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

Question

The method Arrays.asList(<T>...A) returns a List representation of A. The returned object here is a List backed by an array, but is not an ArrayList object.

I'm looking for the differences between the object Arrays.asList() returns and an ArrayList object-- a quick source to tell these without diving into the code.

TIA.

Answer 1

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.