How to copy a generic Collection in Java using wildcards

Question

Consider this java class:

import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedList;


public class NumberSet {

    private Collection<? extends Number> numbers;

    public NumberSet(Collection<? extends Number> numbers) {
        this.numbers = numbers;
    }

    public NumberSet(NumberSet other) {

        //copy other.numbers to this.numbers
        numbers = new LinkedList<>();
        for (Iterator<? extends Number> it = other.numbers.iterator(); it.hasNext();) {
            numbers.add(it.next()); // Here's Syntax Error near `it.next()`
        }

    }
}

There is This Syntax Error inside for loop:

actual argument Number cannot be converted to CAP#1 by method invocation conversion
where E is a type-variable:
    E extends Object declared in interface Collection
where CAP#1 is a fresh type-variable:
    CAP#1 extends Number from capture of ? extends Number

I Understand the meaning of PECS but I want to implement a copy-constructor for this class. the copied instance would be used Just as an snapshot of other. any Idea?

Answer 1

Change :

 Collection<? extends Number> dst 

To :

 Collection<? super Number> dst

Addition is not allowed using ? extends. There is something known as the Get and Put principle. Quoting from the Generics and Collections book from Philip Wadler :

The Get and Put Principle: use an extends wildcard when you only get values out of a structure, use a super wildcard when you only put values into a structure, and don’t use a wildcard when you both get and put

Also take a look at PECS for a more common explanation of this phenomenon. (However, I prefer the GET and PUT principle over the Producer and Consumer principle as it is less confusing)