Generics Help

authorityaction

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

Expand|Select|Wrap|Line Numbers

 MyStatisticsClass.java:27: standardDeviation(java.util.ArrayList<T>) in MyStatisticsClass<T> cannot be applied to (java.util.ArrayList<java.lang.Double>)

        Double result = standardDeviation(numbers);

Here are some tid bits of the code I have:

Expand|Select|Wrap|Line Numbers

     public static void main(String[] args)

    {

        ArrayList<Double> numbers = new ArrayList<Double>();

        //some random numbers are assigned to numbers here

        Double result = standardDeviation(numbers);//this is causing the error

    }
 
public double standardDeviation(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);

    }

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

BigDaddyLH

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

authorityaction

Quote:

Originally Posted by BigDaddyLH

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

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

Expand|Select|Wrap|Line Numbers

 MyStatisticsClass.java:37: non-static class T cannot be referenced from a static context

    public static double standardDeviation(ArrayList<T> a)

BigDaddyLH

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

Expand|Select|Wrap|Line Numbers

 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(Collection<V> c){

        return sum(c)/c.size();

    }
 
    public static <V extends Number> double standardDeviation(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<Double>();

        Collections.addAll(data, 1.0, 2.0, 3.0, 4.0, 5.0);

        double std = standardDeviation(data);

    }

}

authorityaction

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

Expand|Select|Wrap|Line Numbers

 public static <T extends Number> double standardDeviation(ArrayList<T> a)
 
private static <T extends Number> double calcAverage(ArrayList<T> a)

Generics Help

re: Generics Help

re: Generics Help

re: Generics Help

re: Generics Help

Similar Java bytes

/bytes/development

/bytes/it

/bytes/about