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i am using reflection to copy values of fields from object of Class A to object of Class B. However method in A returns Number, where as setter in B requires Long. Is there any generic way i could set the value. As of now as expected i get illegalArgumentException: argument type mismatch

class a
{
    Number value1;
    Number value2;
    public Number getValue1(){return value1;}
    public Number getValue2(){return value2;}

}

class b
{
    Double value1;
    Long   value2;
    public void setValue1(Double value){this.value1 = value;}
    public void setValue2(Long value){this.value2 = value;}

}

Not sure if my question is unclear.

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  • 1
    It would be good if your code corresponded to your question. You don't have any getters or setters here. Commented Dec 9, 2010 at 17:16
  • BTW: I suggest you use primitives like double and long, unless you know you need an object. Commented Dec 9, 2010 at 19:57

3 Answers 3

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You could do

b.setValue2(a.getValue2().longValue());

But if a.value2 isn't actually an integer (e.g. it's a Double with a fractional component) this will lose data.

Correspondingly

b.setValue1(a.getValue1().doubleValue());

Edit

Ok I think I've got a grasp on your situation. Here's a dirty way to go about what you want to do. Basically you need to have a transform method which will transform a Number into another Number based on a chosen class. That class you get from the Method itself. So it will be something like this:

   public static void main(String[] args) throws Exception {
      A a = new A();
      a.setValue1(1.0);
      a.setValue2(5);

      B b = new B();

      Method[] methods = b.getClass().getMethods();
      for ( Method m : methods ) {
         if ( m.getName().equals("setValue2") ) {
            m.invoke(b, transform(a.getValue2(), m.getParameterTypes()[0]));
         }
      }
      System.out.println(b.getValue2());
   }

   private static Number transform(Number n, Class<?> toClass) {
      if ( toClass == Long.class ) {
         return n.longValue();
      } else if ( toClass == Double.class ) {
         return n.doubleValue();
      }
      //instead of this you should handle the other cases exhaustively
      return null;
   }

The reason you would otherwise get an IllegalArgumentException in the above is because with a, value2 is not being set to a Long, it's being set to an Integer. They are disjoint types. If a.value2 was actually set to be a Long instead, you wouldn't have that error.

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2 Comments

@icon666: How are you invoking that setter such that you don't know what setter you're invoking?
@icon666 so do you have multiple setters?
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You need to do the conversion:

// get the Number 'number'
Long l = new Long(number.longValue());
// store the Long

You could do it even more efficiently using autoboxing.

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11 Comments

Autoboxing or not, you should at least be using Long.valueOf(...).
And check its not a Long already which can be simply cast (Long).
@Peter Lawrey I think runtime type identification via instanceof and having an if-else would be more expensive that calling always the "virtual" method longValue.
Hmm, this is basically what I already said, which the OP already said doesn't work for him/her.
@fortran: I don't think it's calling the method that's expensive, it's potentially creating an unnecessary object. Depends on the range of values and the size of the Long cache.
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You can't do this in a 'generic' way, because a Number could be a Float, Byte etc.

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