List.stream().filter() vs removeIf - java-8

Question

Please help me understand the difference.
I'm trying to remove elements from list. If I use stream().filter(), this works but when I use removeIf, it throws UnsupportedOperationException.
private void filterEmployees(EmployeeResponse employeeResponse) {
List<Employee> employees = employeeResponse.getEmployees();
List<Employee> employeesFiltered = employees
.stream()
.filter(employee -> employee.getRole().equals("01") || employee.getRole().equals("02") || employee.getRole().equals("03"))
.collect(Collectors.toList());
employeeResponse.setEmployees(employeesFiltered);
}
Below code throws UnsupportedOperationException.
private void filterEmployees(EmployeeResponse employeeResponse, List<String> rolesList) {
List<Employee> employees = employeeResponse.getEmployees();
employees.removeIf(employee -> !rolesList.contains(employee.getRole()));
employeeResponse.setEmployees(employees);
}
Exception:
java.lang.UnsupportedOperationException: remove
at java.base/java.util.Iterator.remove(Iterator.java:102)
at java.base/java.util.Collection.removeIf(Collection.java:545)

Answer 1

Streaming list with a filter simply filters the current list, skipping the unwanted values, and creates a new one. I would be equivalent to you iterating the list and when you see the value, skip it. Otherwise add it to the new list.
removeIf is a new method in Java 8. It is in the Collections interface which List extends. That method is not supported in any immutable collection or in List.of which creates an ImmutableCollection.
I can't tell if that is the problem so you may want to check if that instance is immutable by doing the following:
List<Employee> employees = employeeResponse.getEmployees();
System.out.println(employees.getClass().getName());
One way to possibly solve that problem is to do the following:
List<Employee> employee = new ArrayList<>(employeeResponse.getEmployees());
Which creates a mutable ArrayList and populates it with the employee list. Then removeIf should work as expected.