Java - issue with generics

Question

How do I properly design this, so it would work:

In class Property<E>:

public void addPropertyChangedListener(OnPropertyChangedListener<E> listener) {
    listeners.add(listener);
}

In class ViewModelBase:

public void addPropertyChangedListener(String propertyName, OnPropertyChangedListener<?>   listener) {
    Property<?> property = properties.get(propertyName);
    property.addPropertyChangedListener(listener); // I get error here
}

Error I get:

The method addPropertyChangedListener(Property.OnPropertyChangedListener<capture#5-of ?>)
in the type Property<capture#5-of ?> is not applicable for the arguments
(Property.OnPropertyChangedListener<capture#6-of ?>)

Just to clarify, properties.get(PropertyName) is a Property, correct? — Jason C
– Jason C, Commented Mar 29, 2014 at 20:57
Generalizing Jason Cs comment: Some more code showing the involved classes may be helpful - particularly, showing where E is defined. You can't add a ...Listener<?> where a ...Listener<E> is expected, but it's hard to say more based on the current code. — Marco13
– Marco13, Commented Mar 29, 2014 at 20:58
The ViewModelBase.addPropertyChangedListener and use the type variable everywhere you are using ?. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 29, 2014 at 21:02

Judge Mental · Accepted Answer · 2014-03-29 21:49:41Z

1

public void <T> addPropertyChangedListener(String propertyName, OnPropertyChangedListener<T>   listener) {
    Property<T> property = (Property<T>)properties.get(propertyName);
    property.addPropertyChangedListener(listener);
}

There is no way to avoid an unchecked cast warning if Properties is a heterogeneous collection. How does it even make sense (in a type system as weak as that of Java) to statically typecheck something whose type can only be known at runtime?

edited Mar 29, 2014 at 21:49

answered Mar 29, 2014 at 21:35

Judge Mental

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