Question

In some way I do understand the Handler, but I’m not sure what to do with the params and how to let the code wait until the job is done in the background. I want the UI to be normally working and in the background I want to do an exchange rate calculation.

I have the following:

I call new getOnlineExchangeRate().execute(""); //Get Exchange Rate in BG

After that I want to have a result=amount*exchangerate, but the code is not waiting for the result.
Can somebody tell me how the calculation waits till we have an exchangerate. Do I have to send some params and how would that look?

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public double getYahooExchangeRate(String ER){

        double exchangerate=0;

    try {

        s = getJson("http://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20yahoo.finance.xchange%20where%20pair%20in%20(%22"+ER+"%22)&format=json&diagnostics=true&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys&callback=");                       
        //s = getJson("http://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20yahoo.finance.xchange%20where%20pair%20in%20(%22"+val[from]+val[to]+"%22)&format=json&diagnostics=true&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys&callback=");                      

        JSONObject jObj;
        jObj = new JSONObject(s);
        String exResult = jObj.getJSONObject("query").getJSONObject("results").getJSONObject("rate").getString("Rate");

        exchangerate=Double.parseDouble(exResult);

        } catch (JSONException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            ALS.Toast(myContext.getString(R.string.conversionerror), false);
        }
        catch (ClientProtocolException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            ALS.Toast(myContext.getString(R.string.conversionerror), false);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            ALS.Toast(myContext.getString(R.string.conversionerror), false);
        }                                   
    return exchangerate;

}       



public String getJson(String url)throws ClientProtocolException, IOException {

StringBuilder build = new StringBuilder();
HttpClient client = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(url);
HttpResponse response = client.execute(httpGet);
HttpEntity entity = response.getEntity();
InputStream content = entity.getContent();
BufferedReader reader = new BufferedReader(new InputStreamReader(content));
String con;
while ((con = reader.readLine()) != null) {
    build.append(con);
}
return build.toString();
}



public class getOnlineExchangeRate extends AsyncTask<String, Void, String> {


      @Override

      protected void onPostExecute(String result) {

        // execution of result of Long time consuming operation
          ALS.Toast(myContext.getString(R.string.exchangeratesupdated), true);


      }


    @Override
    protected String doInBackground(String... params) {
        // TODO Auto-generated method stub
         // perform long running operation operation

         getYahooExchangeRate(USDEUR);
        return null;
    }

Answer 1

I think your problem is in this line:

@Override
protected String doInBackground(String... params) {
    getYahooExchangeRate(USDEUR);
    return null;

You want to return the result of getYahooExchangeRate and not null 🙂
So change this and the return-value should be a double. So change this to:

@Override
protected Double doInBackground(String... params){
    return getYahooExchangeRate(USDEUR);
}

You also have to change your class header:

public class getOnlineExchangeRate extends AsyncTask<String, Void, Double> {


AsyncTask<Params, Progress, Result>

The generic part tells the AsyncTask which Informationstypes are handled.
The first is the type for the params of doInBackground(Params... )
The second is the type of the progress-Information
The last explains which type is returned by doInBackground(), so it changes the method-header from

protected Result doInBackground(Params... params){ };

to
protected double doInBackground(Params… params){};

To bring back the Result i would use and Observer oder Callback-Pattern.

Edit: changed double to Double, because primitives cannot be used for Generics.