All 15 Arrays & Multidimensional Arrays questions
More in Arrays & Multidimensional Arrays
How do you declare, initialize, and access elements of a one-dimensional array in Java? How do you declare and initialize an array of objects (reference types) versus an array of primitives, and what is the key difference in their default values? How do multidimensional arrays work in Java, and how do they differ from truly rectangular arrays in languages like C? What utility methods does java.util.Arrays provide for common array operations like sorting, searching, filling, and comparing? 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? 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]? 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?
Advertisement
Java › Arrays & Multidimensional Arrays

How do you convert between an array and a List (ArrayList) in both directions, and what pitfalls exist with Arrays.asList()?

Intermediate

Answer

Arrays.asList(array) returns a fixed-size List backed directly by the original array (not a true resizable ArrayList -- calling add()/remove() throws UnsupportedOperationException, and mutating the array is reflected in the list view and vice versa); to get a genuinely independent, resizable list, wrap it in new ArrayList<>(Arrays.asList(array)); converting back from a List to an array uses list.toArray(new String[0]) (or the newer list.toArray(String[]::new)).
Integer[] arr = {1, 2, 3};
List<Integer> fixedView = Arrays.asList(arr);  // backed by arr, fixed-size
// fixedView.add(4);  // throws UnsupportedOperationException!

List<Integer> resizable = new ArrayList<>(Arrays.asList(arr));  // truly independent, resizable copy
resizable.add(4);  // works fine

Integer[] backToArray = resizable.toArray(new Integer[0]);
Real-world example A developer's attempt to call .add() on a list obtained directly from Arrays.asList() throws an unexpected UnsupportedOperationException in production, a common pitfall traced to Arrays.asList() returning a fixed-size view rather than a genuine resizable ArrayList.

Common follow-ups: Why does Arrays.asList() exist as a fixed-size view instead of just returning a full ArrayList directly?;What's the difference in behavior between toArray(new T[0]) and the newer toArray(T[]::new) method reference syntax?

Collections Framework;Generics
All 15 Arrays & Multidimensional Arrays questions