Comparing two object arraylists

Question

First time here so I hope this makes sense!

I have two object arrays say l1 and l2, I want to run a compare between these two lists and get a the unmatched value in say in l3. User class contains 2 Strings:

userEnteredValue 
valueReturnedFromDatabase

Say, l1 contains: Java, JSF, JAXR, foo l2 contains: JSF, JAXR

I could run a compare for matching values, but for not for non-matching values. The logic seems to be flawed. Any help?

For matching values:

for(User u1 : l1) {
   for(User u2: l2) { 
      if(u1.getUserEnteredValue().equals(u2.getValueReturnedFromDatabase())) {
        l3.add(u1);
      }
}

But, for the non-matching when I say not equal to, instead of getting only the unique values I get all values. A couple of similar posts on Stackoverflow suggest to implement the equals and hashcode method in the User class. Is this necessary, since my arraylist size don't go beyond 5 to 10.

Can't you use 2 HashSets for this and find the overlapping parts using that? — selten98
– selten98, Commented May 18, 2016 at 6:10

Titus · Accepted Answer · 2016-05-18 06:36:58Z

3

You can do something like this:

for(User u1 : l1) {
    boolean unique = true;
    for(User u2: l2) { 
        if(u1.getUserEnteredValue().equals(u2.getValueReturnedFromDatabase())) {
            unique = false;
            break;
        }
    } 
    if(unique){
        l3.add(u1);
    }
}

answered May 18, 2016 at 6:36

Titus

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1 Comment

Ani Over a year ago

Cheers! I used this snippet, and it works like a charm.

user5789865 · Accepted Answer · 2016-05-18 06:13:19Z

2

You can use the contains() method from java.util.ArrayList to determine whether your list contains the object.

Example:

for(User u1:l1) {
    if(!l2.contains(u1)) {
        l3.add(u1);
    }
}

answered May 18, 2016 at 6:13

user5789865

1 Comment

Titus Over a year ago

l2.contains(u1) is not the same as u1.getUserEnteredValue().equals(u2.getValueReturnedFromDatabase()) he will need to override equals and hashCode in the User class in order for that to be true.

user1933888 · Accepted Answer · 2016-05-18 06:14:43Z

2

https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#contains(java.lang.Object)

Iterate through one array and check if it "contains()" the element from other array, using the above method provided with ArrayList in java.

answered May 18, 2016 at 6:14

user1933888

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Comments

Olimpiu POP · Accepted Answer · 2016-05-18 06:35:53Z

2

If you can use Java 8 and you would like some shorter format:

 List<String> l1 = Arrays.asList("a", "b", "c");
 List<String> l2 = Arrays.asList("b");

 List<String> l3 = l1.stream().filter(e -> !l2.contains(e)).collect(Collectors.toList());

answered May 18, 2016 at 6:35

Olimpiu POP

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Comments

afzalex · Accepted Answer · 2016-05-18 06:25:16Z

1

Let's make your problem even smaller.
I have 2 arrays
l1 = [1, 2]
l2 = [2, 3]

for(int i : l1) {
    for(int j : l2) {
        if(i != j) {
            l3.add(i);
        }
    }
}

Now let's examine this code.
When i = 1, j = 2 then i != j i.e. 1 != 2 is true and 1 will be inserted in l3.
When i = 1, j = 3 then i != j i.e. 1 != 3 is true and 1 again will be inserted in l3.
When i = 2, j = 2 then i != j i.e. 2 != 2 is false
When i = 2, j = 3 then i != j i.e. 1 != 2 is true and 2 will be inserted in l3.
So the final array will be l3 = [1, 2] (if it is a set. or [1, 1, 2] if it is a list) i.e. all the elements of l1 will be inserted in l2.

To get unique elements of l1 you will have to check all elements of l2 for same element in l1 1 by 1 and if it is not found in complete l2 then add it in l3.

rootloop:
for(int i : l1) {
    for(int j : l2) {
        if(i == j) {
            continue rootloop;
        }
        l3.add(i);
    }
}

Now change the above code to work for your problem.

But these kind of searching are so common that their implementation is already given in collection framework.

for(int i : l1) {
    if(!l2.contains(i)){
        l3.add(i);
    }
}

answered May 18, 2016 at 6:25

afzalex

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1 Comment

Ani Over a year ago

Thank you for the explanation. It would have good if I had put a couple of console messages and traced the snippet. Hope this would help someone else.

uniknow · Accepted Answer · 2016-05-18 06:33:57Z

1

One could use the Array of matching entities for the creation of unmatched entities as follows:

User[] matchingUsers = ...
User[] AllUsers = ...

List<User> listOfMatchingUsers = Arrays.asList(matchingUsers);
List<User> listOfAllUsers = Arrays.asList(allUsers);

List<User> unmatchedUsers = listOfAllUsers.removeAll(listOfMatchingUsers);

answered May 18, 2016 at 6:33

uniknow

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Comments

Amer Qarabsa · Accepted Answer · 2016-05-18 06:39:38Z

1

I see many answers using list.contains, i do not agree as contains use direct loop so the performance will be an issue for large lists O(n^2).

Instead you can use maps, add each list to a map with key and value the same then do the following:

Iterator it = map1.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry pair = (Map.Entry)it.next();
    if(map2.get(pair.getValue())!=null) {
            list3.add(map2.get(pair.getValue()));
     }
}

answered May 18, 2016 at 6:39

Amer Qarabsa

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