ArrayList vs the List returned by Arrays.asList()

When you call Arrays.asList it does not return a java.util.ArrayList. It returns a java.util.Arrays$ArrayList which is a fixed size list backed by the original source array. In other words, it is a view for the array exposed with Java's collection-based APIs.

    String[] sourceArr = {"A", "B", "C"};
    List<String> list = Arrays.asList(sourceArr);
    System.out.println(list); // [A, B, C]

    sourceArr[2] = ""; // changing source array changes the exposed view List
    System.out.println(list); //[A, B, ]

    list.set(0, "Z"); // Setting an element within the size of the source array
    System.out.println(Arrays.toString(sourceArr)); //[Z, B, ]

    list.set(3, "Z"); // java.lang.ArrayIndexOutOfBoundsException
    System.out.println(Arrays.toString(sourceArr));

    list.add("X"); //java.lang.UnsupportedOperationException

    list.remove("Z"); //java.lang.UnsupportedOperationException

You cannot add elements to it and you cannot remove elements from it. If you try to add or remove elements from them you will get UnsupportedOperationException.

I'll expand my comment a little bit.

One problem that can occur if you use asList as it wasn't different from ArrayList object:

List<Object> list = Array.asList(array) ;
list.remove(0); // UnsupportedOperationException :(

Here you cannot remove the 0 element because asList returns a fixed-size list backed by the specified array. So you should do something like:

List<Object> newList = new ArrayList<>(Arrays.asList(array));

in order to make the newList modifiable.