Java ArrayList从两个列表中删除重复项

Question

I have 2 ArrayList of custom objects. I want to remove duplicate entries from both the ArrayList.

The objects have three fields: fName, lName, and id. If the id occurs multiple times, I want to remove from both the lists.

How can I do that?

I am fine with merging both the lists and removing the 2 duplicate entries as well.

Answer 1

If you want to merge: simply copy the content of both lists to a map. Then you don't have the duplicates anymore (but you loose your actual ordering):

Map<Integer, MyObject> temp = new HashMap<Integer, MyObject>();
for (MyObject obj:firstList) {
   temp.put(obj.getId(), obj);
}
for (MyObject obj:secondList) {
   temp.put(obj.getId(), obj);
}
List<MyObject> result = new ArrayList<MyObject>(temp.values());

Answer 2

If your classes have a correct implementation of the equals and hashCode method, turn the list into a HashSet to eliminate the duplicates. The HashSet<T> constructor accepts a Collection<T> so you should be good to go.

If you need some custom comparator function (like the one in your case which compares only id ), pass in a custom Comparator<T> implementation when creating the TreeSet . To sum it up, just create a comparator which compares id s of both the objects and pass it to the TreeSet constructor. Then adding items from both the lists to this set will take care of eliminating the duplicates. Something like:

public class Test {

    public static void main(String[] args) {
        Person p1 = new Person("first", "id1");
        Person p2 = new Person("dummy", "id1"); // same id as above
        Person p3 = new Person("second", "id2");
        Person p4 = new Person("third", "id1");
        List<Person> asList = Arrays.asList(p1, p2, p3, p4);
        CustomComparator comparator = new CustomComparator();
        TreeSet<Person> ts = new TreeSet<Person>(comparator);
        TreeSet<Person> duplicates = new TreeSet<Person>(comparator);
        for (Person p : asList) {
            if (ts.contains(p) || duplicates.contains(p)) {
                duplicates.add(p);
                ts.remove(p);
            } else {
                ts.add(p);
            }
        }
        System.out.println(ts);
    }

}

class Person {

    public Person(String name, String id) {
        super();
        this.name = name;
        this.id = id;
    }

    public String name;

    public String id;

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("Person [id=");
        builder.append(id);
        builder.append(", name=");
        builder.append(name);
        builder.append("]");
        return builder.toString();
    }

}

class CustomComparator implements Comparator<Person> {

    @Override
    public int compare(Person o1, Person o2) {
        return o1.id.compareTo(o2.id);
    }

}

Answer 3

Use Set for this purpose.

Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.

Answer 4

HashSet<Integer> list_1_ids = new HashSet<Integer>();
HashSet<Integer> list_2_ids = new HashSet<Integer>();
for (CustomObject x : list1) list_1_ids.add(x.id);
for (CustomObject x : list2) list_2_ids.add(x.id);
HashSet<Integer> both_ids = list_1_ids;
both_ids.retainAll(list_2_ids);
List<CustomObject> pruned_list_1 = new ArrayList<CustomObject>();
for (CustomObject x : list1) if (!both_ids.contains(x.id)) pruned_list_1.add(x);
List<CustomObject> pruned_list_2 = new ArrayList<CustomObject>();
for (CustomObject x : list2) if (!both_ids.contains(x.id)) pruned_list_2.add(x);