Find Missing items from arraylist in java

Question

Hi I am newbie to java and was trying my hand on collections part, I have a simple query I have two array list of say employee object

class Employee
{
    private int emp_id;
    private String name;
    private List<String> mobile_numbers;

    //.....getters and setters
}

say listA and listB have the following data

    List<Employee> listA = new ArrayList<Employee>(); 
    List<Employee> listB = new ArrayList<Employee>(); 

    listA.add(new Employee("101", "E1", listOfMobileNumbers));
    listA.add(new Employee("102", "E2", listOfMobileNumbers1));
    listA.add(new Employee("103", "E3", listOfMobileNumbers2));
    listA.add(new Employee("104", "E4", listOfMobileNumber4));
    listA.add(new Employee("105", "E5", listOfMobileNumbers5));

    listB.add(new Employee("101", "E1", listOfMobileNumbers1));
    listB.add(new Employee("102", "E2", listOfMobileNumbers2));
    listB.add(new Employee("106", "E6", listOfMobileNumber6));
    listB.add(new Employee("107", "E7", listOfMobileNumber7));
    listB.add(new Employee("108", "E8", listOfMobileNumbers8));

where listOfMobileNumbers is a List<String>

Now I want to find the additional elements from the individuals list. i.e

 List<Employee> additionalDataInListA = new ArrayList<Employee>(); 
    // this list should contain 103, 104 and 105

  List<Employee> additionalDataInListB= new ArrayList<Employee>(); 
    // this list should contain 106, 107 and 108

How do i achieve this?

Your help is appreciated.

Edit:

I don't want to use any internal functions i want to achieve it writing some manual function kind of comparison function.

I also cant override the equals function because the in my usecase i have FieldNo which is int and Values which is a List<String>.

the occurrences of field is 'n' times and every time the Values associated with that field will different.

For example : FieldNo=18 Values=["A"];

FieldNo=18 Values=["B"]; and so on...

The employee id that I had used was for only illustration purposes where it makes sense to override equals and hash code.

What have you tried so far? Have you compared `Employee`s? Done any iteration through a list? — thegrinner, Jun 10 '13 at 14:11
possible duplicate of [How to union, intersect, difference and reverse data in java](http://stackoverflow.com/questions/3590677/how-to-union-intersect-difference-and-reverse-data-in-java) — Stijn Geukens, Jun 10 '13 at 14:14
@user2437809 Please see my solution using lambdaj and hamcrest libraries (this can be done in a few lines). — Gaston Flores, Jun 10 '13 at 18:08

score 5 · Answer 1 · answered Jun 10 '13 at 14:14

5

You can use boolean removeAll(Collection<?> c) method.

Just note that this method modifies the List on which you invoke it. In case you to keep the original List intact then you would have to make a copy of the list first and then the method on the copy.

e.g.

List<Employee> additionalDataInListA = new ArrayList<Employee>(listA);
additionalDataInListA.removeAll(listB);

answered Jun 10 '13 at 14:14

Bhesh Gurung

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1

It also requires `equals()` and `hashCode()` to be overridden in the `Employee` class. – GriffeyDog Jun 10 '13 at 15:35
@GriffeyDog: You are right that's the basic requirement that the objects should fulfill, in order for the collection(s) to work properly. – Bhesh Gurung Jun 10 '13 at 16:33

arshajii · Answer 2 · 2013-06-10T16:30:22.480

3

Override equals in Employee to test for equality between ids (and remember that when you override equals you should also override hashCode). Then:

List<Employee> additionalDataInListA = new ArrayList<Employee>(listA);
additionalDataInListA.removeAll(listB);

List<Employee> additionalDataInListB = new ArrayList<Employee>(listB);
additionalDataInListB.removeAll(listA);

Relevant documentation:

removeAll

edited Jun 10 '13 at 16:30

answered Jun 10 '13 at 14:14

arshajii

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score 1 · Answer 3 · edited May 23 '17 at 10:31

1

You can use the Collection interface's removeAll method as shown in the top answer to this question (How can I calculate the difference between two ArrayLists?).

edited May 23 '17 at 10:31

Community

1
1

answered Jun 10 '13 at 14:14

David Futcher

320
1
7

score 1 · Answer 4 · answered Jun 10 '13 at 18:01

You can use lambdaj (download here, website) and hamcrest (download here, website), this libraries are very powerfull for managing collections, the following code is very simple and works perfectly. With this libraries you can solve your problem in one line. You must add to your project: hamcrest-all-1.3.jar and lambdaj-2.4.jar Hope this help serve.

import static ch.lambdaj.Lambda.filter;
import static ch.lambdaj.Lambda.having;
import static ch.lambdaj.Lambda.on;
import java.util.Arrays;
import java.util.List;
import static org.hamcrest.Matchers.isIn;
import static org.hamcrest.Matchers.not;

public class Test2{
        public static void main(String[] args) {
            List<String> oldNames =  Arrays.asList("101","102","103","104","105");
            List<String> newNames = Arrays.asList("101","102","106","107","108");

            List<String> newList = filter(not(having(on(String.class), isIn(oldNames))),newNames);
            List<String> newList2 = filter(not(having(on(String.class), isIn(newNames))),oldNames);
            System.out.println(newList);
            System.out.println(newList2);
            /*out
            [106, 107, 108]
            [103, 104, 105]
            */
    }
}

This example work with a simple List of Strings but can can be adapted for your object Employee.

score 0 · Answer 5 · answered Jun 10 '13 at 14:11

0

for(int j = 0; j < listB.size(); j++){
   if (!listA.contains(listB.get(j))
       additionalDataInListB.add(listB.get(j))
}

Just repeat that with opposite values to fill up the additionalDataInListA object. This should do the trick :)

EDIT: This will only work if these are the same object. That may not be the case. If this is not the case, you should compare one of the traits probably emp_id to see if those are the same. If they are not the same, add the different one to the new list.

answered Jun 10 '13 at 14:11

Jake Gascho

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contains will only work if you override `equals` in `Employee` – Majid Laissi Jun 10 '13 at 14:14
This will not work if `equals ` was not implemented using only the employee id. – Stijn Geukens Jun 10 '13 at 14:14
this would work if you have equals implemented but i guess the api methods are preferred – tgkprog Jun 10 '13 at 14:19

score 0 · Answer 6 · answered Jun 10 '13 at 14:19

class Employee {
    private int empId;
    ...
    @Override
    public int hashCode() { return empId; }

    @Override
    public boolean equals(Object other) {
        return other != null && other instanceof Employee && other.empId == empId;
    }
}

The above would allow you to make a Set<Employee> employees = new HashSet<Employee>().

Sets can do set operations, like removeAll and retainAll.

That would be optimal (which is not the case for List).

Himanshu Tewari · Answer 7 · 2017-05-18T10:55:49.677

0

Its very simple , first of all iterate the array and get the sum of all number , As we know sum of natural numbers from 1 to n we can write as n*(n+1)/2. Now we have to subtract the sum of the array from [n*(n+1)/2] .

Here we get the missing number .

you can implement this logic in your code, I hope you got my point.

Source with example

edited May 18 '17 at 10:55

answered May 13 '15 at 05:18

Himanshu Tewari

313
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Find Missing items from arraylist in java

7 Answers7