What is the difference between an array and an ArrayList in Java?

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An array is a way of storing information, which can vary from simple integers to strings and characters. In everyday life, we would probably call this a list. A traditional array of integers in java is defined like so:

int[] myArray = new int[3];

Here we have declared an integer array of size 3. The size of an array in Java is immutable; this means that myArray can only ever hold 3 items. If I then wanted to add the number 3 in the first position, I would simply do:

myArray[0] = 3;

If we then deleted 3 from our array, the size of the array would stay the same. We are simply deleting the value at position 0, not the position itself. 

An ArrayList is Java's answer to "mutable" arrays. We don't need to define a length when we instantiate them, and we can add as many elements as we want: the size of the array grows or shrinks. We declare an ArrayList of strings like so:

ArrayList myArrayList = new ArrayList();

We then have several methods, such as add(), remove(), or size(), which allow us to manipulate the ArrayList. To get an element from the array at a certain position, we use its get() method:

myArrayList.get(index);

Overall, we would use simple arrays when we have a pre-defined length for our array, and we know that the number of items we want to add won't surpass this length. ArrayLists are ways of getting around that, and therefore allow more flexibility. One last difference to notice between these two data types: ArrayList cannot store primitive data types. ArrayLists can only store objects, such as strings. Since Java 5, autoboxing was introduced which allowed you to add primitve types, such as integers, to ArrayLists; but they are in fact converting them into objects. So think carefully about your problem before you choose which version of the array you would like to use!

Jonathan W. A Level Computing tutor, A Level French tutor, GCSE Maths...

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