Why Does Creating an Array of Generic Lists in Java Cause a Compiler Error?

Arrays of Generic Lists

In Java, utilizing generics and arrays can pose challenges. While certain constructions may appear valid, the compiler may raise objections.

Consider the following example:

<code class="java">ArrayList<Key> a = new ArrayList<Key>(); // Compiles without issue</code>

However, when attempting to create an array of generic lists:

<code class="java">ArrayList<Key>[] a = new ArrayList<Key>[10]; // Compiler error</code>

This error arises due to type erasure. To resolve it, one can either cast the array as follows:

<code class="java">ArrayList<Key>[] a = (ArrayList<Key>[]) new ArrayList[10];</code>

Or employ a list of lists:

<code class="java">ArrayList<ArrayList<Key>> b = new ArrayList<ArrayList<Key>>(); // No error</code>

To understand the rationale behind these observations, it's crucial to comprehend Java's type erasure mechanism. During compilation, type information is erased for generic types, thereby treating them as their raw counterparts. This is because arrays can only work with raw types.

In the first example, ArrayList is erased to ArrayList, which is a valid raw type. However, in the second example, the compiler encounters a raw type ArrayList[], which is not allowed. The cast employed ensures that the resulting array's component type aligns with the defined type.

The list of lists approach eliminates these issues because ArrayList is not an array and hence does not suffer from the same restrictions.

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