How to get index of List Object

Question

here Is Code there any way to get Index of the object from the list by calling a method in the list.

for example something like this:

class A{
    String a="";
    String b="";
}

List<A> alist= new ArrayList();

for (A a : alist) {
    a.getIndexInList();
}

No, there isn't a way to call a method on the object and get the index. You can loop over the indexes instead, and that's probably the best way to do it. — Louis Wasserman, Sep 05 '19 at 05:12
Yes you can do something like implement `hashCode` and `equals` method in `A` class. And then use `List alist= new ArrayList(); for (A a : alist) { alist.indexOf(a);// This will give you the index of element }` — Sudhir Ojha, Sep 05 '19 at 05:19

score 3 · Answer 1 · answered Sep 05 '19 at 05:23

3

Why not use indexOf? If I recall correctly it is a built-in function of list.

 List<A> alist= new ArrayList<>();
 for (A a : alist) {
     int index = alist.indexOf(a);
 }

Only the list can give you the index. Unless the object in the array knows it's in an array it can't give you it's index.

answered Sep 05 '19 at 05:23

Richard Barker

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It's slower than using your own counter. We know the ordering, `indexOf` will perform lookup every time. – Andronicus Sep 05 '19 at 05:29
1

Also you did not mention `equals` and `hashCode`. There are reasons, when `hashCode` is left returning constant value - this won't work then. – Andronicus Sep 05 '19 at 05:31
1

@Richard Baker what if there are duplicate elements? indexOf() returns first occurrence. – raviraja Sep 05 '19 at 07:14
All of which are implementation details the OP didn't mention. Yes, it's slower than making your own loop, yes if hashcode and equals are incorrect the results will be as well, and yes duplicates will return the first - all of which is true for just about every other function on a list. – Richard Barker Sep 05 '19 at 12:53

Andronicus · Answer 2 · 2019-09-05T05:21:50.943

There is no builtin solution, you can use external counter:

List<A> alist= new ArrayList();
int counter = 0;
for (A a : alist) {
    // logic
    counter++;
}

You could also create a map with indices as keys, something like:

IntStream.range(0, alist.size()).mapToObj(Integer::valueOf)
    .collect(Collectors.toMap(
            Function.identity(),
            alist::get
    ));

but alist needs to be effectively final.

How to get index of List Object

2 Answers2