ArrayList vs the List returned by Arrays.asList() [duplicate]

Answer 1

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.

Answer 2

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.