Question

    ArrayList<classA> aList = /*I then fill this array*/
    ArrayList<classB> bList = /* I then fill this array too*/

    /*Now put them both in the following ArrayList of Objects*/
    ArrayList<Object> myObjs =  new ArrayList<Object>();
    myObjs.add(aList);
    myObjs.add(bList);

    /*The following two lines however fails at run time*/
    ArrayList<classA> x = (ArrayList<classA>) myObjs.get(0);
    ArrayList<classB> x = (ArrayList<classB>) myObjs.get(1);

I just read from another thread that “Downcasting is allowed when there is a possibility that it succeeds at run time “

can any one please tell me the problem of the above code ?
thank you very much!

Answer 1

From the Java Tutorials on Generics:

Consider the following method:

public void boxTest(Box<Number> n) { /* ... */ } 

What type of argument does it accept? By looking at its signature, you can see that it accepts a single argument whose type is Box<Number>. But what does that mean? Are you allowed to pass in Box<Integer> or Box<Double>, as you might expect? The answer is “no”, becaue Box<Integer> and Box<Double> are not subtypes of Box<Number>.

In other words: Even though classA is a subclass of Object, ArrayList<classA> is not a subtype of ArrayList<Object>.