Sort an ArrayList

[überfuzz]

Hi!

Is there any function/method in the arraylist class able to sort an arraylist like this?

arraylist

[0] = object 1
[1] = object 2
[2] = null
[3] = object 4
[4] = null
[5] = object 6
[6] = object 7

Sorted arraylist

[0] = object 1
[1] = object 2
[2] = object 4
[3] = object 6
[4] = object 7

Edit:
I tried this, but the sort part don't work.

          arrayList.trimToSize();
          arrayList.sort();

[Petr]

you can create yourself comparator implementation Comparator<> interface and it use in your code as follows Collections.sort(List, Comparator);

[überfuzz]

Thinking about it, it not really a sort stunt I'm trying to pull. I just want to reindex the array. Erasing all index corresponding to a value of null, or in my case no object.

[Petr]

So and do you have some problems with that?
Can you show your code what are you doing.

[jomypgeorge]

you can use COllections.sort(ArrayList_obj)

[Petr]

The any case him need implementations or yourself comparator or implementation Compare interface for object that use method Collection.sort(List)

[JosAH]

you can use COllections.sort(ArrayList_obj)

You can't use it because the List can contain null elements. The OP has to implement a simple Comparator<T> that anticipates on the fact that one of, or both of, its arguments can be null in the compare( ... ) method.

kind regards,

Jos

[überfuzz]

Since I didn't understand your posts I came up with an other way of solving this. For now I'll just set status == false on my objects, then I use a if statement to check status.

Thanks

[Petr]

see

package javatalk;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class SortEx {
    private static class User {
        private String name;
        private String soname;

        private User(String name, String soname) {
            this.name = name;
            this.soname = soname;
        }

        public String getName() {
            return name;
        }

        public String getSoname() {
            return soname;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            User user = (User) o;

            if (name != null ? !name.equals(user.name) : user.name != null) return false;
            if (soname != null ? !soname.equals(user.soname) : user.soname != null) return false;

            return true;
        }

        @Override
        public int hashCode() {
            int result = name != null ? name.hashCode() : 0;
            result = 31 * result + (soname != null ? soname.hashCode() : 0);
            return result;
        }

        @Override
        public String toString() {
            return "User: name - " + name + "; soname - " + soname;
        }
    }

    private static class UserCompare implements Comparator<User> {
        @Override
        public int compare(User o1, User o2) {
            if (o1 == null && o2 == null) {
                return 0;
            } else if (o1 == null) {
                return -1;
            } else if (o2 == null) {
                return 1;
            } else {
                return o1.getName().compareTo(o2.getName());
            }
        }
    }

    public static void main(String[] arg) {
        List<User> users = new ArrayList<User>();
        users.add(new User("Petr", "Pert"));
        users.add(null);
        users.add(new User("User_3", "User_3"));
        users.add(new User("User_2", "User_2"));
        users.add(null);
        users.add(new User("User_1", "User_1"));
        users.add(new User("Auser", "Auser"));
        printList(users);
        Collections.sort(users, new UserCompare());
        printList(users);
    }

    private static void printList(List<User> list) {
        for (User user : list) {
            System.out.println(user);
        }
        System.out.println();
    }
}

[JosAH]

@Petr, spoonfeeding code like you did is not going to help the OP much; give a man a fish, or teach him to fish etc. Also, spoonfeeding is considered not done in this forum (and other forums).

kind regards,

Jos

[j2me64]

Is there any function/method in the arraylist class able to sort an arraylist like this?

not methods or functions but interfaces called Comparator and Comparable. do you know why: imagine your object has multiple members. how should the sort method know which member(s) to use for sorting? right: by implementing the interface Comparator or Comparable you specify how your objects are to be sorted. for and example see the post of petr.

[j2me64]

@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;

corresponding to the book "Effective Java" of J. Bloch the last if can be written in a more effective way with

if (!(o instanceof User)) return false;

since instanceof checks for null and instanceof.

[JosAH]

not methods or functions but interfaces called Comparator and Comparable. do you know why: imagine your object has multiple members. how should the sort method know which member(s) to use for sorting? right: by implementing the interface Comparator or Comparable you specify how your objects are to be sorted. for and example see the post of petr.

In this case (List elements can be null) it is no use to make the elements implement the Comparable<T> interface, because a null element may be chosen as a target by the sort method to call the Comparable.compareTo( ... ) method on. A Comparator<T> has to do the job.

kind regards,

Jos

[j2me64]

@Petr, spoonfeeding code like you did is not going to help the OP much; give a man a fish, or teach him to fish etc. Also, spoonfeeding is considered not done in this forum (and other forums).

kind regards,

Jos

furthermore, even if you are a experienced java programmer "the things you think you know might work just a little different than you imagined. It isn't enough to be able to get your code to work correctly; you must understand the core fundamentals in a deep way " (OCP Java SE 6 Programmer Practice Exams, p. xv). so it doesn't hurt to open the the api-html about Arrays or Collections.