How to compare and reorder an ArrayList of objects based on a list of another objects?

Question

I have 2 arraylists, one of type String and the other is a custom class Person.

List names = new ArrayList<String>(); 
List people = new ArrayList<Person>();

Both lists are populated like so:

names.add("bob");
names.add("joe");
names.add("tom");
people.add(new Person("joe")); //Assuming Person has a name property
people.add(new Person("tom"));
people.add(new Person("bob"));

Notice that the same names have been used in both lists, but added in different order. How can I sort the people arraylist in the same order as the names?

mkyong.com/java/… may be a good read. You need either a Comparator or make Person Comparable. — JP Moresmau
– JP Moresmau, Commented May 18, 2015 at 16:03
Do you mean "sort" or simply "reorder"? As in, "reorder the people array to match the order in the names" array? — Ted Hopp
– Ted Hopp, Commented May 18, 2015 at 16:04

Luiggi Mendoza · Accepted Answer · 2015-05-18 16:10:46Z

3

Strange requirement, but you can do it by using a Map:

Map<String, Person> personMap = new HashMap<>();
//Assuming people is declared rightfully as List<Person> rather than just List
for (Person people : people) {
    personMap.put(person.getName(), person);
}
List<Person> results = new ArrayList<>();
for (String name : names) {
    if (personMap.containsKey(name)) {
        results.add(personMap.get(name));
    }
}
//in case you need to work with people only
people.clear();
people.addAll(results);

answered May 18, 2015 at 16:10

Luiggi Mendoza

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2 Comments

Ted Hopp Over a year ago

Any reason for using a separate results list? Why not simply clear people and add directly to that when iterating through names?

Luiggi Mendoza Over a year ago

@TedHopp well, I was thinking to use a Java 8 solution as proposed here in this other answer I posted and used that way.

Ted Hopp · Accepted Answer · 2015-05-18 16:11:31Z

Since the names array can apparently be in an arbitrary order, the concept of "sorting" isn't very applicable. The most direct approach, I think, is to rebuild the people array from the given names array by using a map. Something like this might work:

void reoderPeople(ArrayList<Person> people, ArrayList<String> names) {
    // first build the map
    Map<String, Person> map = new HashMap<>();
    for (Person p : people) {
        map.add(p.getName(), p);
    }
    // now re-create the people array
    people.clear();
    for (String name : names) {
        people.add(map.get(name));
    }
}

This assumes that there is a one-to-one correspondence between the elements of names and people based on the name. If that's not a correct assumption, then this approach would have to be modified accordingly.

tj-recess · Accepted Answer · 2015-05-18 16:09:35Z

0

Use a comparator for sorting people which uses ordering of names list. (Untested code below)

Collections.sort(people, new Comparator<Person>(){
  public int comapre(Person a, Person b) {
    Integer indexA = names.indexOf(a.getName());
    Integer indexB = names.indexOf(b.getName());
    return indexA.compareTo(indexB);
  }
});

answered May 18, 2015 at 16:09

tj-recess

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1 Comment

Ted Hopp Over a year ago

Finally, a comparison-based approach that will will work (provided one changes public int comapre to public int compare). But this seems like a strikingly inefficient compared to building a map.

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