Nullpointer Exception on reading data using google spreadsheet oauth in Java

Question

Getting nullpointer exception on trying to read data using google spreadsheet API in Java. Created a spreadsheet and enabled Google Spreadsheet API. Also downloaded client_services.json file and added inside src/resources/ path. Still getting nullpointer exception on running.

package main.java;
import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp;
import com.google.api.client.extensions.jetty.auth.oauth2.LocalServerReceiver;
import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow;
import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.util.store.FileDataStoreFactory;
import com.google.api.services.sheets.v4.SheetsScopes;
import com.google.api.services.sheets.v4.model.*;
import com.google.api.services.sheets.v4.Sheets;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.List;

public class Quickstart {
    /** Application name. */
    private static final String APPLICATION_NAME =
        "Google Sheets API Java Quickstart";

    /** Directory to store user credentials for this application. */
    private static final java.io.File DATA_STORE_DIR = new java.io.File(
        System.getProperty("user.home"), ".credentials/sheets.googleapis.com-java-quickstart");

    /** Global instance of the {@link FileDataStoreFactory}. */
    private static FileDataStoreFactory DATA_STORE_FACTORY;

    /** Global instance of the JSON factory. */
    private static final JsonFactory JSON_FACTORY =
        JacksonFactory.getDefaultInstance();

    /** Global instance of the HTTP transport. */
    private static HttpTransport HTTP_TRANSPORT;

    /** Global instance of the scopes required by this quickstart.
     *
     * If modifying these scopes, delete your previously saved credentials
     * at ~/.credentials/sheets.googleapis.com-java-quickstart
     */
    private static final List<String> SCOPES =
        Arrays.asList(SheetsScopes.SPREADSHEETS_READONLY);

    static {
        try {
            HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
            DATA_STORE_FACTORY = new FileDataStoreFactory(DATA_STORE_DIR);
        } catch (Throwable t) {
            t.printStackTrace();
            System.exit(1);
        }
    }

    /**
     * Creates an authorized Credential object.
     * @return an authorized Credential object.
     * @throws IOException
     */
    public static Credential authorize() throws IOException {
        // Load client secrets.
        InputStream in =
            Quickstart.class.getResourceAsStream("//client_secret.json");

        GoogleClientSecrets clientSecrets =
            GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in));

        // Build flow and trigger user authorization request.
        GoogleAuthorizationCodeFlow flow =
                new GoogleAuthorizationCodeFlow.Builder(
                        HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, SCOPES)
                .setDataStoreFactory(DATA_STORE_FACTORY)
                .setAccessType("offline")
                .build();
        Credential credential = new AuthorizationCodeInstalledApp(
            flow, new LocalServerReceiver()).authorize("user");
        System.out.println(
                "Credentials saved to " + DATA_STORE_DIR.getAbsolutePath());
        return credential;

    }

    /**
     * Build and return an authorized Sheets API client service.
     * @return an authorized Sheets API client service
     * @throws IOException
     */
    public static Sheets getSheetsService() throws IOException {
        Credential credential = authorize();
        return new Sheets.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential)
                .setApplicationName(APPLICATION_NAME)
                .build();
    }

    public static void main(String[] args) throws IOException {
        // Build a new authorized API client service.
        Sheets service = getSheetsService();

        // https://docs.google.com/spreadsheets/d/1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms/edit
        String spreadsheetId = "12tEQZqb-12N-a8hF-4TG-1E0qqJuhrYTl5CYAeIg0Sc";
        String range = "Sheet1!A1:E";
        ValueRange response = service.spreadsheets().values()
            .get(spreadsheetId, range)
            .execute();
        List<List<Object>> values = response.getValues();
        if (values == null || values.size() == 0) {
            System.out.println("No data found.");
        } else {
          for (List row : values) {
            // Print columns A and E, which correspond to indices 0 and 4.
            System.out.printf("%s, %s\n", row.get(0), row.get(4));
          }
        }
    }


}

Also, added the "client_secret.json" file inside src/main/resources/ path. This is the trace. Want to know why it is showing as null even after client_secret file has data. Want to know the proper reason why it is showing as null even if client_secret has proper data. Also it has been added in the right path.

    > java.lang.NullPointerException
    at java.io.Reader.<init>(Reader.java:78)
    at java.io.InputStreamReader.<init>(InputStreamReader.java:72)
    at main.java.Quickstart.authorize(Quickstart.java:73)
    at main.java.Quickstart.getSheetsService(Quickstart.java:102)
    at main.java.Quickstart.main(Quickstart.java:110)
Exception in thread "main" com.google.api.client.googleapis.json.GoogleJsonResponseException: 403 Forbidden
{
  "code" : 403,
  "errors" : [ {
    "domain" : "global",
    "message" : "The request is missing a valid API key.",
    "reason" : "forbidden"
  } ],
  "message" : "The request is missing a valid API key.",
  "status" : "PERMISSION_DENIED"
}
    at com.google.api.client.googleapis.json.GoogleJsonResponseException.from(GoogleJsonResponseException.java:146)
    at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:113)
    at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:40)
    at com.google.api.client.googleapis.services.AbstractGoogleClientRequest$1.interceptResponse(AbstractGoogleClientRequest.java:321)
    at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:1065)
    at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:419)
    at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:352)
    at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:469)
    at main.java.Quickstart.main(Quickstart.java:118)