会话setAttribute抛出空指针

Question

I'm writing a Java code where in if I enter a country name, the session should set an attribute to corresponding country code, else it should not save the session.

Below is my code

Temp Class

package onlyJava;

import java.util.Locale;
import java.util.Scanner;

import com.amazon.speech.speechlet.Session;

public class Temp {

    String cityName;

    public void getTheCurrentWeather(Session session) {
        System.out.println("Enter a city Name\n");
        Scanner sc = new Scanner(System.in);
        cityName = sc.next();
        getCountryCode(cityName, session);
        System.out.println(session.getAttribute("cCode").toString());
    }

    public String getCountryCode(String countryName, Session session) {
        System.out.println(countryName + "\t" + session);
        String result = "";
        String[] locales = Locale.getISOCountries();

        for (String countryCode : locales) {

            Locale obj = new Locale("", countryCode);
            if (countryName.equals(obj.getDisplayCountry().toString())) {
                result = obj.getCountry();
                session.setAttribute("cCode", result);
            }
        }

        return result;
    }

}

Test Class (Main)

package onlyJava;

import com.amazon.speech.speechlet.Session;

public class Test {

    public static void main(String[] args) {

        Temp temp = new Temp();
        temp.getTheCurrentWeather(null);

    }

}

When I run this code with input as India Country , I get the below result.

India   null
Exception in thread "main" java.lang.NullPointerException
    at onlyJava.Temp.getCountryCode(Temp.java:30)
    at onlyJava.Temp.getTheCurrentWeather(Temp.java:16)
    at onlyJava.Test.main(Test.java:10)

When I enter it as Hyderabad City , I get the below result.

Hyderabad   null
Exception in thread "main" java.lang.NullPointerException
    at onlyJava.Temp.getTheCurrentWeather(Temp.java:17)
    at onlyJava.Test.main(Test.java:10)

Please let me know where am I going wrong and how can I fix this.

Thanks

Answer 1

In both cases the NPE is thrown due to the fact that you are giving a null as a parameter in the call to the method getWeather from the main.

The main class is expecting to receive a non null Session instance, and it is never checked wether it is null or not.

In the India test case, the NullPointerException is thrown from the getCountry () method, as India is in fact a valid country, so this method tries to write down it's corresponding country code in the session object. It just in this moment when it falls in the NPE.

In the second case, as Hyderabad is not a country, nothing is written in the Session, and the NPE is thrown while it tries to recover the session attribute from a null Session instance.

Both errors would be fixed if you pass a valid Session instance in the main method of the test class