Android，Mandrill和Parse.com：Java ClassCastException：HashMap无法转换为类

Question

I don't know whether the problem is sourced this far back, so I will tell you about it anyway. I have a function stored as Parse Cloud Code which retrieves the user's list of scheduled emails, as per Parse and Mandrill API documentation. However, the returned object from Mandrill, according to their docs, is an array of structs containing String key-value pairs. You can see the relevant docs here: https://mandrillapp.com/api/docs/messages.JSON.html#method=list-scheduled

Parse Cloud Code can only be in Javascript, so how the returned array is interpreted by it, I am not sure. Either way, I have the cloud function return the returned array directly back to the Java method which called it in my Android application. I thought it would arrive as a JSONArray, but that did not seem to be the case. My editor, Android Studio, insisted that the return type was of ArrayList<ScheduledEmail> , and would not let me use any other parameter type, so I set it to that. (ScheduledEmail is the class I created in Java to match the elements contained in a returned struct from Mandrill, for this method. Was this part of where I went wrong? How do I convert those structs in the returned array into Java-compatible key-value pairs?) However, when I try to access an object contained in the array, I receive this error:

    java.lang.ClassCastException: java.util.HashMap cannot be cast to com.****.******.ScheduledEmail

Nowhere does there seem to have been a HashMap used. What am I missing? Should I have first done something to the returned array before having it sent back to the Java method? Has Mandrill returned a HashMap? If so, why would it be interpreted as the Java class which I intended it to match to?

The relevant portions of the Java method in question are as follows:

HashMap<String, Object> params = new HashMap<String, Object>();
params.put("recipient", "koolstr@gmail.com");
ParseCloud.callFunctionInBackground("listScheduledEmails", params, new FunctionCallback<ArrayList<ScheduledEmail>>() {
    public void done(ArrayList<ScheduledEmail> schedEmailsFromJS, ParseException e) {
        if (e == null) {
            for (int i=0; i < schedEmailsFromJS.size(); i++) {

                //This is the line which produces the error
                Log.d("sEmail", schedEmailsFromJS.get(i).get_id());
            }
        }
    }
});

This is the Parse Cloud Code Javascript function in which I retrieve the list of scheduled emails from Mandrill:

Parse.Cloud.define("listScheduledEmails", function(request, response) {
Parse.Cloud.httpRequest({
    method: 'POST',
    headers: {
        'Content-Type': 'application/json;charset=utf-8'
    },
    url: "https://mandrillapp.com/api/1.0/messages/list-scheduled.json",

    body: {
        key: "***************",
        to: request.params.recipient
    },
    success: function(httpResponse) {
        console.log("The scheduled emails have been successfully retrieved.");
        console.log(httpResponse.data);
        response.success(httpResponse.data);        
        console.log("Returned the list successfully.");
    },
    error: function(httpResponse) {
        console.error('Request failed with response code ' + httpResponse.status);
        response.error("Failed to retrieve messages.");
    }
});
});

Any help, ideas, clarifications, or criticisms would be appreciated. I've been stuck for half a day on this one problem and I can't seem to solve it. I don't know where I am going wrong.

Answer 1

You first need to figure out what type is passed down to the FunctionCallback :

ParseCloud.callFunctionInBackground("listScheduledEmails", params, new FunctionCallback<Object>() {
    public void done(Object o, ParseException e) {
        if (e == null) {
            Log.i("Tag", "Return type = " + o.getClass().getName());
        }
    }
});

After doing that, change your FunctionCallback to correctly handle the returned type, and, if needed, map it to your own bean type. I assume the returned type will be some sort of list of hash maps.

If that's indeed the case, you can change your implementation to something like this to get your list of ScheduledEmail objects:

ParseCloud.callFunctionInBackground("listScheduledEmails", params, new FunctionCallback<List<Map>>() {
    public void done(List<Map> list, ParseException e) {
        if (e == null) {
            ArrayList<ScheduledEmail> scheduledEmails = new ArrayList<>();

            for(Map<String, ?> data : list) {
                ScheduledEmail scheduledEmail = new ScheduledEmail();
                scheduledEmail.set_id(data.get("_id"));
                // ... extract rest of data

                scheduledEmails.add(scheduledEmail);
            }
        }
    }
});