How do I compare two lists?

Question

CompareList removedList = new CompareList();
CompareList addedList = new CompareList();

This is how I'm adding elements inside

addedList.add(new Objec("Var_a", "1"));

and class Objec has two strings.

How can I compare that? I can't use contains like I could with ArrayList of Strings?

In CompareList I have

public boolean equals(CompareList l) {

    if (l.containsAll(this)) {
        if (this.containsAll(l)) {
            return true;
        }
    }
    return false;
}

and in Objec

public Objec(String n, String s) {
    this.name=n;
    this.surname=s;
}

public String toString() {
    return " Name: " + name +  ", Surname: " + surname;
}

I see that many people are confused with my question. So what I want?

List 1:

Samy Joe
Emma Than
Julia Rob

List 2:

Samy Joe
Emma Than
Anna Sky

Removed Julia Rob and added Anna Sky. But I don't know how to do it when my lists contains of object that have two strings?

`CompareList` seems like an object defined by your code, without information on that class we really can't say anything. — Claudio Brasser, Aug 24 '17 at 06:21
Check below link: https://stackoverflow.com/questions/2762093/java-compare-two-lists It might help you. — Prakash, Aug 24 '17 at 06:25
There's no `CompareList` in the standard collections. Does it extend `AbstractList`? — shmosel, Aug 24 '17 at 06:35
Then how are you adding `Objec` to it? Does `Objec` extend/implement `CompareObject`? — shmosel, Aug 24 '17 at 06:39
Btw, we really shouldn't be having this conversation. You're wasting everyone's time by not providing this information up front. — shmosel, Aug 24 '17 at 06:40
Okay ignore my code, tell me tip how do i compare list of object who contain than two strings for example name and surname, how do i compare that objects... @shmosel — Alen, Aug 24 '17 at 06:42
@Lalija So you want to compare the two lists. Do you want to check **whether they contain the same elements** (and nothing more or less)? Or do you want to do something with the disjoint elements? — MC Emperor, Aug 24 '17 at 07:09

MC Emperor · Answer 1 · 2017-08-24T07:21:05.510

This piece of code compares if the lists are equal, that is, contains the same elements.

static boolean same(Collection<?> a, Collection<?> b) {
    if (a.size() != b.size()) {
        return false;
    }
    List<?> c = new ArrayList<>(a);
    c.removeAll(b);
    return c.isEmpty();
}

If the sizes are not equal, then the lists are never equal.
Else, if the sizes are equal, then we know that both lists contain one or more elements that are not present in the other list. So we make a new list from one of the lists (list a in my case), and then we remove the elements of b.

You don't need to use your own class CompareList, instead you could just use an ArrayList or something.

In order to compare your Objec to another one, you'll need to implement equals(Object) and hashCode() correctly.

If you want to know which elements are not contained in the other list, then you can use this:

static HashMap<Collection<?>, Collection<?>> disjoints(Collection<?> a, Collection<?> b) {
    List<?> aa = new ArrayList<>(a);
    aa.removeAll(b);
    List<?> bb = new ArrayList<>(b);
    bb.removeAll(a);

    HashMap<Collection<?>, Collection<?>> map = new HashMap<>();
    map.put(a, aa);
    map.put(b, bb);
    return map;
}

It returns a map with as keys the two collections and as values the elements of the collection specified by the key, which are not contained in the other collection. For example, if you want to know the elements of a not present in b, then call disjoints(a, b).get(a).

Note: I call the lists collections, because they are. In Java, a List is a subtype of Collection.

d3rbastl3r · Answer 2 · 2017-08-24T07:06:34.837

You need to override the equals method in your custom object like this:

public class MyObject {
     private String name;
     private String surname;

    @Override
    public boolean equals(MyObject myObject) {
         // assert that name and surename can not be null or check for this
         if (!this.name.equals(myObject.name)) {
             return false;
         }

         if (!this.surname.equals(myObject.surname)) {
             return false;
         }

         return true;
    }
}

The contains-Method of a list will be use the equals method to check the equality of your objects.

But the link that @Prakash has posted is the better way do do that. I think no one understand what your attention is to do with yout CompareList and why you use a custom one ;)

EDIT: Sorry, had mistake in the Signature.

@Override
    public boolean equals(Object obj) {
        if (!(obj instanceof MyObject) {
            return false;
        }
        // else cast and use code above
    }

shmosel is hinting that the equals() method does not have the right signature. You are overloading it instead of overriding. — MC Emperor, Aug 24 '17 at 06:53

score 1 · Answer 3 · answered Aug 24 '17 at 07:06

You're question: "How can I compare that?"

The first thing to keep in mind is that you can compare in 2 ways. so think about that first.

Equality: When comparing for equality

use equals() and while you're at it implement hashCode().

Comparing / sorting: When are 2 objects considered lower then, same, or higher then the other?

implement "Comparable" interface and override compareTo.

You're post contains the equals() so lets say that is what you want to do. If you want to use containsAll(), keep in mind that it uses equals from the class the list contains to figure out to return true/false. So you should probably:

add equals() to your "Objec" class.

How do I compare two lists?

3 Answers3