Compare two arraylist contents and store unmatched contents in another arraylist

Question

I want to compare two arraylist contents .

I am storing objects in them in this way.

for Arraylist 1:

Employee e1=new Employee();

e1.setID("1");
e1.setID("2");

ArrayList<Employee>list1 = new ArrayList<Employee>(); 

if(e1!=null){
    list1.add(e1);
}

for Arraylist 2:

Employee e2=new Employee();

e2.setID("1");
e2.setID("2");
e2.setID("4");

ArrayList<Employee>list2 = new ArrayList<Employee>(); 

if(e2!=null){
    list2.add(e2);
}

Now I am trying to compare the above arraylist contents in this way

ArrayList<Employee>unmatchedList = new ArrayList<Employee>();   

for (Employee l1 : list1){                               
    if(!list2.contains(l1.getID())){    
        System.out.println("list2 does not contains this ID"+l1.getID());   
        Employee e3=new Employee();          
        e3.setID(l1.getID());

        if(unmatchedList==null){            
        unmatchedList=new ArrayList<Employee>();            
        unmatchedList.add(e3);          
        }

        if(unmatchedList!=null){                            
            unmatchedList.add(e3);              
        }                       
    }
}

But I am not gettting correct unmatchedList contents as "4" only . I am getting unmatchedList as "1" and "2" which is wrong. So how can I get unmatched contents only in "unmatchedList"

Answer 1

If your class Employee is defined like this :

public class Employee {

private int id;

public Employee(int id){
    this.id = id;
}

public int getId() {
    return id;
}

@Override
public String toString() {
    return "Id : " + this.id;
}

@Override
public boolean equals(Object obj) {
    return (obj instanceof Employee) && this.id == ((Employee)obj).getId();
}

in the Main method, you can retrieve the unmatch contents like this :

 public static void main( String[] args )
{
   List<Employee> l1 = new ArrayList<Employee>();
   l1.add(new Employee(1));
   l1.add(new Employee(2));
   l1.add(new Employee(3));
   l1.add(new Employee(4));
   l1.add(new Employee(5));


   List<Employee> l2 = new ArrayList<Employee>();
   l2.add(new Employee(4));
   l2.add(new Employee(5));


   l1.removeAll(l2);
   System.out.println(l1);

}

This will print : [Id : 1, Id : 2, Id : 3]

Note that for this work you must override the equals method.

Answer 2

The problem is this code: -

if(!list2.contains(l1.getID())){

Your list2 is an ArrayList<Employee>, and you are checking for containment of l1.getId(). So, your if condition will always be true.

---

You should rather override equals and hashCode method in your Employee class, and just use : -

if(!list2.contains(l1))

for checking whether list2 contains employee l1.

---

And why are you setting your id 3 times back to back before adding it to list. It will not add all the three id's, but only a single Employee with last value set for id. You need to correct it: -

e2.setID("1");
e2.setID("2");
e2.setID("4");

list.add(e2);  // Will only add one Employee with id = 4

Answer 3

Add an equals method to your employees that compares them by their ID, make a copy of list2 and call removeAll:

List<Employee> list1 = ...;
List<Employee> list2 = ...;
List<Employee> unmatchedList = new ArrayList<Employee>(list2);
unmatchedList.removeAll(list1);

For a complete example, see @Dimitri's answer.