java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to

Question

I am getting java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to ...model.MyClassAttributes exception at for(MyClassAttributes mc : atList) , even if I have done a cast to the entity.

List<MyClassAttributes> atList = new ArrayList<MyClassAttributes>();
atList=(List<MyClassAttributes>)session.createSQLQuery(SELECT_QR_SQL).setLong("rec", rec).list();
for(MyClassAttributes mc : atList)
{

}

When I checked the compiled class it seems cast didnt update.

atList = session.createSQLQuery("Select col from....").setLong("rec", rec.longValue()).list();

What is the issue here?

select col1,col2 from some_table where col3=:some_val - returns object list — Nidheesh
– Nidheesh, Commented Nov 17, 2015 at 17:53
Try casting it to a normal List without generics. It just returns a simple list. Click here — Yassin Hajaj
– Yassin Hajaj, Commented Nov 17, 2015 at 17:54
What's the point of assigning a new ArrayList if you immediately replace it by the result of the other call? — Clashsoft
– Clashsoft, Commented Nov 17, 2015 at 18:00
Yeah, that's redundant, but shouldn't be part of the actual problem. But of course, that initialization of the local variable should be removed for being pointless. — Wormbo
– Wormbo, Commented Nov 17, 2015 at 18:30

Wormbo · Accepted Answer · 2015-11-17 18:29:35Z

Java does not know any generics at runtime. This is called "type erasure". Since the list() method already returns a List<A>, typecasting it to some other List<B> (where B is a subclass of A) doesn't generate any additional bytecode.

Note how the ClassCastException occurs in the for loop, not the assignment. With the loop variable you (and also the compiler, since you casted before) assume that the List contains only null and MyClassAttributes objects, but apparently that isn't the case, since the exception is thrown.

Ravindra Mijar · Accepted Answer · 2015-11-17 17:57:51Z

-1

Can you try this? Don't initialize the List object. Just declare it eg -

List<MyClassAttributes> atList;

The reason the cast isn't working is - You have a List instance with an ArrayList Object in it. And the sqlQuery<>.list() function is returning a List.

Casts work only when the 'is-a' relationship is satisfied.

ArrayList is-a(always) List. So you can cast an ArrayList instance as a List. But the List may not be an ArrayList all the time. So the cast doesn't work.

answered Nov 17, 2015 at 17:57

Ravindra Mijar

1,3122 gold badges9 silver badges17 bronze badges

Actually, it doesn't matter what was assigned to the atList variable before assigning something else. Particularly the value of a variable doesn't affect any unrelated typecasting. The ClassCastException really tells that the list doesn't contain MyClassAttributes objects, at least not exclusively.

Wormbo
– Wormbo

11/17/2015 18:25:22
Commented Nov 17, 2015 at 18:25

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