How to remove element from one list by comparing two lists

Question

I have two Lists, say List1 and List2. I need to remove elements from List2 if that is already present in List1

To avoid ConCurrentModificationException, I have tried with ListIterator

employeeList1 // data present
employeeList2 // data present
ListIterator<Employee> zeroList=employeeList2.listIterator();

//remove employee if already present in list1
while(zeroList.hasNext()){
for(Employee employee : employeeList1){
                    if(zeroList.next().getID().equals(employee.getId())){
zeroList.remove();
                    }
                }
            }

I am getting the below exception inside the if condition

java.util.NoSuchElementException

It is possible that element might not be present in List1, but checking the condition is necessary.

Answer 1

You can use removeAll method on collection from which you want to remove the elements and pass the collection as arguments which contains the elements you want to remove.

List<String> list1 = new ArrayList<>();
list1.add("a");
list1.add("b");
list1.add("c");
List<String> list2 = new ArrayList<>();
list2.add("a");
list2.add("p");
list2.add("q");
list2.removeAll(list1);
System.out.println(list2);

Removes a from list2 as it was present in list1 and prints p and q only,

[p, q]

Edit: Here is a sample code for Employee class while yours might be a different one but as you said your key is employeeId hence equals and hashCode methods needs to play on employeeId only.

static class Employee {
    private long employeeId;
    private String name;
    // whatever more variables

    public Employee(long employeeId, String name) {
        this.employeeId = employeeId;
        this.name = name;
    }

    public String toString() {
        return String.format("Employee[employeeId=%s, name=%s]", employeeId, name);
    }

    @Override
    public boolean equals(Object o) {
        if (o instanceof Employee) {
            return this.employeeId == ((Employee) o).employeeId;
        }
        return false;
    }

    @Override
    public int hashCode() {
        return new Long(employeeId).hashCode();
    }
}

public static void main(String[] args) throws Exception {
    List<Employee> list1 = new ArrayList<>();
    list1.add(new Employee(1, "a"));
    list1.add(new Employee(2, "b"));
    list1.add(new Employee(3, "c"));
    List<Employee> list2 = new ArrayList<>();
    list2.add(new Employee(1, "a"));
    list2.add(new Employee(4, "d"));
    list2.add(new Employee(5, "e"));
    list2.removeAll(list1);
    System.out.println(list2);
}

Try using this code and see what it prints and then just comment both equals and hashCode method and then see what happens. After you comment those two methods, the object present in list1 will not be removed as list doesn't know when the two objects are equal.