Question

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Java how to: Generic Array creation
Error generic array creation

I have been tasked with writing a Hash Table in Java, which must work with any data type. The rules on the code I am writing are as follows:
– The hash table must have an array as the underlying data structures, of a size determined at the time the object is constructed
– When a collision occurs, the element that collides should be placed into a linked list, which holds all of the elements at that index (key) in the hash table

Thus, for the underlying data type, I have made an array of type LinkedList (custom, not the Java API LinkedList).

private LinkedList<T>[] table;

The issue is, of course, instantiating this array. Here are some of my attempts:

public HashTable(int size) {
  table = new LinkedList<T>[size];
}

This throws a compile-time generic array creation error.

public HashTable(int size) {
  table = (LinkedList<T>[])(new Object[size]);
}

That causes a ClassCastException error at runtime (java.lang.Object cannot be cast to LinkedList).

The person heading the project is also unsure of how to deal with this issue. Is there any way I can change my code so that the hash table still has an array as its underlying data structure with the collisions being placed in a LinkedList?

Answer 1

This worked for me:

public class HashTable<T> {

    private LinkedList<T> table[];

    @SuppressWarnings("unchecked")
    public HashTable(int size) {
        table = new LinkedList[size];
    }

}

For example:

HashTable<String> t = new HashTable<String>(10);
t.table[0] = new LinkedList<String>();
t.table[0].add("test");
System.out.println(t.table[0].get(0));

Yes, the constructor generated a warning (that explains the “unchecked” annotation), but afterwards the code works without more warnings.