Create an Array of Arraylists

Question

I am wanting to create an array of arraylist like below   ArrayList lt Individual gt    group   new ArrayList lt Individual gt    4    But it s not compiling  How can I do this

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ArrayList lt String gt  al     new ArrayList n 1   for int i   0 i lt n i        al i    new ArrayList lt String gt

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To declare an array of ArrayLists statically for  say  sprite positions as Points   ArrayList lt Point gt    positionList   new ArrayList 2    public Main ---        positionList 0    new ArrayList lt Point gt        Important  or you will get a NullPointerException at runtime     positionList 1    new ArrayList lt Point gt         dynamically   ArrayList lt Point gt    positionList  int numberOfLists   public Main ---        numberOfLists   2      positionList   new ArrayList numberOfLists       for int i   0  i  lt  numberOfLists  i              positionList i    new ArrayList lt Point gt               Despite the cautions and some complex suggestions here  I have found an array of ArrayLists to be an elegant solution to represent related ArrayLists of the same type

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I find this easier to use     static ArrayList lt Individual gt  group           void initializeGroup int size     group new ArrayList size    for int i 0 i lt size i         group i  new ArrayList lt Individual gt

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You can create Array of ArrayList  List lt Integer gt    outer   new List number   for  int i   0  i  lt  number  i          outer i    new ArrayList lt  gt         This will be helpful in scenarios like this  You know the size of the outer one  But the size of inner ones varies  Here you can create an array of fixed length which contains size-varying Array lists  Hope this will be helpful for you   In Java 8 and above you can do it in a much better way   List lt Integer gt    outer   new List number   Arrays setAll outer  element - gt  new ArrayList lt  gt        Even better using method reference  List lt Integer gt    outer   new List 10   Arrays setAll outer   ArrayList    new

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ArrayList lt String gt    lists    ArrayList lt String gt    new ArrayList 10

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This works  array of ArrayList  Give it a try to understand how it works   import java util     public class ArrayOfArrayList       public static void main String   args                Put the length of the array you need         ArrayList lt String gt    group   new ArrayList 15           for  int x   0  x  lt  group length  x                  group x    new ArrayList lt  gt                          Add some thing to first array         group 0  add  Some            group 0  add  Code               Add some thing to Secondarray         group 1  add  In here               Try to output  em         System out println group 0            System out println group 1              Credits to Kelvincer for some of codes

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you can create a List   and initialize them by for loop  it compiles without errors   List lt e gt    l  for int i   0  i  lt  l length  i         l i    new ArrayList lt e gt         it works with arrayList   l as well

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I totally do not get it  why everyone is suggesting the genric type over the array particularly for this question   What if my need is to index n different arraylists   With declaring List lt List lt Integer gt  gt  I need to create n ArrayList lt Integer gt  objects manually or put a for loop to create n lists or some other way  in any way it will always be my duty to create n lists   Isn t it great if we declare it through casting as List lt Integer gt       List lt Integer gt     new List lt   gt  somenumber   I see it as a good design where one do not have to create all the indexing object  arraylists  by himself  Can anyone enlighten me why this  arrayform  will be a bad design and what are its disadvantages

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Creation and initialization  Object   yourArray   new Object ARRAY LENGTH    Write access  yourArray i   someArrayList    to access elements of internal ArrayList     ArrayList lt YourType gt   yourArray i   add elementOfYourType     or other method  Read access  to read array element i as an ArrayList use type casting   someElement   ArrayList lt YourType gt   yourArray i     for array element i  to read ArrayList element at index j  arrayListElement    ArrayList lt YourType gt   yourArray i   get j

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You can t create array of generic type  Create List of ArrayLists     List lt ArrayList lt Individual gt  gt  group   new ArrayList lt ArrayList lt Individual gt  gt       or if you REALLY need array  WARNING  bad design      ArrayList   group   new ArrayList 4

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As the others have mentioned it s probably better to use another list to store the ArrayList in but if you have to use an array   ArrayList lt Individual gt    group    ArrayList lt Individual gt    new ArrayList 4

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You can do this      Create an Array of type ArrayList   ArrayList lt Integer gt    a   new ArrayList n        For each element in array make an ArrayList  for int i 0  i lt n  i          a i    new ArrayList lt Integer gt

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You can create like this  ArrayList lt Individual gt    group    ArrayList lt Individual gt    new ArrayList 4    You have to create array of non generic type and then cast it into generic one

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As per Oracle Documentation       You cannot create arrays of parameterized types    Instead  you could do   ArrayList lt ArrayList lt Individual gt  gt  group   new ArrayList lt ArrayList lt Individual gt  gt  4     As suggested by Tom Hawting - tackline  it is even better to do   List lt List lt Individual gt  gt  group   new ArrayList lt List lt Individual gt  gt  4

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The problem with this situation is by using a arraylist you get a time complexity of o n  for adding at a specific position  If you use an array you create a memory location by declaring your array therefore it is constant

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ArrayList lt Integer gt    graph   new ArrayList numCourses  It works

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You can create a class extending ArrayList  class IndividualList extends ArrayList lt Individual gt         and then create the array  IndividualList   group   new IndividualList 10

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This works   ArrayList lt String gt    group   new ArrayList 4

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List   listArr   new ArrayList 4    Above line gives warning   but it works  i e it creates Array of ArrayList

[java] Create an Array of Arraylists

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