Why Can\'t I Declare an Array of Generic Lists in Java?

Incomplete Erasure in Generic Arrays

Generic arrays pose a unique challenge in Java due to the concept of type erasure. This article explores the differences between two seemingly similar code fragments and sheds light on the underlying reasons for the compiler's behavior.

Syntax and Semantics

Consider the following code:

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

This code compiles without issue, as it creates an instance of a generic ArrayList. However, the following code results in a compilation error:

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

Unveiling Type Erasure

To understand the discrepancy, we must delve into the concept of type erasure. During compilation, the generic type information is erased, which means that at runtime, only the raw type (in this case, ArrayList) is available.

Raw Arrays vs. Type-Specific Lists

An array requires a raw type, while the first code fragment creates a reference to a generic list (ArrayList) without any additional type information. Therefore, the compiler has no problem resolving the type.

However, the second code fragment attempts to create an array of references to generic lists. This breaks the type safety rules because arrays are not parametric types and cannot accept type variables or parameterized types.

A Typecast Solution

To bypass this limitation, we can typecast the array as follows:

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

By explicitly specifying the generic type information in the cast, we inform the compiler about the intended type of the array elements. This satisfies the compiler's type safety requirements.

List of Lists (LL) Approach

Alternately, we can use a list of lists:

<code class="java">ArrayList<ArrayList<Key>> b = new ArrayList<ArrayList<Key>>();</code>

This is legal because ArrayList is not an array. Each element in the outer ArrayList is an ArrayList, which is a fully instantiated type.

Type Safety and Rationale

The compiler's restrictions on generic arrays are in place to enforce type safety. Without these checks, it would be easy to introduce subtle runtime errors by assigning lists of different types to an array of generic lists.

By understanding the reason behind the compiler's behavior, we can make informed decisions about when and how to use generic arrays and lists of lists, ensuring both code correctness and maintainability.

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