Type safety: Unchecked cast from Object to List<MyObject>

Question

I have a ListView listing a custom object (let's say MyObject).

I want to filter it dynamically through an EditText so I had to implement a getFilter() with a publishResults method:

@Override
protected void publishResults(CharSequence constraint, FilterResults results) {
    MyObjectAdapter.this.setItems((List<MyObject>) results.values);
    MyObjectAdapter.this.notifyDataSetChanged();
}

At this point, Eclipse complains: Type safety: Unchecked cast from Object to List<MyObject>

I am sure this cast will always be true, but Eclipse only suggests to add @SuppressWarnings("unchecked") but I'm totally against SuppressWarnings because it's only hiding the problem, not a solution...

I tried adding:

if(results.values instanceof List<MyObject>)

But Eclipse complains again, and this solves nothing...

Cannot perform instanceof check against parameterized type List<MyObject>. Use the form List<?>

I know the casting will always be correct, but which is the proper way to make the code to be sure results.values is actually a List<MyObject> ?

Thanks in advance!

Solution 1

Well, I finally managed to find a solution.

Just as @Medo42 said:

Another option is to check for and cast to List as the instanceof error suggests. Then you can iterate over the elements in the list and check if they are actually all instances of MyObject, and copy them to a new List.

Even though I did not went through the process of creating a whole new object in order to make this particular case to work "warning-less" this was the right direction to go.

So I took @lokoko 's idea and use it in a new setItems() method, with an Object parameter instead of a List<MyObject> in order to make sure

The result code is the following:

public void setItems(List<MyObject> var){
    this.list = var;
}

public void setItems(Object var){
    List<MyObject> result = new ArrayList<MyObject>();
    if (var instanceof List){
        for(int i = 0; i < ((List<?>)var).size(); i++){
            Object item = ((List<?>) var).get(i);
            if(item instanceof MyObject){
                result.add((MyObject) item);
            }
        }
    }
    setItems(result);
}

Thanks everyone for your help!

Solution 2

If all you have to work from is an Object, then you can't check at runtime that you actually have a List<MyObject>, because the generic type MyObject is only used for compile-time type checking, it is not available at runtime. This is why you get an error when you try to add the instanceof check.

If you are sure that your Object really is always a List<MyObject> then I'd say the @SuppressWarnings is OK, if you document why you are sure it is not a problem.

If you absolutely want to avoid a warning though, you could create your own List implementation (say, MyObjectList) that is not itself generic but implements List<MyObject>. Then you can do an instanceof check against MyObjectList at runtime.

Another option is to check for and cast to List<?> as the instanceof error suggests. Then you can iterate over the elements in the list and check if they are actually all instances of MyObject, and copy them to a new List<MyObject>.

Solution 3

Try something like this :

List<?> result = (List<?>) results.values;
for (Object object : result) {
    if (object instanceof MyObject) {
        tempList.add((MyObject) object); // <-- add to temp
    }
}

filteredItems = tempList; // <-- set filtered

Solution 4

You can perform the checking before passing it to setItems().

final Object myListObj = reuslts.values;
if(myListObj instanceof List<?>) {
    if(((List<?>)myListObj).get(0) instanceof MyObject)
        // You can safely suppress the warning here because you made sure it is a List containing MyObject
        MyObjectAdapter.this.setItems((List<? extends MyObject>) myListObj);

}

However, you need to change your setItems() method accordingly:

public void setItems(List<? extends MyObject> list) {
    // Your code here
}

View more solutions

Comments

[ROMANIA_engineer]

stackoverflow.com/questions/262367/type-safety-unchecked-cas‌​t

[Medo42]

This check only ensures that the first object in the list is a MyObject, so the only thing you have shown is that the list can contain such elements - not that it actually is a List<MyObject> or even a List<? extends MyObject>. Rather, you have shown that it is a List<? super MyObject>.

[Lawrence Choy]

You are correct. However, a List<String> can never contains an element of Integer right? If MyObject has no subclass this works well. Even if the objects inside extends MyObject, they all contains necessary interface or functions that MyObjectAdapter required, because method signature decladed MyObject, not other subclasses.

[Medo42]

Technically, a List<String> can contain an Integer because the generic type of the list is not enforced at runtime. But even if you had that guarantee and even if MyObject has no subclasses your suggestion isn't valid. Imagine a List<Object> that contains one MyObject and one String. Your check would cast that to List<? extends MyObject> because the first element happens to be a MyObject, but trying to get the second element throught that casted list would cause a ClassCastException.

[Eloi Navarro]

Your solution helped me to figure out how to solve my problem, thx!

[Android Dev]

For me I did: val imageList: List<Any> = snapshot.value as ArrayList<Any>