Java Reflection: Casting an Object to another without knowing their types

Question

I am using reflections to map an ResultSet to a Field in bean.

field = clazz.getDeclaredField(str);
field.setAccessible(true);
Object resultSetObject = rs.getObject(str);
Class fieldType = field.getType();
field.set(clazzInst, fieldType.cast(resultSetObject));

The problem is resultSetObject is of type Integer and fieldType is of Long and I cannot cast Intger to Long and getting ClassCastException.

This question might help you stackoverflow.com/questions/6690745/converting-integer-to-long — Gallo2fire
– Gallo2fire, Commented Jun 30, 2014 at 5:03
In that question you explicitly know that you have to cast Integer to Long, but here i don't know about their types — Dheerendra
– Dheerendra, Commented Jun 30, 2014 at 5:04
Why is the ResultSet-derived object of type Integer while the target field is of type Long? (NB I suspect it's actually the other way around.) The real solution here is to make sure your Java and SQL data models match. — user207421
– user207421, Commented Jun 30, 2014 at 5:14

Jim Garrison · Accepted Answer · 2014-06-30 05:09:25Z

0

You are essentially asking to duplicate the type knowledge that is available at compile-time with which the compiler generates the correct conversion. The runtime doesn't have this knowledge, so you will have to provide it by building a matrix of types and coding all the conversions you want, explicitly.

You can define this as a matrix indexed by type along both axes (from-type and to-type) or more likely as a Map whose key is a ConversionType object each instance of which defines fromType, toType and a convert() method.

answered Jun 30, 2014 at 5:09

Jim Garrison

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Sotirios Delimanolis · Accepted Answer · 2014-06-30 05:10:08Z

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Whether you have

Object ref = new String("object of type string");

or

String ref = new String("still object of type string");

the object referenced will be of type String. Doing

Object obj = (Object) new String("still a string");

does not change that the referenced object is a String. In your case, you'll probably need a conversion strategy to convert between types.

answered Jun 30, 2014 at 5:10

Sotirios Delimanolis

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2 Comments

Kick Buttowski Over a year ago

that is interesting, yet could you please share your source so I can learn more? thank you

Sotirios Delimanolis Over a year ago

@kick I don't have any right now. You have to differentiate between instances, references and variables.

Frank ZHENG · Accepted Answer · 2014-06-30 05:11:30Z

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You still have another option, convert long to string ,and then convert to integer.

answered Jun 30, 2014 at 5:11

Frank ZHENG

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5 Comments

Dheerendra Over a year ago

But since I don't know their types explicitly, I must be prepared for the cases where fieldType and resultSetObject can be of different Types like Date, int, Double Boolean or any other types

Frank ZHENG Over a year ago

You don't know the exact data type in your database? You can read it from database MetaData.

Dheerendra Over a year ago

I can do that but then I have to define a conversion strategy from every possible type to every other types

Frank ZHENG Over a year ago

NO ,YOU ONLY NEED TO DEFINE THE CONVERSION RULE BETWEEN YOUR TYPE WITH STRING.

Dheerendra Over a year ago

but why? don't I have to convert string to myType again?

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