由于试图捕获多个异常，出现了错误“局部变量年龄可能尚未初始化”

Question

错误“局部变量 age 可能尚未初始化”是由于尝试捕获多个异常而发生的。

你好。在前一个问题得到解决之后，我决定对程序做一个小小的改动，使用JOptionPane将控制台中的打印转换成一个简单的GUI，这时出现了一个新的错误。我试图捕捉两个名为NumberFormatException和NullPointerException的特定异常。NumberFormatException是在JOptionPane.showMessageDialog的空白处输入数字以外的字符的结果，因为该变量已被声明为double。单击X符号或取消按钮会导致NullPointerException。为了捕捉NullPointerException，我在NumberFormatException后面添加了一些额外的代码。但是，当测试运行该程序时，控制台中会出现以下错误:

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized
The local variable age may not have been initialized

at Welcome.main(Welcome.java:10)

实际上，在类的开头，我已经将变量age声明为Double，尽管如此，“可能没有被初始化”的错误仍然存在。你能提供一个解决这个错误的方法吗？如果代码catch（NullPointerException e）{JOptionPane.showMessageDialog（null，“程序已取消。”）不存在，则不会发生错误。

顺便说一下，除了主要问题之外，你能在JOptionPane.showMessageDialog.的括号内解释一下“null”的含义和用法吗？非常感谢。我的代码如下：

import javax.swing.*;

public class Welcome {
    public static void main(String[] args) {
        double age;
        while (true) {
            try {
                age = Double.parseDouble(JOptionPane.showInputDialog(null, "please enter your age and we will define your age group"));
            } catch (NumberFormatException e) {
                JOptionPane.showMessageDialog(null, "Input Error. "
                        + "Please enter a number corresponding to your age only.", "error", JOptionPane.ERROR_MESSAGE);
            } catch (NullPointerException e) {
                JOptionPane.showMessageDialog(null, "Program canceled.");
                continue;
            }
            if (age < 0) JOptionPane.showMessageDialog(null, "Warning!Negative values cannot hold true.",
                    "warning", JOptionPane.WARNING_MESSAGE);
            else if (age <= 0.1) JOptionPane.showMessageDialog(null, "You are a newborn.");
            else if (age <= 1) JOptionPane.showMessageDialog(null, "You are an infant.");
            else if (age <= 3) JOptionPane.showMessageDialog(null, "You are a toddler.");
            else if (age <= 5) JOptionPane.showMessageDialog(null, "You are a preschooler.");
            else if (age <= 13) JOptionPane.showMessageDialog(null, "You are a school-aged child.");
            else if (age <= 19) JOptionPane.showMessageDialog(null, "You are an adolescent and a denarian.");
            else if (age <= 29) JOptionPane.showMessageDialog(null, "You are a vicenarian.");
            else if (age <= 39) JOptionPane.showMessageDialog(null, "You are a tricenarian.");
            else if (age <= 49) JOptionPane.showMessageDialog(null, "You are a quadragenarian.");
            else if (age <= 59) JOptionPane.showMessageDialog(null, "You are a quinquagenarian.");
            else if (age <= 69) JOptionPane.showMessageDialog(null, "You are a sexagenarian.");
            else if (age <= 79) JOptionPane.showMessageDialog(null, "You are a septuagenarian.");
            else if (age <= 89) JOptionPane.showMessageDialog(null, "You are a octogenarian.");
            else if (age <= 99) JOptionPane.showMessageDialog(null, "You are a nonagenarian.");
            else if (age <= 109) JOptionPane.showMessageDialog(null, "You are a centenarian.");
            else if (age <= 150) JOptionPane.showMessageDialog(null, "You are a supercentenarian.");
            else if (age > 150)
                JOptionPane.showMessageDialog(null, "Warning!The value entered is too large to be processed.", "warning", JOptionPane.WARNING_MESSAGE);
        }
    }
}

Answer 1

问题是，如果您的代码输入以下代码行，则变量 age 实际上不会被值：

     catch (NumberFormatException e) {
        JOptionPane.showMessageDialog(null, "Input Error. "+ "Please enter a number corresponding to your age only.", "error", JOptionPane.ERROR_MESSAGE);
    }

Java 在编译时检查是否已正确初始化所有变量。如果编译器认为变量在下一条指令之前可能尚未初始化，则会获得此错误。

您可以通过简单地初始化变量来解决此问题：

     catch (NumberFormatException e) {
        age = 0D;
        JOptionPane.showMessageDialog(null, "Input Error. "+ "Please enter a number corresponding to your age only.", "error", JOptionPane.ERROR_MESSAGE);
    }

但我认为正确的方式是:

     catch (NumberFormatException e) {
        JOptionPane.showMessageDialog(null, "Input Error. "+ "Please enter a number corresponding to your age only.", "error", JOptionPane.ERROR_MESSAGE);
        continue;
    }

关于您的第二个问题，关于show MessageDialog，您可以在留档中阅读：

public static void showMessageDialog(Component parentComponent, Object message) throws HeadlessException
    Brings up an information-message dialog titled "Message".
    Parameters:
        * parentComponent - determines the Frame in which the dialog is displayed; if null, or if the parentComponent has no Frame, a default Frame is used
        * message - the Object to display

Answer 2

假设有人在弹出窗口中输入“hello”。< code > joptionpane . showinputdialog 方法返回的结果很好，并且< code >“Hello”被传递给< code>Double.parseDouble。这导致引发< code > NumberFormatException ，因此代码切换到catch块。

此catch块将打印一条关于需要输入数字的消息，然后..代码继续。毕竟，捕获意味着您要处理它（如果您想做一些事情，但随后继续执行异常过程，然后重新抛出它，或者做一些其他事情来影响控制流，例如继续；，这是您在NullPointer捕获块中做的！）

代码继续到＜Code＞if（年龄

这里的主要解决方案似乎是编写控制流，使得 if（age）

或者，选择一个用作默认值或占位符的期限，以指示用户尚未输入内容。但这听起来不像是这种情况的最佳解决方案 - 在用户实际输入年龄之前，您不想处理年龄。

Answer 3

尝试将所有年龄验证登录移到Try块中，其中年龄，变量是100%定义的。像这样：

try {
    // here you initialise the "age" variable
    final double age = Double.parseDouble(JOptionPane.showInputDialog(null, "please enter your age and we will define your age group"));
    
    // here you work with the "age" variable
    if (age < 0) JOptionPane.showMessageDialog(null, "Warning!Negative values cannot hold true.",
                    "warning", JOptionPane.WARNING_MESSAGE);
    else if (age <= 0.1) JOptionPane.showMessageDialog(null, "You are a newborn.");
    else if (age <= 1) JOptionPane.showMessageDialog(null, "You are an infant.");
    else if (age <= 3) JOptionPane.showMessageDialog(null, "You are a toddler.");
    else if (age <= 5) JOptionPane.showMessageDialog(null, "You are a preschooler.");
    else if (age <= 13) JOptionPane.showMessageDialog(null, "You are a school-aged child.");
    else if (age <= 19) JOptionPane.showMessageDialog(null, "You are an adolescent and a denarian.");
    else if (age <= 29) JOptionPane.showMessageDialog(null, "You are a vicenarian.");
    else if (age <= 39) JOptionPane.showMessageDialog(null, "You are a tricenarian.");
    else if (age <= 49) JOptionPane.showMessageDialog(null, "You are a quadragenarian.");
    else if (age <= 59) JOptionPane.showMessageDialog(null, "You are a quinquagenarian.");
    else if (age <= 69) JOptionPane.showMessageDialog(null, "You are a sexagenarian.");
    else if (age <= 79) JOptionPane.showMessageDialog(null, "You are a septuagenarian.");
    else if (age <= 89) JOptionPane.showMessageDialog(null, "You are a octogenarian.");
    else if (age <= 99) JOptionPane.showMessageDialog(null, "You are a nonagenarian.");
    else if (age <= 109) JOptionPane.showMessageDialog(null, "You are a centenarian.");
    else if (age <= 150) JOptionPane.showMessageDialog(null, "You are a supercentenarian.");
    else if (age > 150)
                JOptionPane.showMessageDialog(null, "Warning!The value entered is too large to be processed.", "warning", JOptionPane.WARNING_MESSAGE);
} catch (NumberFormatException e) {
     JOptionPane.showMessageDialog(null, "Input Error. "
                        + "Please enter a number corresponding to your age only.", "error", JOptionPane.ERROR_MESSAGE);
} catch (NullPointerException e) {
    JOptionPane.showMessageDialog(null, "Program canceled.");
                continue;
}