Array Reification II

Overview

This post is a continuation of the previous post on Array Reification. Today, I am going to discuss about some important principles on Array Reification.

Example

Let’s have a look at the following example code to convert a collection to an array.

public final class Helper {

  	public static void main(final String[] args) {
  		final List<String> strings = asList("a", "b", "c");
  		final String[] array = toArray(strings);
  		System.out.println(array);
  	}

  	public static <T> T[] toArray(final Collection<T> collection) {
  		final T[] arr = (T[]) new Object[collection.size()];
  		int i = 0;
  		for (final T val : collection) {
  			arr[i++] = val;
  		}
  		return arr;
  	}
}

If you run the aforementioned example in Java 1.8.0_71 (as it is the currently installed Java JDK in my local machine), you would encounter a ClassCastException at runtime.

Along with it, you would also encounter an Unchecked Cast warning as follows.

Type safety: Unchecked cast from Object[] to T[]

If you look closely to the exception that it encounters,

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String; at com.amitinside.practice.Helper.main(Helper.java:13)

it clearly states that Object array can not be casted to String array because the actual type of the array is an Object array and in runtime while casting it to String array, causes the exception. Even though the actual array provided in main method is a String array, while assigning the converted array to a String Array fails because the actual reified type is an Object array. The actual reified type should have been String array.

Principle of Truth in Advertising

The reified type of an array must be a subtype of its erasure

So, according to this principle, in the previous problem, T has been bound to String but the reified type of the array is still Object and hence the exception occurs.

Cast-Iron Guarantee

Casts inserted automatically by erasure will never fail unless your application code has Unchecked Cast Warning message

This clearly states that due to erasure, the casts will never fail in runtime unless your code has the Unchecked Cast Warning messages and in our example, we do have an Unchecked Cast Warning which causes the ClassCastException in runtime.

Unchecked Cast Warning

Just as a reference to what Unchecked Cast Warning is. It is a compiler warning message due to any cast from a non-qualified type to a generic type or vice-versa.