Java 将字符串的值保存到另一个类并检索它
我目前正在尝试将字符串的值保存到JAVA中另一个名为Memory.JAVA的类文件中。我之所以这样做是因为变量不会保存在Try-Catch块之外。因此,我使用以下代码在try-catch块中启动该类:Java 将字符串的值保存到另一个类并检索它,java,string,Java,String,我目前正在尝试将字符串的值保存到JAVA中另一个名为Memory.JAVA的类文件中。我之所以这样做是因为变量不会保存在Try-Catch块之外。因此,我使用以下代码在try-catch块中启动该类: Memory mem = new Memory(); 然后,当我想要保存字符串时,我使用以下命令: mem.brother1ID = "Whatever"; 我之所以不将其创建为新字符串,是因为在Memory类中,我已经启动了该字符串。为了测试是否已保存,我使用了System.out.pri
Memory mem = new Memory();
然后,当我想要保存字符串时,我使用以下命令:
mem.brother1ID = "Whatever";
我之所以不将其创建为新字符串,是因为在Memory类中,我已经启动了该字符串。为了测试是否已保存,我使用了System.out.println
打印结果,在本例中,结果是“Whatever”,但当我尝试在同一类中打印相同的结果时,结果是“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:
下面是一些代码:
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);
}
}
这就是我试图获得与以前相同的System.out.println结果的时候:
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);
}
}
内存对象中的值brother1ID
为null
的原因是您正在使用该对象的新实例
我强烈建议您阅读Java中对象实例的作用域,以及创建新实例与使用现有实例的区别
例如,执行此操作时:
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。如果希望在多个方法调用中保持这些值,最好将内存
对象保存为包含所显示方法的任何类的实例变量。i、 e:
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'
}
请看这里
try {
Memory mem = new Memory();
String sql = "select * from userInfo where id=?";
pst=conn.prepareStatement(sql);
String IDNO = mem.brother1ID;
System.out.println(IDNO);
您正在创建内存对象,而没有设置任何值。
然后,您试图打印将为null的字符串,所有字符串变量将初始化为null,这就是为什么您得到“null”。您可以做的是将打印语句从底部的try-catch块中取出,如果数据库中有任何记录,它将打印最后的记录。到目前为止,您所解释的情况非常混乱。我认为您需要发布您遇到问题的整个代码片段。好的,我将添加到我的帖子中。与其试图描述您的代码,为什么不简单地发布一些示例代码呢?在这里,我已经添加了一些示例代码。每次您执行Memory mem=new Memory()
您都在创建一个全新的,完全不同的内存实例
。它们不共享任何状态,因此您设置为一个的任何值都无法从另一个读取。请您解释一下我将如何使用此。内存将变量保存到内存类?已添加。同样,我认为您可能应该使用一些小型Java类,并使用实例变量进行实验。这是一个需要掌握的非常重要的概念,虽然大型项目可以通过查看功能获得回报,但您可能一次处理的内容有点太多。只是想知道,当您可以只执行memory.brother1ID时,为什么要使用This.memory.brother1ID?只是为了显式地访问This
的实例变量。在这种特殊情况下,这没有什么区别,但在我看来,最好先用this
作为实例变量访问的开头,以防您意外地拥有一个同名的局部变量。