Index of an object in the ArrayList according to one of its properties value

Question

I have an ArrayList contains Book objects, how can I get the index of a specific object according to its property "ID" value ?

public static void main(String[] args) {
   ArrayList<Book> list = new ArrayList<>();
   list.add(new Book("foods", 1));
   list.add(new Book("dogs", 2));
   list.add(new Book("cats", 3));
   list.add(new Book("drinks", 4));
   list.add(new Book("sport", 5));

   int index =  
}

this Book class :

public class Book {
    String name;
    int id;

    public Book(String name, int Id) {
        this.name=name;
        this.id=Id;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

}

You wish to get the index of which item? Please clarify your question for us. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Mar 3, 2018 at 12:16
i want to get the index of an object according to its ID value — M.khanousse
– M.khanousse, Commented Mar 3, 2018 at 12:18
Use for loop to iterate, test each item to find the index. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Mar 3, 2018 at 12:20
Ideally put them into a map by Id, otherwise the stream API answer below will do. — Oleg Sklyar
– Oleg Sklyar, Commented Mar 3, 2018 at 12:21
just a note, you can keep 2 lists separately name to ids and ids to name, saves time in iterating as number of searches will be large(as far as i guess) — Polish
– Polish, Commented Mar 3, 2018 at 12:22

Ousmane D. · Accepted Answer · 2018-03-03 12:23:19Z

2

You can do so by using an IntStream to generate the indices then use a filter operation for your given criteria then retrieve the index using .findFirst()... as shown below:

int index = IntStream.range(0, list.size())
                     .filter(i -> list.get(i).id == searchId)
                     .findFirst()
                     .orElse(-1);

edited Mar 3, 2018 at 12:23

answered Mar 3, 2018 at 12:19

Ousmane D.

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Przemysław Moskal · Accepted Answer · 2018-03-03 15:39:23Z

2

For Java Versions Below 8 Solution

As an addition to other, Java 8 working solution, there is a solution for those working with Java versions earlier than 8:

int idThatYouWantToCheck = 12345; // here you can put any ID you're looking for
int indexInTheList = -1; // initialize with negative value, if after the for loop it becomes >=, matching ID was found

for (int i = 0; i < list.size(); i++) {
    if (list.get(i).getId == idThatYouWantToCheck) {
        indexInTheList = i;
        break;
    } 
}

edited Mar 3, 2018 at 15:39

answered Mar 3, 2018 at 15:33

Przemysław Moskal

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Comments

bdsg4D · Accepted Answer · 2018-03-03 16:05:51Z

1

This is precisely what you are looking for:

private static int findIndexById(List<Book> list, int id) {
    int index = -1;
    for(int i=0; i < list.size();i++){
        if(list.get(i).id == id){
            return i;
        }
    }
    return index;
}

And call it like this:

int index = findIndexById(list,4);

Even if you are using Java 8, for what you are doing streams are not suggested; for-loop is faster than streams. Reference

edited Mar 3, 2018 at 16:05

answered Mar 3, 2018 at 15:50

bdsg4D

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