com.javarush.task.task35.task3511

Solution.java

package com.javarush.task.task35.task3511;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

/* 
Wildcards для коллекций
*/
public class Solution
{

    public static void main(String[] args)
        {
        }

    public static Double sum(List<? extends  Number> list)
        {
        Double result = 0.0;
        for (int i = 0; i < list.size ( ); i++)
            {
                Number numb = (Number) list.get (i);
                result += numb.doubleValue ( );
            }
        return result;
        }

    public static Double multiply(List<? extends  Number> list)
        {
        Double result = 1.0;
        for (int i = 0; i < list.size ( ); i++)
            {
                Number numb = (Number) list.get (i);
                result *= numb.doubleValue ( );
            }
        return result;
        }

    public static String concat(List<?> list)
        {
        StringBuilder builder = new StringBuilder ( );
        for (Object obj : list)
            {
                builder.append (obj);
            }
        return builder.toString ( );
        }

    public static List combine(List<? extends Collection>  list)
        {
        List result = new ArrayList<> ( );
        for (int i = 0; i < list.size ( ); i++)
            {
                Collection collection = (Collection) list.get (i);
                result.addAll (collection);
            }
        return result;
        }
}