Java: Comparing two lists

Question

I have two lists. For example:

A = {21, 41, 96, 02}
B = {21, 96, 32, 952, 1835}

What I want is this as a result: R = {32, 952, 1835}

Like this:

After this I want to add the result R to A :

A = {21, 41, 96, 02, 32, 952, 1835}

Answer 1

It's simple :)

List<Integer> a = new ArrayList<>(Arrays.asList(21, 41, 96, 02));
List<Integer> b = new ArrayList<>(Arrays.asList(21, 96, 32, 952, 1835));

b.removeAll(a)

// now list b contains (32, 952, 1835)

a.addAll(b);

// now list a contains (21, 41, 96, 02, 32, 952, 1835)

Answer 2

So, you want the set reunion of those two collections. That's what sets are for.

HashSet<Integer> set = new HashSet<Integer>();
set.addAll(a);
set.addAll(b);

Answer 3

So you want to make A = AU B. To do this, you can search for each element of B in A and add the element to A if the search fails. However, making too much search on a list is not recommended.

I would recommend using a HashMap. Create a HashMap and loop through A and B putting anything you see into the map. After that, you can convert map back to a list.

[If you want the order to be the same as in the example, you should convert A to a map. Then you should go through B and for each failed map.get(element) you should be adding that element of B to A.]

Answer 4

Try this :

import org.apache.commons.collections.CollectionUtils;

[...]
    //  Returns a Collection containing the exclusive disjunction (symmetric difference) of the given Collections.
    Collection disJointList = CollectionUtils.disjunction(a, b);  
    //To check
    for (Object object : disJointList) {  
            System.out.println(disJointList);  
            //output is {32, 952, 1835} 
    }   

    a.addAll( disJointList );