Compare two arraylist contents and store unmatched contents in another arraylist

arraylist, compare, java

Solution

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.

Problem

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"

Original source