Lists with wildcards cause Generic voodoo error

Question

Does anyone know why the following code does not compile? Neither add() nor addAll() works as expected. Removing the \"? extends\" part makes everything work, but then I wou

Answer 1

Let me try to explain in what case you might need to use .

So, lets say you have 2 classes:

class Grand {
    private String name;

    public Grand(String name) {
        this.setName(name);
    }

    public Grand() {
    }

    public void setName(String name) {
        this.name = name;
    }
}

class Dad extends Grand {
    public Dad(String name) {
        this.setName(name);
    }

    public Dad() {
    }
}

And lets say you have 2 collections, each contains some Grands and some Dads:

    List dads = new ArrayList<>();
    dads.add(new Dad("Dad 1"));
    dads.add(new Dad("Dad 2"));
    dads.add(new Dad("Dad 3"));


    List grands = new ArrayList<>();
    dads.add(new Dad("Grandpa 1"));
    dads.add(new Dad("Grandpa 2"));
    dads.add(new Dad("Grandpa 3"));

Now, lets asume that we want to have collection, which will contain Grand or Dad objects:

        List resultList;
        resultList = dads; // Error - Incompatable types List List
        resultList = grands;//Works fine

How we can avoid this? Simply use wildcard:

List resultList;
resultList = dads; // Works fine
resultList = grands;//Works fine

Notice, that you cant add new items in such (resultList) collection. For more information you can read about Wildcard and PECS conseption in Java