Ondrej Tokar Ondrej Tokar - 3 months ago 6
Java Question

Java generic method doesn't work as expected

I am trying to make a generic method for data deserialization.

My code:

public <T> ExportedData<T> getExportData(T classType, String exportUri) {
Response response = _client.get(exportUri);
// System.out.println(response.body.toString());
ExportedData<T> exportedData = GsonSingleton.getGson().fromJson(response.body.toString(), new TypeToken<ExportedData<T>>() {
}.getType());
return exportedData;
}


The
response.body
:

{"totalResults":2,"limit":50000,"offset":0,"count":2,"hasMore":false,"items":[{"DevicesIDs":"","EmailAddress":"zatokar@gmail.com"},{"DevicesIDs":"","EmailAddress":"oto@increase.dk"}]}


The way I call the generic method:

ExportedData<AccengageOutboundContact> exportedData = generalBulkHelper.getExportData(new AccengageOutboundContact(), uriLimitAndOffset);


The
AccengageOutboundContact
:

public class AccengageOutboundContact {

public String EmailAddress;
public String DevicesIDs;

}


And the
ExportedData
:

public class ExportedData<T> {
public int totalResults;
public int limit;
public int offset;
public int count;
public boolean hasMore;
public List<T> items;
}


I would expect to get an ArrayList of
AccengageOutboundContact
objects. What I am getting is ArrayList of
StringMap
.

Any idea what am I doing wrong?

Answer

I've seen this one a bunch of times, but there does not seem to be a good duplicate to link.


Basically the problem is that T is erased to Ojbect, in your generic method. So the TypeToken that is created, does not hold the needed information.

This results in deserializtion to a StringMap.


You can fix this by passing a complete TypeToken to your method:

public <T> ExportedData<T> getExportData(TypeToken<ExportedData<T>> tt, String exportUri) {
    Response response = _client.get(exportUri);
    // System.out.println(response.body.toString());
    ExportedData<T> exportedData = GsonSingleton.getGson().fromJson(response.body.toString(),
        tt.getType());
    return exportedData;
}

Then call like:

generalBulkHelper.getExportData(new TypeToken<ExportedData<AccengageOutboundContact>>(){},
    uriLimitAndOffset);
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