java - JOptionPane.showMessageDialog 无效 AnnotationName

Question

在学习一些Java初级类(class)后，遇到了这个问题:

Multiple markers at this line
    - Syntax error on token "showMessageDialog", invalid 
     AnnotationName
    - Syntax error on token "(", { expected after this token
    - Syntax error on tokens, ConstructorHeaderName expected instead

该行是:

JOptionPane.showMessageDialog(null, "The answer is " +sum, "The Title", JOptionPane.INFORMATION_MESSAGE);

Java 文档说此方法接受 4 个参数，因此我发送 4 个参数。我还导入了所需的库:

import javax.swing.JOptionPane;

JOptionPane.showInputDialog 工作正常，但 JOptionPane.showMessageDialog 不行。我缺少什么？ 谢谢。

---

完整代码

package java_practice;
import javax.swing.JOptionPane;

public class GraphicalUserInterface {
    String first_number = JOptionPane.showInputDialog("Enter first number");
    String second_number = JOptionPane.showInputDialog("Enter second number");

    int num1 = Integer.parseInt(first_number);
    int num2 = Integer.parseInt(second_number);
    int sum = num1 + num2;

    JOptionPane.showMessageDialog(null, "The answer is " +sum, "The Title", JOptionPane.INFORMATION_MESSAGE);
}

Answer 1

您错过了方法声明。您的代码位于类的主体内部，这会混淆编译器/IDE，例如有关无效注释等的错误

需要位于某种形式的方法、 block 或静态 block 内。

public class GraphicalUserInterface {
    public static void main(String [] args) {  //  <=========

        String first_number = JOptionPane.showInputDialog("Enter first number");
        String second_number = JOptionPane.showInputDialog("Enter second number");

        int num1 = Integer.parseInt(first_number);
        int num2 = Integer.parseInt(second_number);
        int sum = num1 + num2;

        JOptionPane.showMessageDialog(null, "The answer is " +sum, "The Title", JOptionPane.INFORMATION_MESSAGE);

   } //  <=========
}