Generic Array Creation Error Message

[Kevin Simonson]

I've got a piece of code I'm including here that attempts to create an array of "Map< Side, String>" objects:

import java.util.Map;
import java.util.EnumMap;

public class Ma
{
  enum Side { LOWER, HIGHER };

  public static void main ( String[] arguments)
  {
    if (1 < arguments.length && arguments.length % 2 == 0)
    { Map< Side, String>[] values
                     = new EnumMap< Side, String>[ arguments.length >> 1];
      int pair;
      for (pair = 0; pair < values.length; pair++)
      { values[ pair].put( Side.LOWER , arguments[ pair << 1]);
        values[ pair].put( Side.HIGHER, arguments[ (pair << 1) + 1]);
      }
      for (pair = 0; pair < values.length; pair++)
      { System.out.println
          ( "values[ " + pair + "].get( Side.LOWER ) == \""
                       + values[ pair].get( Side.LOWER ) + "\".");
        System.out.println
          ( "values[ " + pair + "].get( Side.HIGHER) == \""
                       + values[ pair].get( Side.HIGHER) + "\".");
      }
    }
    else
    { System.out.println( "Usage is");
      System.out.println( "  java Ma (<low-string> <high-string>)+");
    }
  }
}

When I try to compile it I get the message:

C:\Users\kvnsmnsn\Java\Wade>javac Ma.java
Ma.java:12: generic array creation
= new EnumMap< Side, String>[ arguments.length >> 1];
^
1 error

C:\Users\kvnsmnsn\Java\Wade>

Why is it illegal to create an array of "EnumMap< Side, String>"s?

Kevin Simonson

[John de Michele]

Kevin:

Because generics in Java don't work that way. You can't assign an array of EnumMap<T, U> to an array of Map<T, U>.

John.

[abani patra]

Hi,
In a map you can store any object so why you need the array of map.

[Rob Spoor]

John de Michele wrote:Kevin:

Because generics in Java don't work that way. You can't assign an array of EnumMap<T, U> to an array of Map<T, U>.

Actually you can, once you are able to create one. Because Map is a super type of EnumMap, Map[] is a super type of EnumMap[].
Creating the array is the problem, because generics don't allow you to create arrays of generic types. The reason is because the compiler cannot guarantee type safety anymore. Consider, assuming line 1 would not create a compiler error:

List<Integer>[] intLists = new List<Integer>[3];
intLists[0] = new ArrayList<Integer>();
Object[] objects = intLists; // valid
List<String>[] stringLists = (List<String>[])objects; // generates a warning but compiles
stringLists[0].add("Hello World");
int i = intLists[0].get(0); // ClassCastException

With non-generic types line 3 would have already created a ClassCastException, and therefore it's easier to find the cause of the problem. In this case the ClassCastException comes at a point where you no longer know why it went wrong.

Anyway, with only one compiler warning you can create the values map:

Map<Side, String>[] values = (Map<Side, String>[])new EnumMap[ arguments.length >> 1];