编译/捕获异常

Question

我在使用这段代码时遇到了很多麻烦。在我试图放入代码以捕获异常之前，代码按照预期进行了编译和运行。然后我再也无法编译它了。假设在用户输入负数的情况下捕获异常并生成错误消息。

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

您不需要尝试catch来检查负数，下面就足够了。但在进行格式化时，您可能需要尝试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.");
}

Answer 2

最好使用：

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

Answer 3

Netbeans表示"异常java.io.IOException从未在相应的if语句中抛出"。

基本上，if语句不抛出异常。

Answer 4

抛出自定义异常。。

              try
                {
                    if(NumberX >=0);
                    else
                      throw new Exception("Number Negative");
                }
                catch(Exception e)
                {
                    System.out.println("Number can not be negative."+e.getMessage());
                }

Answer 5

运行良好。

        package com.test;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class SquareRoot extends JFrame {
  /**
   * 
   */
  private static final long serialVersionUID = 1L;
  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;
        try {
          NumberX = Double.parseDouble(NumberField.getText());
          if (NumberX < 0) {
            AnswerField.setText("Please enter positive value.");
          }
          else if (NumberX >= 0) {
            double Answer = Math.sqrt(NumberX);
            AnswerField.setText(String.valueOf(Answer));
          }
        }
        catch (NumberFormatException e) {
          AnswerField.setText("Incorrect number entered.");
          e.printStackTrace();
        }
      }
    }
    ActionListener listener = new CalculateListener();
    button.addActionListener(listener);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
  }
}