Java MySQL空结果集

Question

I'm trying to write a small java application that returns the details for an employee. Here's my Employee class.

public class Employees {

private int id;
private Date dateofBirth;
private String firstName;
private String lastName;
private enum gender{
    M, F;
}
private gender employeeGender;
private Date dateHired;

public String getEmployeeGender() {
    return this.employeeGender.name();
}

public void setEmployeeGender(String employeeGender) {
    this.employeeGender = gender.valueOf(employeeGender);
}

/*Getters, setters omitted*/

Here's my DAO class

public class EmployeeDao {

final String TABLE_EMPLOYEES = "employees";
final String COLUMN_EMPLOYEES_ID = "emp_no";
final String COLUMN_EMPLOYEES_DOB = "birth_date";
final String COLUMN_EMPLOYEES_FIRST_NAME = "first_name";
final String COLUMN_EMPLOYEES_LAST_NAME = "last_name";
final String COLUMN_EMPLOYEES_GENDER = "gender";
final String COLUMN_EMPLOYEES_HIRE_DATE = "hire_date";

final String QUERY_EMPLOYEES = "SELECT * FROM " + TABLE_EMPLOYEES + " WHERE " + COLUMN_EMPLOYEES_ID + " = ?";

public Employees getEmployeeDetails(int employeeId) {
    Employees employee = new Employees();
    try (DbConnection dbConnection = new DbConnection();
         Connection databaseConnection = dbConnection.getConn();
         PreparedStatement selectFromEmployees = databaseConnection.prepareStatement(QUERY_EMPLOYEES)) {
        selectFromEmployees.setInt(1, employeeId);
        try (ResultSet result = selectFromEmployees.executeQuery()) {

            if (result.next() == false) {
                System.out.println("Empty Resultset");
            }
            while (result.next()) {
                employee.setId(result.getInt(COLUMN_EMPLOYEES_ID));
                employee.setFirstName(result.getString(COLUMN_EMPLOYEES_FIRST_NAME));
                employee.setLastName(result.getString(COLUMN_EMPLOYEES_LAST_NAME));
                employee.setDateofBirth(result.getDate(COLUMN_EMPLOYEES_DOB));
                employee.setEmployeeGender(result.getString(COLUMN_EMPLOYEES_GENDER));
                employee.setDateHired(result.getDate(COLUMN_EMPLOYEES_HIRE_DATE));
            }
        }

    } catch (Exception e) {
        e.printStackTrace();
    }
    return employee;
}

}

But when I try to run the app in main method like this, I get an output with null values.

public static void main(String[] args) {
    EmployeeDao employeeDao = new EmployeeDao();
    Employees employees = employeeDao.getEmployeeDetails(39256);
    System.out.println(employees.getId() + " \n" + employees.getFirstName() + " \n" + employees.getLastName() + " \n" + employees.getDateofBirth() + " \n" + employees.getDateHired());
}

This is the output.

This is how the corresponding row looks like in the database

Answer 1

You should not call next twice, since it will move the cursor forward again. Try this:

if (result.next() == false) {
    System.out.println("Empty Resultset");
} else {
    employee.setId(result.getInt(COLUMN_EMPLOYEES_ID));
    employee.setFirstName(result.getString(COLUMN_EMPLOYEES_FIRST_NAME));
    employee.setLastName(result.getString(COLUMN_EMPLOYEES_LAST_NAME));
    employee.setDateofBirth(result.getDate(COLUMN_EMPLOYEES_DOB));
    employee.setEmployeeGender(result.getString(COLUMN_EMPLOYEES_GENDER));
    employee.setDateHired(result.getDate(COLUMN_EMPLOYEES_HIRE_DATE));
}

Answer 2

Calling ResultSet#next moves the cursor forward a row, so your if condition loses the first row. Since you know your query can return at most one row, you don't need the while loop at all, however:

public Employees getEmployeeDetails(int employeeId) throws SQLException {
    Employees employee = null;
    try (DbConnection dbConnection = new DbConnection();
         Connection databaseConnection = dbConnection.getConn();
         PreparedStatement selectFromEmployees = 
            databaseConnection.prepareStatement(QUERY_EMPLOYEES)) {

        selectFromEmployees.setInt(1, employeeId);
        try (ResultSet result = selectFromEmployees.executeQuery()) {

            if (result.next()) {
                employee = new Employees();
                employee.setId(result.getInt(COLUMN_EMPLOYEES_ID));
                employee.setFirstName(result.getString(COLUMN_EMPLOYEES_FIRST_NAME));
                employee.setLastName(result.getString(COLUMN_EMPLOYEES_LAST_NAME));
                employee.setDateofBirth(result.getDate(COLUMN_EMPLOYEES_DOB));
                employee.setEmployeeGender(result.getString(COLUMN_EMPLOYEES_GENDER));
                employee.setDateHired(result.getDate(COLUMN_EMPLOYEES_HIRE_DATE));
            }
        }
    }
    return employee;
}

Answer 3

No need to add extra result.next() comparison.

if (result.next() == false) {
 System.out.println("Empty Resultset");
}
while (result.next()){

}

while will execute only if there are any rows.

Check the size of list generated before using to check if it contains value or not.