Why doesn't my constructor work? (Java)

Question

I have the following class implementation

public class PublisherHashMap
{
     private static HashMap<Integer, String> x;

     public PublisherHashMap()
     {
         x.put(0, "www.stackoverflow.com");
     }
}

In my test function, I am unable to create an object for some reason.

@Test
void test()
{ 
   runTest();
}

public static void runTest()
{
    PublisherHashMap y = new PublisherHashMap();
}

EDIT: I didn't construct the HashMap.

Answer 1

You are attempting to use x , the private HashMap , before it has been constructed. Hence you need to construct it first. You may do this by any of the following:

1) In the constructor:

x = new HashMap<Integer, String>(); 
// or diamond type  
x = new HashMap<>();

2) In the class as a field of this class:

private static HashMap<Integer, String> x = new HashMap<>();

3) In the initializer block:

static { 
    x = new HashMap<>();
}
// or the no-static block
{
    x = = new HashMap<>();
}

Answer 2

You must change your declaration from

private static HashMap<Integer, String> x;

to

private static HashMap<Integer, String> x = new HashMap<Integer, String>();