Array backed list

In this tutorial we will examine what is array backed list and how it behave.

Creation

Array backed lists are lists, which were constructed directly from arrays:

String[] array = {"cat", "dog", "canary"}
List<String> list = Arrays.asList(array);

List<String> list = Arrays.asList("cat", "dog", "canary");

Modification
Any modification through list reference will be visible via the array instance also – the same happens in the opposite direction.

Let’s check this by code:

String[] array = {"mouse", "cat", "canary"};
List list = Arrays.asList(array);
System.out.println("array: " + Arrays.toString(array));
System.out.println("list: " + list);

list.set(0, "cat");
System.out.println("nafter replacing first element in the list:");
System.out.println("array: " + Arrays.toString(array));
System.out.println("list: " + list);

array[1] = "dog";
System.out.println("nafter replacing second element in the array:");
System.out.println("array: " + Arrays.toString(array));
System.out.println("list: " + list);

the output should be:

array: [mouse, cat, canary]
list: [mouse, cat, canary]

after replacing first element in the list:
array: [cat, cat, canary]
list: [cat, cat, canary]

after replacing second element in the array:
array: [cat, dog, canary]
list: [cat, dog, canary]

We see that modification on specific element in any of the reference reflects changes in both of them.

Limitations

Lists backed by an arrays are fixed-length – we cannot add or remove elements. Any attempt of changing size:

list.add("cat");

list.remove(0);

will result with the following exception

java.lang.UnsupportedOperationException
 at java.util.AbstractList.add(AbstractList.java:148)
 at java.util.AbstractList.add(AbstractList.java:108)
 at com.pwc.ocp.ListTester.main(ListTester.java:27)

When we check the class returned by the Arrays.asList(), we will get:

class java.util.Arrays$ArrayList

At first glance we see that it is ArrayList, but be careful – after a thorough look we see, that this is a nested class inside the java.util.Arrays class. This nested class provides its own implementation of List, that propagate changes to the array and doesn’t allow any calls of add*() or remove*() methods.