Question

I have a hashmap that contains multiple string arrays. I am trying to output each element in one of the arrays of the hashmap however I seem to always get

java.lang.NullPointerException

Here is my code,

import java.util.HashMap;
public class TestApp {
    private static HashMap<String, String[]> subjects;
    public TestApp() {
        HashMap<String, String[]> subjects = new HashMap<String, String[]>();
        subjects.put("calculus",new String[] {"math","logic"});
        subjects.put("chemisty",new String[] {"ions","electrons"});
        subjects.put("biology",new String[] {"life","bacteria"});
    }
    public static void main(String[] args){
        for(String s:subjects.get("biology")){
            System.out.println(s);
        }
    }


}

How can i stop this issue?

Answer 1

1. You’ve redefined a new local variable subjects inside of TestApp() which is unrelated to the private static variable.
2. Where are you instantiating TestApp()? That code is not getting run in the first place.

Either do all your code in main (or related static functions), or do your code in TestApp() and just instantiate an instance in main. For example:

private static HashMap<String, String[]> subjects;

public TestApp() {
}

public static void main(String[] args){
    subjects = new HashMap<String, String[]>();
    subjects.put("calculus",new String[] {"math","logic"});
    subjects.put("chemisty",new String[] {"ions","electrons"});
    subjects.put("biology",new String[] {"life","bacteria"});
    for(String s:subjects.get("biology")){
        System.out.println(s);
    }
}