How does one instantiate an array of maps in Java?

Question

I can declare an array of maps using generics to specify the map type:

private Map[] myMaps;

However, I can\'t figur

Answer 1

You can't safely create a generic array. Effective Java 2nd Edition goes into the details in the chapter on Generics. Start at the last paragraph of page 119:

Why is it illegal to create a generic array? Because it isn’t typesafe. If it were legal, casts generated by the compiler in an otherwise correct program could fail at runtime with a ClassCastException. This would violate the fundamental guarantee provided by the generic type system.

To make this more concrete, consider the following code fragment:

// Why generic array creation is illegal - won't compile!
List<String>[] stringLists = new List<String>[1]; // (1)
List<Integer> intList = Arrays.asList(42); // (2)
Object[] objects = stringLists; // (3)
objects[0] = intList; // (4)
String s = stringLists[0].get(0); // (5)

Let’s pretend that line 1, which creates a generic array, is legal. Line 2 creates and initializes a List<Integer> containing a single element. Line 3 stores the List<String> array into an Object array variable, which is legal because arrays are covariant. Line 4 stores the List<Integer> into the sole element of the Object array, which succeeds because generics are implemented by erasure: the runtime type of a List<Integer> instance is simply List, and the runtime type of a List<String>[] instance is List[], so this assignment doesn’t generate an ArrayStoreException. Now we’re in trouble. We’ve stored a List<Integer> instance into an array that is declared to hold only List<String> instances. In line 5, we retrieve the sole element from the sole list in this array. The compiler automatically casts the retrieved element to String, but it’s an Integer, so we get a ClassCastException at runtime. In order to prevent this from happening, line 1 (which creates a generic array) generates a compile-time error.

Because arrays and generics don't combine well (as well as other reasons), it's generally better to use Collection objects (in particular List objects) rather than arrays.

Answer 2

In general it is not a good idea to mix generics and arrays in Java, better use an ArrayList.

If you must use an array, the best way to handle this is to put the array creation (your example 2 or 3) in a separate method and annotate it with @SuppressWarnings("unchecked").

Answer 3

Short answer appears to be that you really just can't.

See the following for a blog about it. http://www.bloggingaboutjava.org/2006/01/java-generics-quirks/

One of the comments to the blog states that:

Actually, the engineers made the creation of such an Array illegal. So the creation of an array from generic Class fails. The Collection.toArray method followed by a Cast to the Array works at compile time.

This solves not the problem, that the ArrayStoreCheck can’t be done during Runtime, but you can create an Array of generics in this way.

As suggested by Bill the Lizard, you probably are better off using a

List<Map<String,Integer>>

Answer 4

I had a similar question, best response I got referred to this

Answer 5

myMaps = new HashMap<String, Integer>[10]();

So that's Wrong

Why not make a List of Maps instead of trying to make an array?

List<Map<String, Integer>> mymaps = new ArrayList<Map<String, Integer>>(count);

Answer 6

I know its a bit late to reply but I found this workaround helpful for my case...Hope it helps!

Use an array of HashMap to store HashMaps..

public static void main(String[] args) {
    HashMap[] arr = new HashMap[1];//creating an array of size one..just for sake of example
    HashMap<String, String> arrMap = new HashMap<String, String>();

    //use loops to store desired key-value pairs into the HashMap which will be stored inside the array
    arrMap.put("ABC", "Name");

    //use loop to store your desired hashMap into the array at a particular index
    arr[0] = arrMap;

    //desired manipulation of the stored array.
    System.out.println(arr[0]);
}