CODEWITHSUNDEEP
arraysarraylist
valenth
valenth

Reputation: 19

Arrays inside a Arraylist

i need to build up an ArrayList with Integer in new Arrays in it. How can I setup this and add items?

private final ArrayList<Integer>[] test;

test = new ArrayList<Integer>();

This won't work.
Thank you!

Upvotes: 0

Views: 1438

Answers (4)

slim
slim

Reputation: 41223

An array is an Object, and you can make lists of any object type - so you can make a List of arrays without doing anything special:

// Declare it and initialise as an empty list
List<Integer[]> list = new ArrayList<>();

// Add an item -- it's a new array
list.add(new Integer[5]);

// Access an item -- it's an array
Integer[] array = list.get(0);
Integer x = array[0];

I see few reasons to use arrays of Integer for most situations, however. An array of int is also an Object so you can use that just as well.

List<int[]> list = new ArrayList<>();
list.add(new int[5]);

One of the most common reasons for using the object Integer rather than the primitive int is that you can't have Lists of primitives. But you can have arrays of primitives, so if you're happy with the lesser expressive power of the array, by all means populate it with primitives.

Ask yourself why you want a List of arrays rather than a List of Lists, an array of arrays or an array of Lists, however.

Your question asks for "Arrays inside an ArrayList".

List<Integer[]> is a list of arrays (because, inside the <>, is Integer[]).

List<Integer>[] is an array of lists -- "ArrayLists inside an array" -- because the [] comes after the List<> declaration.

You can initialise your array of lists just like any other array, with new Type[length]:

 // Declare it
 List<Integer>[] array = new List<Integer>[5]; 

 // Put something into an array element
 array[3] = new ArrayList<Integer>();

 // Access an element -- it's a List
 List<Integer> list = array[3];
 Integer x = list.get(0);

Upvotes: 2

Hamid Waezi
Hamid Waezi

Reputation: 897

u can use like this for do this

private ArrayList<Integer>[] test ;

private void test(int n){
    test=new ArrayList[n];
    for(int i=0;i<n;i++){
         test[i]=new ArrayList<Integer>();

         test[i].add(100);
         test[i].add(10);
         test[i].add(10000);
         test[i].add(1);
         ....
    }
}

Upvotes: 0

IRONANDRE
IRONANDRE

Reputation: 26

Looks like you want to create a 2-dimansional integer array.

I assume you have a special use case in which you need a primitive array ob ArrayList-Objects.

Anyways you can create such a thing like any other primitive array like

ArrayList<Integer>[] test = new ArrayList[2];

and access/set it by index..

test[0] = new ArrayList<Integer>();

You also can fill the array on init...

ArrayList<Integer>[] test = new ArrayList[] {
        new ArrayList<Integer>(),
        new ArrayList<Integer>(),
        new ArrayList<Integer>()
};

Upvotes: 0

ayush
ayush

Reputation: 14568

If we want Arrays inside List then -

class Test {

  private final List<int[]> myList;

  public Test() {
      myList = new ArrayList<>();
  }

  public void add(int[] myArray) {
     myList.add(myArray);
  }  
}

You are always allowed to initialize a final variable. The compiler makes sure that you can do it only once.

Upvotes: 0

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