Question

我正在尝试编写一个小的Java应用程序，该应用程序返回员工的详细信息。这是我的员工班。

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*/

这是我的DAO课

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;
}

}

但是当我尝试以这种主要方法运行应用程序时，我得到的输出为空值。

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());
}

这是输出。

这是数据库中对应行的样子

Answer 1

您不应两次调用next，因为它将再次使光标向前移动。试试这个：

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

调用ResultSet#next会将光标向前移动一行，因此您的if条件将丢失第一行。由于您知道查询最多可以返回一行，因此根本不需要while循环，

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

无需添加额外的<object>比较。

result.next()

while仅在有任何行时执行。

在用于检查列表中是否包含值之前，请检查所生成列表的大小。

Java MySQL空结果集

3 个答案: