Generics, arrays, and the ClassCastException

Question

I think there must be something subtle going on here that I don't know about. Consider the following:

public class Foo<T> {
  private T[] a = (T[]) new Object[5];

  public Foo() {
    // Add some elements to a
  }

  public T[] getA() {
    return a;
  }
}

Suppose that your main method contains the following:

Foo<Double> f = new Foo<Double>();
Double[] d = f.getA();

You will get a CastClassException with the message java.lang.Object cannot be cast to java.lang.Double.

Can anyone tell me why? My understanding of ClassCastException is that it is thrown when you try to cast an object to a type that cannot be casted. That is, to a subclass of which it is not an instance (to quote the documentation). e.g.:

Object o = new Double(3.);
Double d = (Double) o; // Working cast
String s = (String) o; // ClassCastException

And it seems I can do this. If a was just a T instead of an array T[], we can get a and cast it without a problem. Why do arrays break this?

Thanks.

Answer 1

Foo<Double> f = new Foo<Double>();

When you use this version of the generic class Foo, then for the member variable a, the compiler is essentially taking this line:

private T[] a = (T[]) new Object[5];

and replacing T with Double to get this:

private Double[] a = (Double[]) new Object[5];

You cannot cast from Object to Double, hence the ClassCastException.

Update and Clarification: Actually, after running some test code, the ClassCastException is more subtle than this. For example, this main method will work fine without any exception:

public static void main(String[] args) {
    Foo<Double> f = new Foo<Double>();
    System.out.println(f.getA());
}

The problem occurs when you attempt to assign f.getA() to a reference of type Double[]:

public static void main(String[] args) {
    Foo<Double> f = new Foo<Double>();
    Double[] a2 = f.getA(); // throws ClassCastException
    System.out.println(a2);
}

This is because the type-information about the member variable a is erased at runtime. Generics only provide type-safety at compile-time (I was somehow ignoring this in my initial post). So the problem is not

private T[] a = (T[]) new Object[5];

because at run-time this code is really

private Object[] a = new Object[5];

The problem occurs when the result of method getA(), which at runtime actually returns an Object[], is assigned to a reference of type Double[] - this statement throws the ClassCastException because Object cannot be cast to Double.

Update 2: to answer your final question "why do arrays break this?" The answer is because the language specification does not support generic array creation. See this forum post for more - in order to be backwards compatible, nothing is known about the type of T at runtime.