How to find the differences between two Array Lists based on a property?

Question

I have two array list. Each has list of Objects of type Employee.

The Employee class looks like below

    public class Employee {

    Employee(String firstname, String lastname, String employeeId) {
        this.firstname = firstname;
        this.lastname = lastname;
        this.employeeId = employeeId;
    }

    private int id; // this is the primary key from employee table

    private String firstname;

    private String lastname;

    private String employeeId; // manually assigned unique id to each employee

    // getters and setters

}

I need to find the differences between the two lists based on a property of the employee object which is employee id.

Employee id is manually generated unique id given to each employee.

    import java.util.ArrayList;
import java.util.List;


public class FindDifferences {

    public static void main(String args[]){
        List<Employee> list1 = new ArrayList<Employee>(); 
        List<Employee> list2 = new ArrayList<Employee>(); 

        list1.add(new Employee("F1", "L1", "EMP01"));
        list1.add(new Employee("F2", "L2", "EMP02"));
        list1.add(new Employee("F3", "L3", "EMP03"));
        list1.add(new Employee("F4", "L4", "EMP04"));
        list1.add(new Employee("F5", "L5", "EMP05"));

        list2.add(new Employee("F1", "L1", "EMP01"));
        list2.add(new Employee("F2", "L2", "EMP02"));
        list2.add(new Employee("F6", "L6", "EMP06"));
        list2.add(new Employee("F7", "L7", "EMP07"));
        list2.add(new Employee("F8", "L8", "EMP08"));

        List<Employee> notPresentInList1 = new ArrayList<Employee>(); 
        // this list should contain EMP06, EMP07 and EMP08

        List<Employee> notPresentInList2= new ArrayList<Employee>(); 
        // this list should contain EMP03, EMP04 and EMP05



    }

}

Answer 1

Override equals() and hashcode() methods of your Employee class to only use employeeId when checking for equality (im not sure about why you need the id field. you might what to incorporate it as well). NetBeans / Eclipse IDEs can do this for you. Then you can create a copy of your original lists and use List.removeAll() to calculate difference.