"Cannot create generic array of .." - how to create an Array of Map<String, Object>?

Question

I would like to use simpleJdbcInsert class and executeBatch method

public int[] executeBatch(Map<String,Object>[] batch)

http://static.springsource.org/spring/docs/2.5.x/api/org/springframework/jdbc/core/simple/SimpleJdbcInsert.html

So I need to pass an array of Map<String,Object> as parameter. How to create such an array? What I tried is

Map<String, Object>[] myArray = new HashMap<String, Object>[10]

It is error: Cannot create generic array of Map<String, Object>

A List<Map<String, Object>> would be easier, but I guess I need an array. So how to create an array of Map<String, Object> ? Thanks

Solution 1

Because of how generics in Java work, you cannot directly create an array of a generic type (such as Map<String, Object>[]). Instead, you create an array of the raw type (Map[]) and cast it to Map<String, Object>[]. This will cause an unavoidable (but suppressible) compiler warning.

This should work for what you need:

Map<String, Object>[] myArray = (Map<String, Object>[]) new Map[10];

You may want to annotate the method this occurs in with @SuppressWarnings("unchecked"), to prevent the warning from being shown.

Solution 2

You can create generic array of map.

1. Create a list of maps.

   List<Map<String, ?>> myData = new ArrayList<Map<String, ?>>();
   

2. Initialize array.

   Map<String,?>[] myDataArray = new HashMap[myData.size()];
   

3. Populate data in array from list.

   myDataArray = myData.toArray(myDataArray);
   

Comments

[Admin]

I would likely start with a List and turn it into an array as needed, since I dislike dealing with Java arrays .. but this is still a valid question nevertheless.

[nneonneo]

@pst: Post that as an answer. I hate casting generic arrays (my last attempt at an answer was incorrect because I forgot how broken generics were).

[Diffy]

when i use LinkedHashMap<String,String>map_array[] = new LinkedHashMap<String,String>[2]; , it also gives this error? Now i dont understand what is 'generic' here?

[Jonathan Callen]

@Diffy The "generic" part is the type parameters <String,String>. To remove the error (and replace it with a warning), use LinkedHashMap<String, String>[] map_array = (LinkedHashMap<String, String>) new LinkedHashMap[2];

[Diffy]

Yes, i got out of the error by using simply new LinkedHashMap[2], but it would have been generic if i would have used an Object datatype or "T" (generic tmplate ). How come <String,String> is generic? I am specifying the proper datatype here

[Jonathan Callen]

"Generic" in this case means "Type that has type parameters, or is defined by a type parameter". So LinkedHashMap<String, String> becomes LinkedHashMap, T becomes Object (normally), etc.