CODEWITHSUNDEEP
javaarraylist
M.khanousse
M.khanousse

Reputation: 33

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

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;
    }

}

Upvotes: 3

Views: 879

Answers (3)

bp4D
bp4D

Reputation: 961

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

Upvotes: 1

Przemysław Moskal
Przemysław Moskal

Reputation: 3609

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;
    } 
}

Upvotes: 2

Ousmane D.
Ousmane D.

Reputation: 56433

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);

Upvotes: 2

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