How to map an object stream to a specific type

Question

I have a JSONArray of objects, and I want to map each object to a JSONObject.

I tried:

JSONArray array; //my json array
array.stream().map(obj -> (JSONObject) obj).forEach((JSONObject prof) -> {
    //code
});

However the type is already encapsulated by Object and thus I cannot seem to cast it down. How can I achieve this with Java 8 streams?

Answer 1

A few issues here. JSONArray is a raw sub type of ArrayList. Therefore the methods it inherits are also raw, all their type parameter uses are erased and reduce to Object. Just that should tell you that things will not be safe going forward.

The invocation of

array.stream().map(..)

is raw. Therefore the Function that map accepts will also be raw. The returned Stream will also be raw. And therefore the forEach invoked will operate on Object, the resulting type from erasure. There's nothing you can do about this except for casting in between operations. Something like

Stream stream = array.stream().map(obj -> (JSONObject) obj);
((Stream<JSONObject>) stream).forEach((JSONObject prof) -> {
    // code
});

But this is in no way safe (unless you know there are only JSONObject objects in the JSONArray). Which brings us to...

I have a JSONArray of objects, and I want to map each object to a JSONObject.

You have to decide how to do this mapping. JSONArray can contain JSONObject, Strings, numbers, etc. Simply casting to a JSONObject won't work.

Answer 2

Your code won't work, since the ArrayList that is the base type of JSONArray has no type parameter specified. You can simply "add" one by casting:

((List<?>)array).stream().map(obj -> (JSONObject) obj).forEach((JSONObject prof) -> {
    //code
});

But in your code example I really don't see any reason not to use only one lambda expression like this:

array.stream().forEach(obj -> {
    JSONObject prof = (JSONObject) obj;
    //code
});