从构造函数(Java、MySQL)设置具有类作用域的变量时出现问题
我试图用构造函数设置一个具有类作用域的变量,每当我从构造函数中打印出来时,该变量被设置为正确的值,但每当我试图在同一类中的另一个方法中使用该变量时,该值为null。如何在构造函数外部正确使用firstNameTest变量 这是构造函数和调用方法:从构造函数(Java、MySQL)设置具有类作用域的变量时出现问题,java,mysql,variables,constructor,scope,Java,Mysql,Variables,Constructor,Scope,我试图用构造函数设置一个具有类作用域的变量,每当我从构造函数中打印出来时,该变量被设置为正确的值,但每当我试图在同一类中的另一个方法中使用该变量时,该值为null。如何在构造函数外部正确使用firstNameTest变量 这是构造函数和调用方法: public class Member extends javax.swing.JFrame { DefaultTableModel rentalsTableModel; String customerID; String firstNameTe
public class Member extends javax.swing.JFrame {
DefaultTableModel rentalsTableModel;
String customerID;
String firstNameTest; //Variable declared with class scope.
public Member() {
initComponents();
rentalsTableModel = (DefaultTableModel)tblRentals.getModel();
}
Member(String firstName, String lastName, String phoneNumber, String email, String over18, String dateJoined, String dateExpired, String fines) throws SQLException {
System.out.println(firstName); // Successfully prints value.
firstNameTest = firstName;
System.out.println(firstNameTest); // Successfully prints value.
}
...
private void btnChangeCustomerActionPerformed(java.awt.event.ActionEvent evt) {
customerID = JOptionPane.showInputDialog(null, "Enter Customer ID.");
MemberDAO member = new MemberDAO();
try {
List membersDetails = member.getMembersDetails(customerID);
System.out.println(firstNameTest); // Value is null.
txtFullName.setText(firstNameTest);
} catch (SQLException ex) {
System.err.println(ex);
System.out.println("Failed to get Details.");
JOptionPane.showMessageDialog(null, "Failed to retrieve data.");
}
}
调用的方法:
public List<Member> getMembersDetails(String ID) throws SQLException{
List<Member> membersDetails = new ArrayList();
String getMembershipDetails = "SELECT first_name, last_name, phone_number, email, over_18, date_joined, date_expire, fines FROM members"
+ " WHERE member_id = " + ID + ";";
try {
DBConnection mc = new DBConnection();
dbConnection = mc.getConnection();
statement = dbConnection.createStatement();
ResultSet rs = statement.executeQuery(getMembershipDetails);
while(rs.next()){
String firstName = rs.getString("first_name");
String lastName = rs.getString("last_name");
String phoneNumber = rs.getString("phone_number");
String email = rs.getString("email");
String over18 = rs.getString("over_18");
String dateJoined = rs.getString("date_joined");
String dateExpired = rs.getString("date_expire");
String fines = rs.getString("fines");
System.out.println(firstName);
System.out.println(dateExpired);
Member m;
m = new Member(firstName, lastName, phoneNumber, email, over18, dateJoined, dateExpired, fines);
membersDetails.add(m);
System.out.println(membersDetails.get(0));
}
} catch (SQLException ex){
System.err.println(ex);
System.out.println("Failed to get Membership Details.");
return null;
} finally{
if (ps != null){
ps.close();
}
if (dbConnection != null){
dbConnection.close();
}
} return membersDetails;
}
公共列表getMembersDetails(字符串ID)抛出SQLException{
List membersDetails=新建ArrayList();
String getMembershipDetails=“选择名字、姓氏、电话号码、电子邮件、18岁以上、加入日期、过期日期、会员罚款”
+“WHERE member_id=“+id+”;”;
试一试{
DBConnection mc=新的DBConnection();
dbConnection=mc.getConnection();
statement=dbConnection.createStatement();
ResultSet rs=statement.executeQuery(getMembershipDetails);
while(rs.next()){
String firstName=rs.getString(“first_name”);
String lastName=rs.getString(“last_name”);
String phoneNumber=rs.getString(“电话号码”);
String email=rs.getString(“电子邮件”);
String over18=rs.getString(“over_18”);
String dateJoined=rs.getString(“date_joined”);
String dateExpired=rs.getString(“date_expired”);
字符串罚款=rs.getString(“罚款”);
System.out.println(名字);
System.out.println(过期日期);
m委员;
m=新会员(名字、姓氏、电话号码、电子邮件、年满18岁、已加入日期、已过期日期、罚款);
成员详细信息。添加(m);
System.out.println(membersDetails.get(0));
}
}catch(SQLException-ex){
系统错误打印项次(ex);
System.out.println(“未能获取成员详细信息”);
返回null;
}最后{
如果(ps!=null){
ps.close();
}
if(dbConnection!=null){
dbConnection.close();
}
}返回成员详细信息;
}
更改此行-
System.out.println(firstNameTest); // Value is null.
到
这里有很多代码,但只有部分代码是相关的-为什么需要这么多属性来演示这个问题?为什么需要一个数据库来演示这个问题?此外,您还没有真正展示您编写的代码的功能。如果没有一个简短但完整的程序来清楚地演示问题,很难帮助您。这似乎缺少一些重要的代码。您有两个构造函数,一个是public no arg,另一个是带参数的包,但是带参数的构造函数既没有链也没有调用类似
initComponents()
的东西。哪里会得到null?我为所有代码道歉,我在筛选和编辑它时遇到问题。Ninad我在btnChangeCustomerActionPerformed中得到null(…在这一行:System.out.println(firstNameTest);//值为null.Ty,用于建议,但当我尝试此操作时,IDE说它找不到符号,有什么想法吗?找不到哪个符号-membersDetails?get(0)之后的firstNameTest。您是一个图例。感谢heaps.+1
System.out.println(((Member)membersDetails.get(0)).firstNameTest); // Value is null.