Bi-directional associations

Address.java

public class Address {

  private final int id;

  public Set<Employee> employees = new HashSet<Employee>();

  public Address(final int id) {
    this.id = id;
  }

  public void removeEmployee(final Employee employee) {
    if(employees.remove(employee)) {
      employee.setAddress(null);
    }
  }

  public void addEmployee(final Employee employee) {
    if(employees.add(employee)) {
      employee.setAddress(this);
    }
  }
  
  public void setEmployees(Set<Employee> employees) {
    for(Employee e : this.employees) {
      if(!employess.contains(e)) {
        removeEmployee(e);
      }
    }
    for(Employee e : employees) {
      addEmployee(e);
    }
  }
}

Employee.java

public class Employee {

  private final int id;
  public Address address;

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

  public void setAddress(Address newAddress) {
    if (address != newAddress) {
      if(this.address != null) {
        address.removeEmployee(this);
      }
      this.address = newAddress;
      if(newAddress != null) {
        newAddress.addEmployee(this);
      }
    }
  }

  @Override
  public String toString() {
    return "[Employee@" + id + "]";
  }
}

Test.java

  public static void main(String[] args) {
    Employee employee=new Employee(1);
    Employee employee2=new Employee(2);
    Address address=new Address(12);
    Address address3=new Address(34);

    address.employees.add(employee);
    address.employees.add(employee2);

    // MANUALLY SETTING THE OTHER END
    employee.address = address;
    employee2.address = address;

    System.out.println(address.employees); // still lists the 2 employees
    System.out.println(address3.employees); // lists nothing

    // Moving the employees, doesn't update the set!
//    employee.address = address3;
//    employee2.address = address3;
//
//    System.out.println(address.employees); // still lists the employees
//    System.out.println(address3.employees); // lists nothing

    // BUT
    employee.setAddress(address3);
    employee2.setAddress(address3);

    System.out.println(address.employees); // No one here
    System.out.println(address3.employees); // Works here notw

    // WORKS BOTH WAYS
    address.addEmployee(employee);
    address.addEmployee(employee2);

    System.out.println(address.employees); // Back here
    System.out.println(address3.employees); // no one here

  }