Generics vs. Arrays

Question

I have seen in many answers (eg, this ) that a way of creating an array using generics is the following:

T[] array = (T[]) new Object[SIZE];

I am trying to do something similar:

EntryRecipient<K,V>[] array = (EntryRecipient<K,V>[]) new Object[MAX_SIZE];

However, this does not work, casting the following error:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LHashTable$EntryRecipient;

My code is:

HashTableEntry (included as private subclass of HashTable)

class HashTableEntry<K,V> {
    private K key;
    private V value;

    public HashTableEntry(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public V value() {
        return value;
    }

    public K key() {
        return key;
    }
}

EntryRecipient (included as private subclass of HashTable)

private class EntryRecipient<K,V> {
    private List<HashTableEntry<K,V>> entries;

    public EntryRecipient() {
        entries = new ArrayList<HashTableEntry<K,V>>();
    }

    // ... other methods
}

HashTable

class HashTable<K,V> {
    private EntryRecipient<K,V>[] map;
    private final int MAX_SIZE = 199; // Prime, to avoid collisions
    private int size;

    public HashTable() {
        map = (EntryRecipient<K,V>[]) new Object[MAX_SIZE];
        size = 0;
    }
    // ... other methods
}

Could you give me some hints to solve this?

Answer 1

I cannot find any reason for using an Object[] for EntryRecipient<K,V>[] in this code. The reference of the array is not a generic reference, so use the array of that class type. Converting an Object array to EntryRecipient array directly is not possible in Java. ( EntryRecipient[] is not same as T[] ). The solution for your problem is modifying your HashTable constructor as shown below:

public HashTable() {
    // Create an array of EntryRecipient
    map = new EntryRecipient[MAX_SIZE];
    size = 0;
    // Initialize the array
    // Otherwise you will get NullPointerException
    for (int i = 0; i < MAX_SIZE; i++) {
        map[i] = new EntryRecipient<K, V>();
    }
}

Answer 2

Start by asking yourself: 'Do I really need an array? Could I just use ArrayList<EntryRecipient<K, V>> ? An array list is a very skinny wrapper on an array; your code has to be awfully performance-sensitive to see any different in speed. Whereas, all this generic type stuff just works.