java - casting generics

Question

I'm having hard time googling this. I found a lot of articles but I can't still solve my problem.

Here is my code:

List<MyMainClass> mySource = (List<MyMainClass>) session.getAttribute("myAttribute");

the session.getAttribute("myAttribute") may return List<MyObject1> or List<MyObject2>. Both MyObject1 and MyObject2 are subclasses of MyMainClass Now I have 2 functions. The first one accepts List<MyObject1> and the other one accepts List<MyObject2>. Now Im getting error in eclipse

The method myMethod1(List<MyObject1>) in the type MyAction is not applicable for the arguments (List<MyMainClass>)

Can you add the code where myMethod1 is defined, and how you are invoking it? It seems myMethod1 needs a MyObject1 list, but you are passing it a MyMainClass list, which will not be accepted. — Hari Menon
– Hari Menon, Commented Oct 30, 2012 at 3:23

Matt Ball · Accepted Answer · 2012-10-30 03:30:01Z

You can't safely store different generic types with the same erasure (List) in a session attribute. So: don't do it.

Instead either refactor your code so that it is compatible regardless of the list type of that session attribute. This may be hard, but long-term seems less smelly to me. IME, it's generally a smell of poor design when you need to store two things of potentially different type in a single variable.

You could alternately use two different session attributes so that you know which more-specific list type to cast to.

List<MyObject1> mySource1 = (List<MyObject1>) session.getAttribute("myAttribute1");
if (mySource1 == null) {
    List<MyObject2> mySource2 = (List<MyObject2>) session.getAttribute("myAttribute2");
    if (mySource2 == null) {
        // ???
    } else {
        // rock and roll
        myMethod2(mySource2);
    }
} else {
    // proceed
    myMethod1(mySource1);
}

If you take the latter approach, I'd recommend writing a wrapper object or method that manages those details for you.

devang · Accepted Answer · 2012-10-30 03:28:57Z

0

That is correct. Generics provides only compile time safeguarding. You are trying to pass a more generic type of list to a more specific type of function of type list.

Understand that an instance of List<MyMainClass> can hold instances of both MyObject1 and MyObject2. When you call function1(List<MyObject1>) this function expects all the elements the list to be of type MyObject1. Therefore you cannot pass a list of type List<MyMainClass>.

You need to have distinguished variables to store different types of lists.

answered Oct 30, 2012 at 3:28

devang

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Yogendra Singh · Accepted Answer · 2012-10-30 03:36:01Z

0

As advised, this is bit tricky. If you really want to do it, I think you can do this as below:

 List<MyMainClass> mySource = 
                           (List<MyMainClass>) session.getAttribute("myAttribute");
 if(mySource != null && !mySource.isEmpty()){
      //Get the first element and check the type
      MyMainClass firstElement = mySource.get(0);
      if(firstElement instanceof MyObject1){
          List<MyObject1> mySourceObj1 = (List<MyObject1>)mySource;
          myMethod1(mySourceObj1);
      }else{
          List<MyObject3> mySourceObj2 = (List<MyObject2>)mySource;
          myMethod1(mySourceObj2);
      }
  }

answered Oct 30, 2012 at 3:36

Yogendra Singh

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Priyank Doshi · Accepted Answer · 2012-10-30 03:40:08Z

0

I would suggest you change the way you declare mySource and your function signature if its possible .

Declare this way:

    List<? extends MyMainClass> mySource = (List<MyMainClass>) 

session.getAttribute("myAttribute");

And change your function signature from

returnType fun(List<Object1> object1List){
}

to

returnType fun(List<? extends MyMainClass> object1List){
}

answered Oct 30, 2012 at 3:40

Priyank Doshi

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