All 15 Arrays & Multidimensional Arrays questions
More in Arrays & Multidimensional Arrays
What utility methods does java.util.Arrays provide for common array operations like sorting, searching, filling, and comparing? How do you convert between an array and a List (ArrayList) in both directions, and what pitfalls exist with Arrays.asList()? How does Java handle array bounds checking, and what exception is thrown when accessing an invalid index? How do you correctly iterate a 2D array using both traditional indexed for-loops and the enhanced for-each loop, and when is each approach preferable? How would you flatten a 2D array into a 1D array, and conversely, reshape a 1D array into a 2D array? How does Java implement covariant array typing, and why is this considered a design flaw that can lead to runtime ArrayStoreException? How would you efficiently copy, resize, or merge large arrays in Java, and what are the performance trade-offs between System.arraycopy(), Arrays.copyOf(), and manual loops? How would you implement an efficient in-place algorithm on a 2D array, such as rotating a matrix 90 degrees, while minimizing extra memory allocation? How would you implement a custom, resizable dynamic array (similar to ArrayList's internal mechanism) from scratch using a raw Java array, including the amortized doubling growth strategy? How would you implement a memory-efficient sparse matrix representation in Java when a large 2D array would mostly contain zero/default values, avoiding the memory waste of a full dense array?
Advertisement
Back to results for “Arrays”
Java › Arrays & Multidimensional Arrays

What is array covariance's relationship to generic arrays, and why can't you directly create a generic array like new T[10] or new List<String>[10]?

Intermediate

Answer

Due to Java's type erasure, generic type information isn't available at runtime, so the JVM couldn't correctly enforce the component type of a genuinely generic array at the point of array creation -- combined with array covariance's already-demonstrated unsoundness (ArrayStoreException), allowing new T[] or new List<String>[] directly would create even more severe, undetectable type-safety holes, so Java simply disallows this syntax entirely at compile time, requiring workarounds like creating an Object[] and unsafely casting, or preferring a generic-friendly collection like ArrayList<T> instead.
public class Container<T> {
    // private T[] items = new T[10];  // COMPILE ERROR: generic array creation

    @SuppressWarnings("unchecked")
    private T[] items = (T[]) new Object[10];  // common workaround, but unsafe and generates a warning

    // Preferred alternative: just use a List instead
    private List<T> betterItems = new ArrayList<>();
}
Real-world example A generic container class initially attempts private T[] items = new T[10] and hits a compile error, leading the team to either use the unsafe Object[]-cast workaround (accepting the unchecked warning) or, preferably, refactor to use ArrayList<T> internally instead, sidestepping the entire generic array limitation.

Common follow-ups: Why does the unsafe (T[]) cast workaround still risk a ClassCastException later, and under what circumstances?;How does the JDK's own source code (like ArrayList's internal implementation) handle this exact limitation internally?

Generics;Collections Framework
All 15 Arrays & Multidimensional Arrays questions