Object type (class) identification
I run into the class misidentifying problem. Scratching my head, I have no clue at all for the cause.
List<MyClass> rl = ...;
for(MyClass e : rl){// <-- java.lang.ClassCastException
logger.debug(e.getId());
}
After I get a list of MyClass objects, how come the ClassCastException is thrown?
If they are not the data type, how I can find out what type of data/object they are?
btw, the list of the objects is retrieved with Hibernate statement which I am quite certain it is correct.
Thanks very much your quick response, Dr. Lasz.
The Eclipse tells me that I can't change the retured type from List<MyClass> to List<Object>. And I get the same exception on
for(Object x : r1) { // <-- here
...
}
I didn't know the ClassCastException could be thrown in an up-casting case.
Message was edited by:
vwuvancouver
Woops! Maybe the first thing is to check to see if r1 is null, and if not, what class it is an instance of:
if (r1 == null)
System.out.println("r1 is null");
else
System.out.println("r1: " + r1.getClass());
It returns the java.util.ArrayList which is corrent. The issue is an array list of what object. The data is passed to a JSP and causes an exception due to an incorrect data type. From the error message
javax.servlet.jsp.JspException: An error occurred while evaluating custom action attribute "value" with value "${entry.consumer.name}": The "." operator was supplied with an index value of type "java.lang.String"
it seems to be a list of a list/collection. That is something totally weird.
After doing some study on the Generics, I try out the wildcard type.
List<?> rl = ...;
for(object e : r1) {
if (e == null)
System.out.println("null");
else
System.out.println(e.getClass());
}
And the output
class [Ljava.lang.Object;
class [Ljava.lang.Object;
class [Ljava.lang.Object;
class [Ljava.lang.Object;
Interesting?!
