static int number(char c)
    {
        switch (c)
        {
            case 'a':
                return 1;
            case 'b':
                return 2;
            case 'c':
                return 3;
            case 'd':
                return 4;
            case 'e':
                return 5;
            case 'f':
                return 6;
            case 'g':
                return 7;
            case 'h':
                return 8;
            case 'w':
                return ID_ENUM_MAP;
            case 'z':
                return ID_MAP;
            default:
                return 0;
        }
    }
Used when the type is an interface (Map/SortedMap).

@author David Yu
@created Apr 24, 2012
protostuff-runtime/src/main/java/io/protostuff/runtime/PolymorphicMapSchema.java