Question

I come from writing a lot of JavaScript, so bear with me.

I’ve got 3 HashMaps, which i reference in a method in a different class. My code (very simply) looks like so:

public class MainClass {
    private HashMap<String,Nation> randomHashMap = new HashMap<String,Nation>();
    DifferentClass d = new DifferentClass(this);
} //with getters/setters

public class DifferentClass {
    private MainClass mc;
    public void randomMethod() {
        System.out.println("randomHashMap is " + (mc.getRandomHashMap() == null));
    } //returns null
    public DifferentClass(MainClass c) {
        this.mc = c;
    }
} 

However, when I call them in my other method, they’re null.

How do I create a new, empty HashMap?

Answer 1

You need to initialize your MainClass mc variable before using it in the DifferentClass#randomMethod method. Also, make sure you’re using the mc variable instead of the MainClass.getRandomHashMap() method (by your actual code, we don’t know how it behaves). Your code will look like this:

public class DifferentClass {

    private MainClass mc = new MainClass();

    public void randomMethod() {
        //assuming getRandomHashMap is the getter of randomHashMap attribute (and non static)
        System.out.println("randomHashMap is " + (mc.getRandomHashMap() == null));
    }
}

public class MainClass {
    private HashMap<String,Nation> randomHashMap = new HashMap<String,Nation>();
    DifferentClass d = new DifferentClass(this);

    public HashMap<String,Nation> getRandomHashMap() {
        return this.randomHashMap;
    }
} //with getters/setters