Question

I am storing objects in ArrayList, where my pojo is as

public class POJOSortableContacts {
    private Long id;
    private String displayName;


    public POJOSortableContacts(Long id, String displayName) {

        super();
        this.id           = id;
        this.displayName  = displayName;
    }

    //Setter and Getters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getDisplayName() {
        return displayName;
    }

    public void setDisplayName(String displayName) {
        this.displayName = displayName;
    }

    //This will be used to sectioned header.
    public String getLabel() {
        return Character.toString(displayName.charAt(0)).toUpperCase();
    }



    //Sortable categories
    //Sort by Contact name
    public static Comparator<POJOSortableContacts> COMPARE_BY_NAME = new Comparator<POJOSortableContacts>() {
        public int compare(POJOSortableContacts one, POJOSortableContacts other) {
            return one.getDisplayName().compareToIgnoreCase(other.getDisplayName());
            //return s1.toLowerCase().compareTo(s2.toLowerCase()); //it returns lower_case word first and then upper_case
        }
    };

    //Sort by id
    public static Comparator<POJOSortableContacts> COMPARE_BY_ID = new Comparator<POJOSortableContacts>() {
        public int compare(POJOSortableContacts one, POJOSortableContacts other) {
            return one.id.compareTo(other.id);
        }
    };
}

and Arraylist structure is as

ArrayList<POJOSortableContacts> contactArrayList = new ArrayList<POJOSortableContacts>() 

, I want to search an object from contactArrayList by id (for example I want an object which id is 20), I want to use binarysearch for this. So how can it will be?

Answer 1

You can use

POJOSortableContacts contact = Collections.binarySearch(contactArrayList,
                                               new POJOSortableContacts(20, ""),
                                               COMPARE_BY_ID);

The new POJOSortableContacts is just a dummy object to act as the key.

Of course, this will only work if your list is sorted by ID to start with – you can’t use a binary search on an unsorted list (or on a list which is sorted in a different way).