How to add an index of an ArrayList to the same Arraylist Java

Question

I want to add the an index of a 2 dimensional ArrayList to the same ArrayList. Code should be something like the following but index numbers and the same ArrayList must be involved:

ArrayList<String> a[0] = new ArrayList<>(Arrays.asList("A", "B", "C"));
ArrayList<String> a[1] = a[0];

Of course this code won't work. Thank you in advance...

a(0) = A, B, C --- a(1) = A, B, C must be done through index numbers or similar — Otag
– Otag, Commented Dec 27, 2015 at 9:44

Eran · Accepted Answer · 2015-12-27 09:36:56Z

3

a[i] is used to access and assign array elements.

Assuming a is declared as :

ArrayList<ArrayList<String>> a = new ArrayList<>();

You assign the inner lists with :

a.set(0,new ArrayList<String>(Arrays.asList("A", "B", "C"));
a.set(1,a.get(0));

Note that this code will work only if a's size is at least 2 (i.e. it already has values for indices 0 and 1). If a is empty, you should use :

a.add (new ArrayList<String>(Arrays.asList("A", "B", "C"));
a.add (a.get(0));

edited Dec 27, 2015 at 9:36

answered Dec 27, 2015 at 9:28

Eran

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21 Comments

SMA Over a year ago

ArrayList is backed by an array and when you instantiate with new ArrayList<>(), it creates an array of size 0 and when you do set with index it would give you IOOBE.

Eran Over a year ago

@SMA That's true. That's why I gave two options. The first option only works if the ArrayList already contains elements at the 0 and 1 indices.

Eran Over a year ago

@Otag a.get(1).get(2)

Otag Over a year ago

During my debugging efforts debugger gives a.add(a.get(donsay - 1)); the following exception => java.lang.RuntimeException: Unable to start activity ComponentInfo{chessactivesekizsatirvetekliarray.com.sekizsatirvetekliarray/chessactivesekizsatirvetekliarray.com.sekizsatirvetekliarray.MainActivity}: java.lang.IndexOutOfBoundsException: Invalid index 1, size is 1 <-------->donsay is the integer variable for looping purposes...Any ideas on what is going on??

Eran Over a year ago

@Otag this means that a contains just one element, whose index is 0, but you are trying to access an element at index 1.

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