How to properly define an array of linked list in Java ?

Question

I tried to define an array of linked list in Java like the following, which compiled fine but it generated 2 warning messages.

 LinkedList<Long> [] hashtable = new LinkedList[10];

warning: [rawtypes] found raw type: LinkedList
    LinkedList<Long> [] hashtable = new LinkedList[10];
                                        ^
  missing type arguments for generic class LinkedList<E>
  where E is a type-variable:
    E extends Object declared in class LinkedList
HashTable.java:13: warning: [unchecked] unchecked conversion
    LinkedList<Long> [] hashtable = new LinkedList[10];
                                    ^
  required: LinkedList<Long>[]
  found:    LinkedList[]

So, I tried

 LinkedList<Long> [] hashtable = new LinkedList<Long>[10];

But this time it would not even compile and generate this error instead.

HashTable.java:13: error: generic array creation
    LinkedList<Long> [] hashtable = new LinkedList<Long>[10];
                                    ^
1 error

So, how should I define my array of linked list properly ?

Solution

This is a proper way to create an array:

@SuppressWarnings("unchecked") LinkedList<Long> [] hashtable = new LinkedList[10];

Cannot Create Arrays of Parameterized Types

You cannot create arrays of parameterized types. For example, the following code does not compile:

List<Integer>[] arrayOfLists = new List<Integer>[2];  // compile-time error

The following code illustrates what happens when different types are inserted into an array:

Object[] strings = new String[2];
strings[0] = "hi";   // OK
strings[1] = 100;    // An ArrayStoreException is thrown.

If you try the same thing with a generic list, there would be a problem:

Object[] stringLists = new List<String>[];  // compiler error, but pretend it's allowed
stringLists[0] = new ArrayList<String>();   // OK
stringLists[1] = new ArrayList<Integer>();  // An ArrayStoreException should be thrown,
                                            // but the runtime can't detect it.

If arrays of parameterized lists were allowed, the previous code would fail to throw the desired ArrayStoreException.

Taken from docs.oracle.com

So what can I store in hashtable[] ?

Does it mean I am now allowed to have a linked list of string in the hashtable[0] and a linked list of Long in hashtable1, if I do LinkedList [] hashtable = new LinkedList[10]?

No, compiler won't allow you to store LinkedList to the hashtable array directly. Following snippet won't compile:

hashtable[0] = new LinkedList<String>();

However you can store the LinkedList without type parameters, or even a subclass of LinkedList:

@SuppressWarnings("unchecked") LinkedList<Long>[] hashtable = new LinkedList[10];

hashtable[0] = new LinkedList<Long>();
hashtable[1] = new MyLinkedList<Long>();
hashtable[2] = new LinkedList();
hashtable[3] = new MyLinkedList();

You can store the LinkedList if you cast your array to LinkedList[]. However you won't be able to store the anything else but a LinkedList:

LinkedList[] rawHashTable = hashtable;
rawHashTable[4] = new LinkedList<String>();

Object[] objectHashTable = rawHashTable;
objectHashTable[5] = "This line will throw an ArrayStoreException ";