How to solve the MismatchedInputException: Cannot construct instance of `java.util.HashSet` exception

Question

In my program I tried to update the user's email. But it gave me an exception saying

nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of java.util.HashSet

But when I update a user with adding skills and email it didn't gave me this exception.

I tried to create a default constructor but it didn't work. I also tried to use Json creator.It also didn't worked for me.I can see that the problem is when I pass the skill set empty this exception is rising.But I don't have a clear idea, how to over come this situation.

This is my Employee entity

@Entity
@Table(name = "Employee")
public class Employee implements Serializable{

    private static final long serialVersionUID = -3009157732242241606L;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long emp_id;

    @Column(name = "emp_fname")
    private String emp_fname;

    @Column(name = "emp_lname")
    private String emp_lname;

    @Column(name = "emp_email")
    private String emp_email;

    @Column(name = "emp_dob")
    private Date emp_dob;

    @ManyToMany(cascade = CascadeType.MERGE)
    @JoinTable(name = "emp_skills",
            joinColumns = @JoinColumn(name = "emp_id", referencedColumnName = "emp_id"),
            inverseJoinColumns = @JoinColumn(name = "s_id",referencedColumnName = "s_id"))
    private Set<Skills> skills;

    protected Employee(){

    }


    public Employee(String emp_fname,String emp_lname,String emp_email,Date emp_dob){
        this.emp_fname = emp_fname;
        this.emp_lname = emp_lname;
        this.emp_email = emp_email;
        this.emp_dob = emp_dob;
    }

    public Employee(Long emp_id,String emp_fname,String emp_lname,String emp_email,Date emp_dob,Set<Skills> skills){
        this.emp_id = emp_id;
        this.emp_fname = emp_fname;
        this.emp_lname = emp_lname;
        this.emp_email = emp_email;
        this.emp_dob = emp_dob;
        this.skills = skills;
    }

//all the getters and setter

This is my Skills entity

@Entity
@Table(name = "Skills")
public class Skills implements Serializable {

    private static final long serialVersionUID = -3009157732242241606L;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long s_id;

    @Column(name = "s_name")
    private String s_name;


    @ManyToMany(mappedBy = "skills",cascade = CascadeType.ALL)
    @JsonIgnore
    private Set<Employee> employees;

    protected Skills(){

    }

    public Skills(String s_name){
        this.s_name = s_name;
    }

//all the getters and setter

This is my controller class method to update user

@Autowired
    EmployeeRepository repository;

    @RequestMapping("/updateEmployee")
    @PostMapping("/updateEmployee")
    @CrossOrigin
    public void updateEmployee(@RequestBody Employee employee, BindingResult bindingResult){
        Employee empToBeUpdated = null;
        for (Employee emp : repository.findAll()){
            if (emp.getEmp_id()==employee.getEmp_id()){
                empToBeUpdated = emp;
            }
        }
        if (!employee.getEmp_fname().equals("")&&employee.getEmp_fname()!=null){
            empToBeUpdated.setEmp_fname(employee.getEmp_fname());
        }
        if (!employee.getEmp_lname().equals("")&&employee.getEmp_lname()!=null){
            empToBeUpdated.setEmp_lname(employee.getEmp_lname());
        }
        if (!employee.getEmp_email().equals("")&&employee.getEmp_email()!=null){
            empToBeUpdated.setEmp_email(employee.getEmp_email());
        }
        if (employee.getEmp_dob()!=null){
            empToBeUpdated.setEmp_dob(employee.getEmp_dob());
        }

        if (employee.getSkills()!= null){
            empToBeUpdated.setSkills(employee.getSkills());
        }

        repository.save(empToBeUpdated);

    }

Here is the error I'm getting

Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of java.util.HashSet (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value (''); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of java.util.HashSet (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('') at [Source: (PushbackInputStream); line: 1, column: 94] (through reference chain: com.ems.assignment1.model.Employee["skills"])]

Answer 1

The problem I faced was, the back end expect a skill set but if when the user didn't pick any skills from the skill list the front end passing an empty string. I resolved this issue by sending an empty array from angular front end if the user didn't select any skills from the skill list.

constructor(private apiService: ApiService) {
    this.nullSkills = [];
  }

if (this.form.controls.skillsSet.value == null || this.form.controls.skillsSet.value === '') {
      this.employee.skills = this.nullSkills;
    } else {
      this.employee.skills = this.form.controls.skillsSet.value;
    }