Adding new element after n items in ArrayList

Question

I am looking for a Java (Android) method to add a new element after every n items. The example is as following: I have two different types of items implementing same base class, let's say A and B. What I would like to achieve is to add item from an array (of B elements) to another array (of A elements) after every, let's say, 10 items. I can achieve it by implementing loop logic, but looking for some built-in functionality. Thank for your help.

DETAILS:

ArrayList<A> aList = new ArrayList<A>(Arrays.asList(a, a, a, a, a, a, a));

ArrayList<B> bList = new ArrayList<B>(Arrays.asList(b, b, b);

What I would like to achive is add element from "bList" to "aList" after every 3rd element of "aList" without replacing items in "aList". So my desired ArrayList is like:

ArrayList<A> aList = new ArrayList<A>(Arrays.asList(a, a, a, b, a, a, a, b, a, b));

You need to provide more details. For example write pseudocode with expected functionality. — Coder-Man
– Coder-Man, Commented Jul 21, 2018 at 20:05
Is this school problem, or Yoy have another practical goal? Or this is XY problen — Jacek Cz
– Jacek Cz, Commented Jul 22, 2018 at 15:06

user8959091 · Accepted Answer · 2018-07-21 20:36:08Z

4

Tested with strings:

    ArrayList<String> aList = new ArrayList<String>(Arrays.asList("a", "a", "a", "a", "a", "a", "a"));

    ArrayList<String> bList = new ArrayList<String>(Arrays.asList("b", "b", "b"));

    int step = 3;

    for (int i = 0; i < bList.size(); i++) {
        int index = (step + 1) * (i + 1) - 1;

        if (index < aList.size()) {
            aList.add(index, bList.get(i));
        } else {
            aList.add(bList.get(i));
        }
    }

    for (String x : aList) {
        System.out.print(x);
    }

answered Jul 21, 2018 at 20:36

user8959091

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3 Comments

Farid Over a year ago

Appreciate your effort. Actually, I'm able to achieve it with loop, looking for something built-in like sort in Collections. "I can achieve it by implementing loop logic, but looking for some built-in functionality"

Coder-Man Over a year ago

@FARID why? You can't achieve that with sorting, how would you even write the comparator for that?

Farid Over a year ago

I'm not saying using sort. I just referenced sort as a built-in method so we don't have to manually sort any ArrayList. I just gave it as an example. So, I am just looking for some built-in method to achieve what I asked in the question. Didn't have high expectation of the existence of such method, but thought maybe there is something like that

SnakeException · Accepted Answer · 2018-07-21 21:12:30Z

0

I don't believe such a method exists. You could use a different approach, like:

Creating an inner class that extends the ArrayList class and overriding the add() method to change an inner global variable (so that you don't declare it final), Declare a method called "specialAdd" or something and make it return a Boolean It will have this line of code at the bottom: return super.add(o); and the method should also increment the global variable you defined earlier and when if it equals three you will add the desired element from bList to aList.

edited Jul 21, 2018 at 21:12

answered Jul 21, 2018 at 21:05

SnakeException

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Comments

SnakeException · Accepted Answer · 2018-07-22 12:14:39Z

-2

If I understood, you have a common class : BaseClass, and two class that extends Baseclass, ClassA and ClassB. To do such array, just create an array of type Baseclass : ArrayList<BaseClass> and write a static method like you said to loop (I do not think there is a built-in method, but not sure) and add ClassA then ClassB, or if you have an array of A type, you can add it using .addAll() method.

I cannot seem to find a way without looping.

edited Jul 22, 2018 at 12:14

SnakeException

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answered Jul 21, 2018 at 20:27

Sami Tahri

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1 Comment

Farid Over a year ago

Actually your answer is kinda re-iteration of my question :))

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