Initialize Empty HashMap

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?

Make sure you initialize them in the constructor or before using them in your code. — Luiggi Mendoza
– Luiggi Mendoza, Commented Nov 18, 2012 at 8:33
The code you've provided won't even compile, unless you've really got a static method called getRandomHashMap() in MainClass. — Jon Skeet
– Jon Skeet, Commented Nov 18, 2012 at 8:40
My condolence for moving from the land of dynamic freedom to Java's type tyranny. — akuhn
– akuhn, Commented Nov 18, 2012 at 9:01
@MarkoTopolnik: The code has changed. The call to getRandomHashMap() was originally MainClass.getRandomHashMap(). (See the edit history.) — Jon Skeet
– Jon Skeet, Commented Nov 18, 2012 at 9:05

Luiggi Mendoza · Accepted Answer · 2012-11-18 08:43:54Z

5

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

edited Nov 18, 2012 at 8:43

answered Nov 18, 2012 at 8:38

Luiggi Mendoza

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4 Comments

Polyov Over a year ago

when i pass the reference in the constructor, i get errors to the stars (stack overflow errors)

Luiggi Mendoza Over a year ago

@SomekidwithHTML it would depend on your design. If you want to use an instance of MainClass to initialize the DifferentClass attribute (like your code shows), it would be better to do it in the MainClass constructor.

Marko Topolnik Over a year ago

I don't see how this is a solution. OP's code clearly initializes the mc field in its constructor, and the constructor is passed a definitely non-null value from MainClass.

Polyov Over a year ago

@MarkoTopolnik this solution does not directly solve my problem, but it made me realize what i needed to do differently, along the lines of "it would depend on your design". I realized that how I am writing this does not comply with how it needs to be designed.

Marko Topolnik · Accepted Answer · 2012-11-18 09:04:16Z

The code you posted is in fact perfectly all right as far as field initialization. I made an SSCCE from it with minimal intervention:

class Nation{}

public class MainClass {
  private HashMap<String,Nation> randomHashMap = new HashMap<String,Nation>();
  DifferentClass d = new DifferentClass(this);
  public Object getRandomHashMap() {
    return randomHashMap;
  }
  public static void main(String[] args) {
    new MainClass().d.randomMethod();
  }
} //with getters/setters

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;
  }
}

and it prints

randomHashMap is false

which proves that randomHashMap is indeed non-null.

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