java - 编译/捕获异常

Question

我在使用这段代码时遇到了很多麻烦。在我尝试放入代码以捕获异常之前，代码按预期编译并运行。然后我就无法再编译它了。如果用户输入负数，它会捕获异常并产生错误消息。

import java.awt.event.ActionEvent; //Next group of lines import various Java classes
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JCheckBox;
import javax.swing.JPanel;
import javax.swing.JTextField;
import java.awt.GridLayout;
import java.text.*;
import java.io.IOException;

public class SquareRoot extends JFrame
{
    public static void main(String[] args)  {
        //Creates Window
        JFrame frame = new JFrame();
        frame.setSize(450, 300);
        frame.setTitle("Find the Square Root");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JLabel Numberlbl = new JLabel("Enter a number:");
        final JTextField NumberField = new JTextField(10);
        NumberField.setText("");

        JLabel Answerlbl = new JLabel("Square Root of your number is:");
        final JTextField AnswerField = new JTextField(10);
        AnswerField.setText("");

        JLabel ButtonLabel = new JLabel("Calculate Square Root");
        JButton button = new JButton("Find Square Root");

        JPanel panel = new JPanel();
        panel.setLayout(new GridLayout(3,2));
        panel.add(Numberlbl);
        panel.add(NumberField);
        panel.add(ButtonLabel);
        panel.add(button);
        panel.add(Answerlbl);
        panel.add(AnswerField);
        frame.add(panel);

        class CalculateListener implements ActionListener {

            public void actionPerformed(ActionEvent event) {

                double NumberX = Double.parseDouble(NumberField.getText());
                try
                {
                    if(NumberX >=0);
                }
                catch(IOException e)
                {
                    System.out.println("Number can not be negative.");
                }
                double Answer = Math.sqrt(NumberX);
                AnswerField.setText(String.valueOf(Answer));

            }
        }

        ActionListener listener = new CalculateListener();
        button.addActionListener(listener);

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

        }
    }

Answer 1

您不需要 try-catch 来检查是否定的，下面应该足够了。但格式化时可能需要 try-catch。

double noX = 0;
try {
   noX = Double.parseDouble(NumberField.getText());
} catch(NumberFormatException e) {
   System.out.println("Not a valid number");
}
if(noX < 0) {
   System.out.println("Number can not be negative.");
}