java null pointer exception hashMap containsKey

Question

I am trying to understand why I get the following error:

Exception in thread "main" java.lang.NullPointerException
    at HashMapClass.person_exists(HashMapClass.java:12)
    at MainClass.main(MainClass.java:7)

I would have expected that by using contains key I would avoid pointing to nothing. Put another way, how do I query by hashMapClass when it is Empty?

MainClass.java

public class MainClass {

    public static void main(String[] args) {

    HashMapClass people = new HashMapClass();
    boolean e = people.person_exists("foo");
    }
}

HashMapClass.java

import java.util.HashMap;

public class HashMapClass {

    private HashMap<String, PersonClass> people;

    HashMapClass() {
    this.people = people;
    }

    public boolean person_exists(String name) {
    if (this.people.containsKey(name)) {
        return true;}
    else {return false;}}
}

PersonClass.java

import java.util.HashMap;

public class PersonClass {

    private String name;
    private HashMap<String, Integer> archive;
    private int x;

    PersonClass(String name) {
    this.name = name;
    this.archive = archive;
    this.x = 10;
    }
}

What exactly do you expect `this.people = people;` to do in the `HashMapClass` constructor? — Jon Skeet, Nov 04 '14 at 17:29
nothing, just wanted an empty constructor as they are not in use yet. I want to be able to add entries as I go along. — beoliver, Nov 04 '14 at 17:29
this.people = people; will do nothing except a self-assignment. You need to initialize your people HashMap or you you will get a NPE. — Michael Krause, Nov 04 '14 at 17:34
@ThemanontheClaphamomnibus: Okay, so if it's not doing anything, remove it. Now, what value do you expect `people` to have, given that no code is initializing it? — Jon Skeet, Nov 04 '14 at 17:34
I would expect it to be `HashMap`, but I know that my expectations do not match reality... — beoliver, Nov 04 '14 at 17:38
@MichaelKrause, how do i initialize my people within a hashMap? — beoliver, Nov 04 '14 at 17:40
Your HashMapClass constructor should replace the line this.people = people with this.people = new HashMap(); — Michael Krause, Nov 04 '14 at 17:43

java null pointer exception hashMap containsKey

0 Answers0