我当前正在尝试将字符串的值保存到 JAVA 中名为 Memory.JAVA 的另一个类文件中。我这样做的原因是因为变量不会保存在 Try - Catch block 之外。因此，我使用以下代码在 try catch block 中启动该类:
Memory mem = new Memory();
然后当我想保存字符串时，我使用以下内容:
mem.brother1ID = "Whatever";
我之所以没有将其创建为新的字符串，是因为在Memory类中，我已经初始化了这个字符串。为了测试是否已保存，我使用 System.out.println 打印出结果，在本例中为“Whatever”，但是当我尝试以相同的方式打印出相同的结果时类中，我得到结果“null”。有人对我的问题有什么建议吗？请随时在下面发表评论。谢谢!
更新:
下面发布了一些代码:
private void searchFieldKeyReleased(java.awt.event.KeyEvent evt) {
try {
Memory mem = new Memory();
String sql = "select * from userInfo where firstName= ? OR lastname = ?";
pst=conn.prepareStatement(sql);
pst.setString(1, searchField.getText());
pst.setString(2, searchField.getText());
rs=pst.executeQuery();
if(rs.next()) {
String firstName = rs.getString("firstName");
String lastName = rs.getString("lastName");
String placeOfResidence = rs.getString("placeOfResidence");
String employmentStatus = rs.getString("employmentStatus");
String currentEmployer = rs.getString("currentEmployer");
String taxStatus = rs.getString("taxStatus");
String dateOfBirth = rs.getString("dateOfBirth");
String mother = rs.getString("mother");
String father = rs.getString("father");
String brother1 = rs.getString("brother1");
String brother2 = rs.getString("brother2");
String brother3 = rs.getString("brother3");
String brother4 = rs.getString("brother4");
String brother5 = rs.getString("brother5");
String sister1 = rs.getString("sister1");
String sister2 = rs.getString("sister2");
String sister3 = rs.getString("sister3");
String sister4 = rs.getString("sister4");
String sister5 = rs.getString("sister5");
mem.brother1ID = rs.getString("brother1ID");
mem.brother2ID = rs.getString("brother2ID");
mem.brother3ID = rs.getString("brother3ID");
mem.brother4ID = rs.getString("brother4ID");
mem.brother5ID = rs.getString("brother5ID");
mem.sister1ID = rs.getString("sister1ID");
mem.sister2ID = rs.getString("sister2ID");
mem.sister3ID = rs.getString("sister3ID");
mem.sister4ID = rs.getString("sister4ID");
mem.sister5ID = rs.getString("sister5ID");
mem.fatherID = rs.getString("fatherID");
mem.motherID = rs.getString("motherID");
System.out.println(mem.brother1ID);
System.out.println(firstName + " " + lastName);
firstNameField.setText(firstName);
lastNameField.setText(lastName);
placeOfResidenceField.setText(placeOfResidence);
employmentStatusField.setText(employmentStatus);
currentEmployerField.setText(currentEmployer);
taxStatusField.setText(taxStatus);
dateOfBirthField.setText(dateOfBirth);
motherField.setText(mother);
fatherField.setText(father);
brothersField1.setText(brother1);
brothersField2.setText(brother2);
brothersField3.setText(brother3);
brothersField4.setText(brother4);
brothersField5.setText(brother5);
sisterField1.setText(sister1);
sisterField2.setText(sister2);
sisterField3.setText(sister3);
sisterField4.setText(sister4);
sisterField5.setText(sister5);
}
} catch(Exception e) {
JOptionPane.showMessageDialog(null, e);
}
}
这是当我尝试获得与之前相同的 System.out.println 结果时:
private void brotherViewButton1ActionPerformed(java.awt.event.ActionEvent evt) {
try {
Memory mem = new Memory();
String sql = "select * from userInfo where id=?";
pst=conn.prepareStatement(sql);
String IDNO = mem.brother1ID;
System.out.println(IDNO);
pst.setString(1, IDNO);
rs=pst.executeQuery();
if(rs.next()) {
String firstName = rs.getString("firstName");
String lastName = rs.getString("lastName");
String placeOfResidence = rs.getString("placeOfResidence");
String employmentStatus = rs.getString("employmentStatus");
String currentEmployer = rs.getString("currentEmployer");
String taxStatus = rs.getString("taxStatus");
String dateOfBirth = rs.getString("dateOfBirth");
String mother = rs.getString("mother");
String father = rs.getString("father");
String brother1 = rs.getString("brother1");
String brother2 = rs.getString("brother2");
String brother3 = rs.getString("brother3");
String brother4 = rs.getString("brother4");
String brother5 = rs.getString("brother5");
String sister1 = rs.getString("sister1");
String sister2 = rs.getString("sister2");
String sister3 = rs.getString("sister3");
String sister4 = rs.getString("sister4");
String sister5 = rs.getString("sister5");
System.out.println(firstName + " " + lastName);
firstNameField.setText(firstName);
lastNameField.setText(lastName);
placeOfResidenceField.setText(placeOfResidence);
employmentStatusField.setText(employmentStatus);
currentEmployerField.setText(currentEmployer);
taxStatusField.setText(taxStatus);
dateOfBirthField.setText(dateOfBirth);
motherField.setText(mother);
fatherField.setText(father);
brothersField1.setText(brother1);
brothersField2.setText(brother2);
brothersField3.setText(brother3);
brothersField4.setText(brother4);
brothersField5.setText(brother5);
sisterField1.setText(sister1);
sisterField2.setText(sister2);
sisterField3.setText(sister3);
sisterField4.setText(sister4);
sisterField5.setText(sister5);
}
} catch(Exception e) {
JOptionPane.showMessageDialog(null, e);
}
}
最佳答案
Memory 对象中的值 brother1ID 为 null 的原因是因为您使用的是新实例 该对象的。
我强烈建议您阅读 Java 中对象实例的范围如何，以及创建新实例与使用现有实例的含义。
例如，当您这样做时:
Memory mem = new Memory();
mem.brother1ID = "1234";
mem = new Memory();
System.out.println(mem.brother1ID);
打印的值将为null。这是因为您正在使用该类的新实例。如果您想在多个方法调用中维护这些值，最好的选择可能是将 Memory 对象保存为包含您所显示的方法的任何类的实例变量。即:
private Memory memory = new Memory();
...
private void searchFieldKeyReleased(java.awt.event.KeyEvent evt) {
// Use 'this.memory'
this.memory.brother1ID = "1234";
//(or)
System.out.println(this.memory.brother1ID);
}
private void brotherViewButton1ActionPerformed(java.awt.event.ActionEvent evt) {
// Use 'this.memory'
}
关于java - 将字符串的值保存到另一个类并检索它，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25984189/