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Asked: 2026-05-24T13:09:12+00:00

Is creating an array and creating an arraylist the same thing in Java? I

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Is creating an array and creating an arraylist the same thing in Java? I hear many different stories with regard to this so if you could point me into the right direction it would much appreciated 🙂

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  1. Editorial Team

    An array is a primitive structure. It is sequential memory and allocated by the JVM.

    An arraylist uses an array behind it, but you don’t have direct access to it. It can “grow” while a regular array cannot.

    Basically, an arraylist starts (by default) with an array of length 10 behind it, and a count of 0. So imagine you had

    ArrayList<Integer> myInts = new ArrayList<Integer>();

    behind this is an array like this 

    count = 0
array = [ null, null, null, null, null, null, null, null, null, null ]

    When you go myInts.add(5); it does this

    count = 1
array = [ 5, null, null, null, null, null, null, null, null, null ]

    If you added more you could get this

    count = 10
array = [ 5, 4, 3, 2, 1, 5, 8, 3, 0, -1 ]

    And if you added one more (which would push it beyond its capacity) it would make a new array to support it

    count = 11
array = [ 5, 4, 3, 2, 1, 5, 8, 3, 0, -1, 9, null, null, null, null, null, null, null, null, null ]

    You can change the size of the backing array if you want to. for example:

    myInts.trimToSize();

count = 11 // hasn't changed
array = [ 5, 4, 3, 2, 1, 5, 8, 3, 0, -1, 9]

myInts.ensureCapacity(15);

count = 11 // hasn't changed
array = [ 5, 4, 3, 2, 1, 5, 8, 3, 0, -1, 9, null, null, null, null,]
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