Can I Create an Array of a Parameterized Type in Java?

Generic Arrays in Java: A Comprehensive Dive

Java arrays and generics may seem incompatible at first glance. While generics allow for type safety and flexibility, arrays in Java are inherently covariant, meaning they retain the exact type of their elements at runtime. This poses a challenge when trying to create arrays of generic types.

Consider the following code snippet:

<code class="java">public class Hash<T extends Comparable<String>> {
    private T[] hashTable;
    // ...
}</code>

This code attempts to create an array of generic objects that extend Comparable. However, Java does not allow casting an Object array to an array of a parameterized type, as seen in the error message that appears during compilation:

java.lang.Object; cannot be cast to [Ljava.lang.Comparable;

To work around this issue, one could use the Array.newInstance() method:

<code class="java">private Comparable[] hashtable;
// ...
hashtable = (Comparable[])Array.newInstance(Comparable.class, tableSize);</code>

However, this approach still does not create an array of the parameterized type. Arrays in Java are type-safe at runtime, so they cannot be cast to another type at will.

Generics in Java undergo type erasure, where type parameters are removed during compilation. This ensures that arrays retain their specific element types, preventing unintended casting. As a result, it is not possible to create an array of a specific parameterized type.

In this particular case, it is recommended to use ArrayList instead of arrays. ArrayLists offer dynamic sizing, generics support, and are more suitable for most scenarios.

For a detailed explanation of this issue, refer to the Java Generics FAQ: https://docs.oracle.com/javase/7/docs/technotes/guides/generics/faq.html#CanIcreateanarraywhosecomponenttypeisaconcreteparameterizedtype

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