Android-使用“ instanceof”是唯一确定“ onItemSelected”中从微调器返回的项目类型的方法吗？

Question

As in the title -

Is there any way to determine the type of the item (typically either a String , if adapted from a resource, or a generic Object , if adapted programmatically) obtained from a Spinner in onItemSelected ? In this case, I'm referring to one filled programmatically in Java, but as far as I know, it would be the same for one created from a String resource array.

Obviously, however, we create our Spinner, we know what type everything in it is. If it's from resources, probably Strings. If done programmatically, some kind of Object or possibly View . However, the signature for onItemSelected looks like this:

@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
    // actions

    // to get the item, you would usually do something like:
    parent.getItemAtPosition(pos);
}

And obviously, the type we get from this is ? , in other words, a generic Object .

So, if we know exactly what type it is, we can, of course, do this (example shown with an Integer ):

Object itemSelected = parent.getItemAtPosition(pos);

if (itemSelected instanceof Integer) doSomeMath((Integer) itemSelected);

But this doesn't exactly strike me as elegant. It works, certainly, at least in the simple cases I've given it (like Integer ). It might even be useful for handling multiple different types of Spinner all running off the same listener - I haven't actually tried that.

But it seems like there should be a way to have the signature simply be something like:

public void onItemSelected(AdapterView<Integer> parent, View view, int pos, long id)

Answer 1

Is using 'instanceof' the only way to determine the type of an item returned from a Spinner in “onItemSelected”?

It is not the only way. But it it probably the best way.

Here are some alternatives:

if (itemSelected.getClass().getName().equals("java.lang.Integer")) {
     doSomeMath((Integer) itemSelected);
}

if (itemSelected.getClass() == Integer.class) {
     doSomeMath((Integer) itemSelected);
}

try {
    doSomeMath((Integer) itemSelected);
} catch (ClassCastException ex) {
    // squash
}

Notes:

1. None of these alternatives is recommended. Seriously.
2. The instanceof operator tests for subclasses as well, unlike the first two alternatives.
3. The third version will also squash ClassCastException if it is thrown in the doSomeMath method.
4. Problems arise with null . The instanceof operator returns false for null , but all of these alternatives will throw an NPE.

---

Unfortunately, your alternative won't work. The AdapterView.OnItemSelectedListener interface declares that method's first parameter with that type. The interface is not generic.

Answer 2

But it seems like there should be a way to have the signature [...]

It seems but it can't be done because AdapterView.getItemAtPosition(int) returns Object anyway. ListView or Spinner don't know or care what type of items the adapter contains.

Obviously, however, we create our Spinner, we know what type everything in it is.

Exactly, since you're the ultimate master of what data you put in the adapter you should be able to cast it safely

@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
     final int item = (int) parent.getItemAtPosition(pos);
     doSomeMath(item);
}

Instead of checking for an always true condition it would just crash when it encounters unexpected data.

Get into this mindset: If something happens if it's not supposed to happen the code should throw an exception. This gives you a chance to fix any errors instead of never knowing if there's anything wrong.

Another option would be having the adapter stored in a member variable and accessing it from the listener

private MyAdapter adapter;

@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
     final int item = adapter.getItem(pos);
     doSomeMath(item);
}

This assumes your adapter overrides getItem(int) and returns the appropriate type.