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Create ArrayList from array in Java

Lewis D.

The Problem

You want to convert your standard Java array, such as the one below, to an ArrayList to take advantage of the dynamically allocated memory.

String[] array = {"new", "String", "array"};

The Solution

The easiest way to convert to an ArrayList is to use the built-in method in the Java Arrays library: Arrays.asList(array). This method will take in a standard array and wrap it in the AbstractList class, exposing all the methods available to objects that implement the List interface.

List<String> list = Arrays.asList(array);

It is important to note, however, that this has no effect on the actual data structure or data from the original code. This can catch some programmers out, as they may expect the new list object to behave like a true List. The example below will cause an UnsupportedOperationException:

List<String> list = Arrays.asList(array);
list.remove(1);

This is because the underlying data structure is still a static array, therefore the size cannot be changed. To create a true ArrayList data structure, we need to instantiate a new instance of the ArrayList object:

List<String> list = new ArrayList<>(Arrays.asList(array));

The code above will create a new ArrayList instance and use the data from our original array to populate its initial values. The memory for our list object will now be dynamically allocated, so method calls such as list.remove or list.add will work as expected.

In conclusion, we can create an ArrayList from a standard array quite easily using methods from core Java libraries as long as we are careful to instantiate the object correctly.

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