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1. Overview

In this short tutorial, we'll take a look at the differences between Arrays.asList(array) and ArrayList(Arrays.asList(array)).

2. Arrays.asList

Let's start with the Arrays.asList method.

Using this method, we can convert from an array to a fixed-size List object. This List is just a wrapper that makes the array available as a list. No data is copied or created.

Also, we can't modify its length because adding or removing elements is not allowed.

However, we can modify single items inside the array. Note that all the modifications we make to the single items of the List will be reflected in our original array:

String[] stringArray = new String[] { "A", "B", "C", "D" };
List stringList = Arrays.asList(stringArray);

Now, let's see what happens if we modify the first element of stringList:

stringList.set(0, "E");
 
assertThat(stringList).containsExactly("E", "B", "C", "D");
assertThat(stringArray).containsExactly("E", "B", "C", "D");

As we can see, our original array was modified, too. Both the list and the array now contain exactly the same elements in the same order.

Let's now try to insert a new element to stringList:

stringList.add("F");
java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractList.add(AbstractList.java:153)
	at java.base/java.util.AbstractList.add(AbstractList.java:111)

As we can see, adding/removing elements to/from the List will throw java.lang.UnsupportedOperationException.

3. ArrayList(Arrays.asList(array))

Similar to the Arrays.asList method, we can use ArrayList<>(Arrays.asList(array)) when we need to create a List out of an array.

But, unlike our previous example, this is an independent copy of the array, which means that modifying the new list won't affect the original array. Additionally, we have all the capabilities of a regular ArrayList, like adding and removing elements:

String[] stringArray = new String[] { "A", "B", "C", "D" }; 
List stringList = new ArrayList<>(Arrays.asList(stringArray));

Now let's modify the first element of stringList:

stringList.set(0, "E");
 
assertThat(stringList).containsExactly("E", "B", "C", "D");

And now, let's see what happened with our original array:

assertThat(stringArray).containsExactly("A", "B", "C", "D");

As we can see, our original array remains untouched.

Before wrapping up, if we take a look at the JDK source code, we can see the Arrays.asList method returns a type of ArrayList that is different from java.util.ArrayList. The main difference is that the returned ArrayList only wraps an existing array — it doesn't implement the add and remove methods.

4. Conclusion

In this short article, we took a look at the differences between two ways of converting an array into an ArrayList. We saw how those two options behave and the difference between how they implement their internal arrays.

As always, the code samples can be found over on GitHub.

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