How can I replace object in java collection?

Question

I am trying to replace element in collection with new modified version. Below is short code that aims to demonstrate what I'd like to achieve.

The whole idea is that I have one object that consists of collections of other objects. At some point in time I am expecting that this objects in collections (in my example phones) might require some modifications and I'd like to modify the code in one place only.

I know that in order to update the object's attributes I can use setters while iterating through the collection as demonstrated below. But maybe there is better, more general way to achieve that.

public class Customer {
    private int id;
    private Collection<Phone> phoneCollection;
    public Customer() {
        phoneCollection = new ArrayList<>();
    }
//getters and setters    

}

and Phone class

public class Phone {
    private int id;
    private String number;
    private String name;
//getters and setters    
}

and

public static void main(String[] args) {
        Customer c = new Customer();

        c.addPhone(new Phone(1, "12345", "aaa"));
        c.addPhone(new Phone(2, "34567", "bbb"));
        System.out.println(c);

        Phone p = new Phone(2, "9999999", "new name");

        Collection<Phone> col = c.getPhoneCollection();
        for (Phone phone : col) {
            if (phone.getId() == p.getId()) {
//             This is working fine
//                phone.setNumber(p.getNumber());
//                phone.setName(p.getName());

//              But I'd like to replace whole object if possible and this is not working, at least not that way
                  phone = p;
            }
        }
        System.out.println(c);
    }
}

Is this possible to achieve what I want? I tried copy constructor idea and other methods I found searching the net but none of them was working like I would expect.

EDIT 1

After reading some comments I got an idea

I added the following method to my Phone class

public static void replace(Phone org, Phone dst){
    org.setName(dst.getName());
    org.setNumber(dst.getNumber());
}

and now my foreach part looks like that

    for (Phone phone : col) {
        if (phone.getId() == p.getId()) {
            Phone.replace(phone, p);
        }
    }

And it does the job. Now if I change the Phone class attributes I only need to change that method. Do you think it is OK solving the issue that way?

Answer 1

You should not modify the collection while you're iterating through it; that's likely to earn you a ConcurrentModificationException. You can scan the collection for the first object that matches your search criterion. Then you can exit the loop, remove the old object, and add the new one.

Collection<Phone> col = c.getPhoneCollection();
Phone original = null;
for (Phone phone : col) {
    if (phone.getId() == p.getId()) {
        original = phone;
        break;
    }
}
if (original != null) {
    Phone replacement = new Phone(original);
    replacement.setNumber(p.getNumber());
    replacement.setName(p.getName());
    col.remove(original);
    col.add(replacement);
}

Alternatively, you could declare a more specific type of collection, such as a List, that would allow you to work with indexes, which would make the replacement step much more efficient.

If your phone IDs are unique to each phone, you should consider using a Map<Integer, Phone> that maps each phone ID to the corresponding phone. (Alternatively, you could use some sort of third-party sparse array structure that doesn't involve boxing each ID into an Integer.) Of course, if your IDs aren't unique, then you might want to modify the above to gather a secondary collection of all matching phones (and reconsider the logic of your existing code as well).

Answer 2

You can also use a Set (HashSet), this is only when you don't want to do the way Mike suggested.

Use the Phone as an item in the set. Don't forget to implement hashCode() and equals() in Phone. hashCode() should return the id, as it is supposed to be unique.

Since you are concerned about replacing the item, here's how HashSet will help you :

1. Create an instance of your object.
2. Remove the object you want to replace from the set.
3. Add the new object (you created in step 1) back to the set.

Both these operations 2 & 3 are guaranteed in O(1) / constant time.

You don't need to maintain a map for this problem, that's redundant.

If you want to get the object from the collection itself and then modify it, then HashMap would be better, search is guaranteed in O(1) time.

Answer 3

Instead of a list, use a map with the Phone's id as the key. Then your code looks like this:

public static void main(String[] args) {
        Customer c = new Customer();

        c.addPhone(new Phone(1, "12345", "aaa"));
        c.addPhone(new Phone(2, "34567", "bbb"));
        System.out.println(c);

        Phone p = new Phone(2, "9999999", "new name");

        Map<Integer, Phone> phoneMap = c.getPhoneMap();
        phoneMap.put(p.getId(), p);

        System.out.println(c);
}

Answer 4

If you take the object out from the collection and update its properties, it will get reflected in the same object in collection too.. Hence, you dont have to technically replace object after updating it. As "Mike M." pointed out, you can use hashmap to retrieve the object quickly without iteration and update the object values.

Answer 5

If order matters to you, you can change Collection to List (Since you're always using an ArrayList anyway) and then:

int index = col.indexOf(phone);
col.remove(phone);
col.add(p, index);