Generics, arrays, and the ClassCastException

by 
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.

More Related Contents:

  1. Why are arrays covariant but generics are invariant?
  2. How to create a generic array in Java?
  3. Generic array creation error
  4. Cannot create an array of LinkedLists in Java…?
  5. How can I convert int[] to Integer[] in Java?
  6. Generic arrays in Java
  7. Error: Generic Array Creation [duplicate]
  8. How does one instantiate an array of maps in Java?
  9. How to create ArrayList (ArrayList) from array (int[]) in Java
  10. Java generics and array initialization
  11. What's the output of this code written in java?
  12. how to replace display array list equals another array list values
  13. java.lang.arrayoutofboundexception length:10 index:10
  14. Convert ArrayList to String[] array [duplicate]
  15. Static method in a generic class?
  16. How to convert an int array to String with toString method in Java [duplicate]
  17. How to find the index of an element in an array in Java?
  18. Variable length (Dynamic) Arrays in Java
  19. How to display a svg byte array as an image in a JasperReport?
  20. ArrayIndexOutOfBoundsException when iterating through all the elements of an array
  21. Why can this generic method with a bound return any type?
  22. Odd method call in java using a dot operator to access a generic list
  23. How to see if an object is an array without using reflection?
  24. Generate Random Numbers in Array [duplicate]
  25. How do generics of generics work?
  26. How to remove last comma and space in array? Java [duplicate]
  27. Return the index of clicked button?
  28. Java: bounded wildcards or bounded type parameter?
  29. Create an array of ArrayList elements
  30. Java nested generic type mismatch

Leave a Comment