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

public class BoundedGenerics {

// Upper bound: works with Number and its subclasses
    public static <T extends Number> double sumList(List<T> list) {
        double sum = 0;
        for (T item : list) {
            sum += item.doubleValue(); // Number method available
        }
        return sum;
    }

// PECS: Producer Extends — read from source
    public static <T> void copy(List<? extends T> source,
                                List<? super T>   dest) {
        for (T item : source) {
            dest.add(item);
        }
    }

// Unbounded wildcard — just needs to print
    public static void printAll(List<?> list) {
        for (Object o : list) {
            System.out.print(o + " ");
        }
        System.out.println();
    }

public static void main(String[] args) {
        List<Integer> ints    = List.of(1, 2, 3, 4);
        List<Double>  doubles = List.of(1.5, 2.5, 3.0);

System.out.println("Sum ints:    " + sumList(ints));
        System.out.println("Sum doubles: " + sumList(doubles));

List<Number> numbers = new ArrayList<>();
        copy(ints, numbers);
        copy(doubles, numbers);
        System.out.print("Copied: ");
        printAll(numbers);
    }
}