Question

I understand the idea behind a hash table, but the implementation is getting frustrating. I have done some reading and gotten mixed answers. I have read that I could implement a generic array of objects as follows:

TableContainer[] classTable = 
    (TableContainer<Object,Object>[]) new TableContainer[256];

where tableContainer is:

class TableContainer<key,val>{
    Object key = null;
    Object val = null;
}

The problem I am having is that when I try to pass the following code in a function:

classTable[i].key = x

I get a null pointer exception. I understand creating generic arrays in java gets messy because of type erasure but I thought the above implementation would work. Is there any way to create a list or array of generics to be utilized as a hashtable? (no hashtable/hashmap classes permitted)

Answer 1

First of all you are creating an empty array:

new TableContainer[0]

and since you cannot resize arrays over time, it doesn’t have much sense. Providing that you actually create an array of some positive size, you must remember one thing: you created an array of references. And by default each array is initialized to default (zero) values, which happens to be null for references.

Thus you need something like this:

classTable = new TableContainer[8];
classTable[0] = new TableContainer();
//...later
if(classTable[0] != null) {  //will pass for [0], but not for [1]
  classTable[0].key
}