Get weather for specific date using Openweather API - parsing JSON response

Question

I am trying to build a simple weather forecasting app using Android studio and Java. I followed some instructions here (https://www.androdocs.com/java/creating-an-android-weather-app-using-java.html) to get up and running and this works. However, I can only get the current weather. Openweather forecasting API calls seem to be for 5 days. That's ok, but how do I get the weather - say temp and wind speed - for a specific date specified by the user that's within the next 5 days?

Below is a sample JSON response (shortened). Even if I could pull out the info around a specific date at 12pm, and get the temp and wind speed for that date, that would be enough. How do I parse this JSON response to get the temp and wind speed for a specific date? Thanks so much... sorry I'm a beginner...

{"cod":"200","message":0,"cnt":40,"list":[{"dt":1574283600,"main":{"temp":281.75,"temp_min":281.68,"temp_max":281.75,"pressure":995,"sea_level":995,"grnd_level":980,"humidity":93,"temp_kf":0.07},"weather":[{"id":501,"main":"Rain","description":"moderate rain","icon":"10n"}],"clouds":{"all":100},"wind":{"speed":4.82,"deg":147},"rain":{"3h":5.38},"sys":{"pod":"n"},"dt_txt":"2019-11-20 21:00:00"},{"dt":1574294400,"main":{"temp":281.79,"temp_min":281.74,"temp_max":281.79,"pressure":995,"sea_level":995,"grnd_level":980,"humidity":91,"temp_kf":0.05},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10n"}],"clouds":{"all":100},"wind":{"speed":5.55,"deg":140},"rain":{"3h":1.75},"sys":{"pod":"n"},"dt_txt":"2019-11-21 00:00:00"},{"dt":1574305200,"main":{"temp":279.48,"temp_min":279.44,"temp_max":279.48,"pressure":994,"sea_level":994,"grnd_level":980,"humidity":95,"temp_kf":0.04},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10n"}],"clouds":{"all":100},"wind":{"speed":2.37,"deg":155},"rain":{"3h":0.94},"sys":{"pod":"n"},"dt_txt":"2019-11-21 03:00:00"},{"dt":1574316000,"main":{"temp":278.56,"temp_min":278.54,"temp_max":278.56,"pressure":995,"sea_level":995,"grnd_level":980,"humidity":94,"temp_kf":0.02},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10n"}],"clouds":{"all":100},"wind":{"speed":1.73,"deg":128},"rain":{"3h":0.06},"sys":{"pod":"n"},"dt_txt":"2019-11-21 06:00:00"},{"dt":1574326800,"main":{"temp":279.19,"temp_min":279.19,"temp_max":279.19,"pressure":995,"sea_level":995,"grnd_level":981,"humidity":95,"temp_kf":0},"weather":[{"id":804,"main":"Clouds","description":"overcast clouds","icon":"04d"}],"clouds":{"all":100},"wind":{"speed":1.79,"deg":104},"sys":{"pod":"d"},"dt_txt":"2019-11-21 09:00:00"},{"dt":1574337600,"main":{"temp":282.2,"temp_min":282.2,"temp_max":282.2,"pressure":995,"sea_level":995,"grnd_level":980,"humidity":85,"temp_kf":0},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10d"}],"clouds":{"all":100},"wind":{"speed":2.78,"deg":129},"rain":{"3h":0.19},"sys":{"pod":"d"},"dt_txt":"2019-11-21 12:00:00"}

Answer 1

JSON-Simple

Here is an example app using the JSON-Simple library to parse the JSON data downloaded from OpenWeatherMap.org.

package work.basil.example;

import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
import java.time.Instant;
import java.time.ZoneId;
import java.time.ZonedDateTime;

public class Weather
{

    public static void main ( String[] args )
    {

        Weather app = new Weather();
        app.demo();
    }

    private void demo ( )
    {
        //Creating a JSONParser object
        JSONParser jsonParser = new JSONParser();
        try
        {
            // Download JSON.
            String yourKey = "b6907d289e10d714a6e88b30761fae22";
            URL url = new URL( "https://samples.openweathermap.org/data/2.5/forecast/hourly?zip=79843&appid=b6907d289e10d714a6e88b30761fae22" + yourKey ); // 79843 = US postal Zip Code for Marfa, Texas.
            URLConnection conn = url.openConnection();
            BufferedReader reader = new BufferedReader( new InputStreamReader( conn.getInputStream() ) );


            // Parse JSON
            JSONObject jsonObject = ( JSONObject ) jsonParser.parse( reader );
            System.out.println( "jsonObject = " + jsonObject );

            JSONArray list = ( JSONArray ) jsonObject.get( "list" );
            System.out.println( "list = " + list );

            // Loop through each item
            for ( Object o : list )
            {
                JSONObject forecast = ( JSONObject ) o;

                Long dt = ( Long ) forecast.get( "dt" );          // Parse text into a number of whole seconds.
                Instant instant = Instant.ofEpochSecond( dt );    // Parse the count of whole seconds since 1970-01-01T00:00Z into a `Instant` object, representing a moment in UTC with a resolution of nanoseconds.
                ZoneId z = ZoneId.of( "America/Chicago" );        // Specify a time zone using a real `Continent/Region` time zone name. Never use 2-4 letter pseudo-zones such as `PDT`, `CST`, `IST`, etc.
                ZonedDateTime zdt = instant.atZone( z );          // Adjust from a moment in UTC to the wall-clock used by the people of a particular region (a time zone). Same moment, same point on the timeline, different wall-clock time.
                LocalTime lt = zdt.toLocalTime() ;
                // … compare with lt.equals( LocalTime.NOON ) to find the data sample you desire. 
                System.out.println( "dt : " + dt );
                System.out.println( "instant : " + instant );
                System.out.println( "zdt : " + zdt );

                JSONObject main = ( JSONObject ) forecast.get( "main" );
                System.out.println( "main = " + main );


                Double temp = ( Double ) main.get( "temp" );  // Better to use BigDecimal instead of Double for accuracy. But I do not know how to get the JSON-Simple library to parse the original string input as a BigDecimal.
                System.out.println( "temp = " + temp );

                JSONObject wind = ( JSONObject ) forecast.get( "wind" );
                System.out.println( "wind = " + wind );

                System.out.println( "BASIL - wind.getCLass: " + wind.getClass() );
                Double speed = ( Double ) wind.get( "speed" );
                System.out.println( "speed = " + speed );

                System.out.println( "\n" );
            }
        }
        catch ( FileNotFoundException e )
        {
            e.printStackTrace();
        }
        catch ( IOException e )
        {
            e.printStackTrace();
        }
        catch ( ParseException e )
        {
            e.printStackTrace();
        }
    }
}

Decimal separator

Notice that this code blows up when encountering a wind speed data point lacking the decimal separator. The publisher of that data should be writing, for example, 1.0 rather than 1 for consistency. If they had done so, the library would have parsed 1.0 as a Double rather than parsing 1 as Long.

JSON-Simple 1 now defunct

Also this code uses the original version 1 of JSON-Simple, now defunct. This project was forked, producing dramatically different versions 2 & 3.

See this page Parsing decimal numbers, some of which lack a decimal separator, in JSON data using JSON-Simple (Java) for details about the parsing-decimal problem and about links to the forked project.

Not for production use

So while I would not recommend this code for production use, it may help you along the way. For real work, consider the later version 3 of JSON-Simple or any of the several other JSON-processing libraries available for Java.

See the example data at this URL. To make it readable, use your text-editor or IDE to reformat the JSON data.

Sample output:

dt : 1553709600
instant : 2019-03-27T18:00:00Z
zdt : 2019-03-27T13:00-05:00[America/Chicago]
main = {"temp":286.44,"temp_min":286.258,"grnd_level":1002.193,"temp_kf":0.18,"humidity":100,"pressure":1015.82,"sea_level":1015.82,"temp_max":286.44}
temp = 286.44
wind = {"deg":202.816,"speed":5.51}
speed = 5.51


dt : 1553713200
instant : 2019-03-27T19:00:00Z
zdt : 2019-03-27T14:00-05:00[America/Chicago]
main = {"temp":286.43,"temp_min":286.3,"grnd_level":1002.667,"temp_kf":0.13,"humidity":100,"pressure":1016.183,"sea_level":1016.183,"temp_max":286.43}
temp = 286.43
wind = {"deg":206.141,"speed":4.84}
speed = 4.84

Answer 2

You can serialize the response JSON string to POJOs by any one of the most popular JSON libraries such as Jackson or Gson, then retrieve fields you want of the object whose date field equals given date. BTW, your JSON string is invalid, ]} is missing at the end of it.

POJOs

@JsonIgnoreProperties(ignoreUnknown = true)
class Response {
    List<Weather> list;

    //general getters and setters
}

@JsonIgnoreProperties(ignoreUnknown = true)
class Weather {
    JsonNode main;
    JsonNode wind;

    @JsonProperty("dt_txt")
    String dtTxt;

    //general getters and setters
}

Use @JsonIgnoreProperties (provided by Jackson) to ignore those fields you don't care while serialization.

Code snippet

ObjectMapper mapper = new ObjectMapper();
Response response = mapper.readValue(jsonStr, Response.class);

String givenDate = "2019-11-21 12:00:00";
response.getList().forEach(e -> {
    if (givenDate.equals(e.getDtTxt())) {
        System.out.println("temp: " + e.getMain().get("temp").asText());
        System.out.println("wind speed:" + e.getWind().get("speed").asText());
    }
});

Console output

temp: 282.2
wind speed:2.78