Generic Arrays in Java: Exploring Covariance and Type Erasure

Introduction

Generic arrays, where the array elements share a common type parameter, present a unique challenge in Java due to the interplay between generics and arrays' covariant behavior.

Question

Can we create an array of generics where the generic type extends Comparable? Attempts to cast an Object array to a generic array that extends Comparable face issues, raising the following question: Is there a workaround?

Answer

Generics and Arrays: A Compatibility Puzzle

Generics and arrays have fundamentally different ways of handling types:

• Arrays: Covariant, meaning their element types retain their type information at runtime.
• Generics: Use type erasure, where type information is removed at runtime.

Due to this mismatch, creating an array of generic types that extend a specific class is not possible.

Array.newInstance(): A Workaround

One potential solution is to use the Array.newInstance() method:

private Comparable[] hashtable;

...

hashtable = (Comparable[])Array.newInstance(Comparable.class, tableSize);

This approach allows you to create an array of the generic type's supertype (Comparable in this case), but it's important to note that this array is not of the desired generic type.

Why Not Use Arrays with Generics?

While there are workarounds, using arrays with generics is generally discouraged due to:

• Type Safety Concerns: Casting an Object array to a generic array can lead to type safety issues since arrays are covariant and generics use type erasure.
• Portability and Maintainability: Different Java Virtual Machines (JVMs) may handle type erasure differently, making the behavior less portable and more difficult to maintain.

Alternative: ArrayList

A more suitable option is to use ArrayList, which provides an efficient and type-safe way to manage collections of generic objects. ArrayLists offer the flexibility of generics and avoid the potential pitfalls associated with arrays and generics.