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[] stringLists = new List[1]; // (1)
List 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 containing a single element. Line 3 stores the List array into an Object array variable, which is legal because arrays are covariant. Line 4 stores the List into the sole element of the Object array, which succeeds because generics are implemented by erasure: the runtime type of a List instance is simply List, and the runtime type of a List[] instance is List[], so this assignment doesn’t generate an ArrayStoreException. Now we’re in trouble. We’ve stored a List instance into an array that is declared to hold only List 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.