static <V extends Number> double sum(Collection< V> c){
double sum = 0;
for(V v : c) {
sum += v.doubleValue() ;
}
return sum;
}

public static <V extends Number> double average(Collect ion<V> c){
return sum(c)/c.size();
}

public static <V extends Number> double standardDeviati on(Collection<V > c){
double average = average(c);
double sum = 0.0;
for(V v : c) {
double diff = v.doubleValue() - average;
sum += diff * diff;
}
return Math.sqrt(sum / average);
}

public static void main(String[] args) {
List<Double> data = new ArrayList<Doubl e>();
Collections.add All(data, 1.0, 2.0, 3.0, 4.0, 5.0);
double std = standardDeviati on(data);
}
}[/CODE]

[authorityaction]

Thanks for that code! I changed to the following and everything worked.

[code=java]public static <T extends Number> double standardDeviati on(ArrayList<T> a)

private static <T extends Number> double calcAverage(Arr ayList<T> a)[/code]