Type Safety warning when casting to generic class in Java

Question

I have a class

class Property<T> {
    value T;

    public void setValue(T value) {
        this.value = value;
    }
}

Now I have a list List<Property<?>> properties with various properties of various Types. I loop through this list and want to set the value of every Property. Therefore I need to cast the generic Property to the correct type like: e.g. for Integer:

if (property.getType().equals("Integer")) {
        Property<Integer> propInt = (Property<Integer>) property;
        propInt.setValue((Integer) values[i++]);
}

where values[] is an Object array which holds the values that I want to set.

Everything works fine, but Java complains about Type Safety "Unchecked cast from Property<?> to Property<Integer>". A check like

if (property instanceof Property<Integer>){...}

is not possible though. How can I change my code to get rid of this warning, or do you know a better practice for my case?

Answer 1

The compiler complains because property is of type Property<?> which could or could not be of type Property<Integer> in general. This is an inherent limitation of the current java language due to type erasure.

In this particular case you are making sure that property is of class Property<Integer> by use of the getType method so it is safe to ignore the warning.

if (property.getType().equals("Integer")) {
        // we have made sure property is of class Property<Integer> so the cast is type safe
        @SuppressWarnings("unchecked")
        Property<Integer> propInt = (Property<Integer>) property;
        propInt.setValue((Integer) values[i++]);
}

It is important to document it with a comment otherwise a peer reviewing your code might not notice that indeed the cast is type safe and could confuse the warning supression as a malpractice.