finding if an element at given index is empty or not , at arrayalist

Question

let's assume that we have an array list of size 3, of some object type (say class named Patient that has attributes of it's own).

so we know about the is Empty() method that checks if the entire given array list is empty or no, but is there a way to loop around the array list and check at which index there is object or not.

to be more clear , Array List<Patient> patient List = new Array List<>(3); patient List .add(patient1); patient List .add(patient2); sorry for the spaces so at index 2 there is no objects but at 0,1 there is — RoronoaNewJavaBoy
– RoronoaNewJavaBoy, Commented Dec 13, 2021 at 19:52
What part of of "loop around the array list and check at which index there is object or not" are you not sure can be done? — Scott Hunter
– Scott Hunter, Commented Dec 13, 2021 at 19:53
@ScottHunter whether i can detect the indices where the arrayList is empty or no — RoronoaNewJavaBoy
– RoronoaNewJavaBoy, Commented Dec 13, 2021 at 20:06

WJS · Accepted Answer · 2021-12-13 20:16:21Z

2

You can do it like this.

stream the indices of the list
filter those locations that are null
put them in an array

List<String> list = Arrays.asList("a",null,"b","c","d",null,"e");

int[] nullLocations = IntStream.range(0,list.size())
        .filter(i->list.get(i) == null)
        .toArray();

System.out.println(Arrays.toString(nullLocations));

prints

[1, 5]

Here is a non-streams version using the same list as above

List<Integer> nullLocations = new ArrayList<>();
for (int i = 0; i < list.size(); i++) {
     if (list.get(i) == null) {
         nullLocations.add(i);
     }
}
System.out.println(nullLocations);

prints

[1, 5]

edited Dec 13, 2021 at 20:16

answered Dec 13, 2021 at 19:56

WJS

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5 Comments

RoronoaNewJavaBoy Over a year ago

thank you so much for the effort!! but i guess this is some advanced solution (at least for me right now when i'm still in the mid of the course)

WJS Over a year ago

@RoronoaNewJavaBoy I added a non-streams version using a simple loop.

RoronoaNewJavaBoy Over a year ago

perfect, you made it sound so simple thank you.

WJS Over a year ago

@RoronoaNewJavaBoy You misunderstand the purpose of the argument to ArrayList. It is the capacity, not the number of elements. An array list is a wrapper for an array that grows dynamically. If there is no more room in the backing array, then it needs to be copied with a new length. When adding lots of values, this can be inefficient due to lots of copying. So you have an option to specify the initial capacity to reduce that activity. I believe the default is 16 but I am not certain. To find the number of elements, use the size() method.

RoronoaNewJavaBoy Over a year ago

i just went back to read my notes, yeah i mistaken the size and the capacity but now i'm wondering about how i can reach or get that capacity , since the arraylist will continue to get adds from the user, but i wanna put a limit to it based on it's capacity. i know in this case the use or arrays is more likely effictient than arraylist but you know how exams be like

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