Why Can't You Directly Create Generic Arrays in Java?

How Generics and Arrays Interact

While you can create an array reference with a generic type (E[] elements), creating an array object with a generic type (elements = new E[10]) is prohibited. This is due to fundamental differences in how generics and arrays handle type enforcement.

Arrays and Covariance

Unlike generics, arrays are covariant. This means that an array of a supertype (e.g., Object[]) can be assigned to an array of a subtype (e.g., String[]). This is not the case with generics, which enforce invariance (e.g., List

Type Enforcement

Generics enforce stronger type checks at compile time, preventing runtime exceptions and preserving type safety. Arrays, however, carry runtime type information, leading to potential ArrayStoreExceptions if an element of an incompatible type is inserted.

Issues with Generic Array Creation

Creating an array with a generic component type (e.g., T[]) is unsafe because the type is not known at runtime. This can lead to type casting errors or ArrayStoreExceptions, as the created array may actually be an Object[] in disguise.

Why Typecasting Works

You may wonder why elements = (E[]) new Object[10] appears to work. This is due to unchecked type casting, which suppresses compile-time warnings but still carries the risk of runtime exceptions. In this case, the cast may succeed if the array elements are compatible with the generic type, but it may not always work.

Workarounds for E[]

To create an array of a generic type without relying on unchecked casting, use the Array#newInstance() method:

public <e> E[] getArray(Class<e> clazz, int size) {
    @SuppressWarnings("unchecked")
    E[] arr = (E[]) Array.newInstance(clazz, size);

    return arr;
}</e></e>

Wildcard Parameterized Types

Creating an array of wildcard parameterized types (e.g., List>[]) is permitted because wildcards are reifiable types that retain their parameterization at runtime. For example, List>[] is equivalent to List[].

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