Combining ArrayList without duplicates

Question

import java.util.ArrayList;
import java.util.Collections;

public class SmartCombining {
    public static void main(String[] args) {
        ArrayList<Integer> list1 = new ArrayList<Integer>();
        ArrayList<Integer> list2 = new ArrayList<Integer>();

        Collections.addAll(list1, 4, 3);
        Collections.addAll(list2, 5, 10, 4, 3, 7);

        smartCombine(list1, list2);
        System.out.println(list1);
        System.out.println(list2);
    }

    public static void smartCombine(ArrayList<Integer> first,
            ArrayList<Integer> second) {
        first.addAll(second);
    }    
}

So, I want to combine two lists into one, but if the second list contains a number from the first it won't be added. So far my method adds them all together.

Answer 1

Well, one way to do it is to iterate through the second list while checking if each element exists in the first list. If it doesn't, add it.

public static void smartCombine(ArrayList<Integer> first, ArrayList<Integer> second) {
     for(Integer num : second) {      // iterate through the second list
         if(!first.contains(num)) {   // if first list doesn't contain current element
             first.add(num);          // add it to the first list
         }
     }
}  

Another way would be for you to hold your values inside a set (like HashSet ) which doesn't allow any duplicates. Then you can combine them like:

first.addAll(second);

One more way you could do it is to first remove all elements from the first list that exist in the second list (the ones that would be duplicated). Then you add all elements of the second list to the first list.

public static void smartCombine(ArrayList<Integer> first, ArrayList<Integer> second) {
    first.removeAll(second); // remove elements that would be duplicated
    first.addAll(second);    // add elements from second list
}   

Answer 2

The simple, no brains solution:

Set<Integer> joinedSet = new HashSet<Integer>();
joinedSet.addAll(list1);
joinedSet.addAll(list2);

Answer 3

Use Set , it has been created for that purpose. A Set cannot contain 2 identical elements, based on the equals method.

Set<Integer> list1 = new HashSet<Integer>();
Set<Integer> list2 = new HashSet<Integer>();

Using a combination of ArrayList and contains method is an antipattern here.

Answer 4

Remove duplicates, then merge both lists:

list1.remove(list2);
list1.addAll(list2);

If you dont want to alter the original list, then first create a backup:

list1BP = new ArrayList(list1);

Another approach is to use HashSet , see other answers.

Answer 5

There are two easy way you can combine two Lists and duplicate will be removed.

1) First and very easiest way you can get your output, by creating equivalent HashSet object of your ArrayList. Since HashSet does not allow duplicates.

public static void main(String[] args) {
    ArrayList<Integer> list1 = new ArrayList<Integer>();
    ArrayList<Integer> list2 = new ArrayList<Integer>();

    Collections.addAll(list1, 4, 3);
    Collections.addAll(list2, 5, 10, 4, 3, 7);
    System.out.println(smartCombine(list1, list2));
}
public static HashSet<Integer> smartCombine(ArrayList<Integer> first, ArrayList<Integer> second) {
    first.addAll(second);
    HashSet<Integer> hs = new HashSet<Integer>(first);
    return hs;

2) There is another way using advanced for loop . Iterate the second list and check if the current element is not in first list and then add the current element.

public static void main(String[] args) {
    ArrayList<Integer> list1 = new ArrayList<Integer>();
    ArrayList<Integer> list2 = new ArrayList<Integer>();
    Collections.addAll(list1, 4, 3);
    Collections.addAll(list2, 5, 10, 4, 3, 7);
    smartCombine(list1, list2);
    System.out.println(list1);
}
public static void smartCombine(ArrayList<Integer> first, ArrayList<Integer> second) {
    for (Integer num : second) {
        if (!first.contains(num)) {
            first.add(num);
        }
    }
}

Note: The second way will work fine only if first list has no duplicates.

Answer 6

use contains(Object) method in ArrayList

public static void smartCombine(ArrayList<Integer> first,
        ArrayList<Integer> second) {
    for(Integer i :second){
       if(!first.contains(i)) { // if first list doesn't contain this item, add item to the first list.
          first.add(i);
       }
    }
}

Answer 7

Have you tried ArrayList.addAll()

Look at this java doc

As pointer out this would not handle duplicates which can easily be removed using a Set