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From: Mike Amling <mamling@chaff.us>
Newsgroups: comp.lang.java.programmer
Subject: Re: generics array type check
Date: Fri, 25 Jul 2014 12:15:54 -0500
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On 7/24/14 3:03 AM, Philipp Kraus wrote:
> Hello,
>
> I have got this signature of a method private static <T> long[] getSize(
> T p_data )
> Depend on the type T the return value is changes e.g. on a non-array type
>
> getSize( 5 ) => new long[1]{1}
> getSize( "test" ) => new long[1]{1}
>
> on an array it should return
>
> double[] p_array
> getSize( p_array ) => new long[1]{ p_data.length }
>
> and on 2-dimensional array it should be
> double[][] p_array
> getSize( p_array ) => new long[2]{ p_array.length, p_array[0].length }
>
> How can I check the generic data type in a correct way to detect the
> array type?

How about something like

static long[] getSize(Object subject) {
   if (subject==null) {
     return new long[0];  // Or whatever
   }
   int dimensions=0;
   long[] answer={1};
   Class klas=subject.getClass();
   while (klas.isArray()) {
     final long[] more=new long[dimensions+1];
     System.arraycopy(answer, 0, more, 0, dimensions);
     answer=more;
     klas=klas.getComponentType();
     if (klas.isPrimitive()) {
       if (klas==Byte.TYPE) {
         answer[dimensions]=((byte[])subject).length;
       } else if (klas==Short.TYPE) {
         answer[dimensions]=((short[])subject).length;
       ...
       } else if (klas==Double.TYPE) {
         answer[dimensions]=((double[])subject).length;
       }
       break;
     }
     final Object[] aray=(Object[])subject;
     answer[dimensions++]=aray.length;
     subject=aray[0];  // Assumed non-null
   }
   return answer;
}

and to cover the primitives,

static long[] getSize(final double whatever) {
   return new long[]{1};
}

--Mike Amling
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