编译/捕获异常[关闭]答案

【问题标题】：Compile/ Catch an exception [closed]编译/捕获异常[关闭]
【发布时间】：2012-03-08 05:42:47
【问题描述】：

我在处理这段代码时遇到了很多麻烦。在我尝试输入代码以捕获异常之前，代码已按预期编译和运行。然后我再也无法编译它。如果用户输入负数，则假设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);

        }
    }

【问题讨论】：

您尝试编译时遇到的错误是什么？
SquareRoot.java:61: 异常 java.io.IOException 永远不会在相应的 try 语句 catch(IOException e) 的主体中抛出 ^
Java unreachable catch block compiler error 的可能重复项

标签： java exception exception-handling try-catch catch-block

【解决方案1】：

你不需要try-catch来检查否定，下面应该足够了。但是你在格式化的时候可能需要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.");
}

【讨论】：

是的，但你应该捕获 NumberFormatException 以防有人试图将“foo”传递给 parseDouble()。

【解决方案2】：

更好用：

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));

【讨论】：

是的，但对于我的任务，我必须捕获一个异常。

【解决方案3】：

Netbeans 说“在相应的 if 语句中永远不会抛出异常 java.io.IOException”。

基本上 if 语句不会抛出异常。

【讨论】：

【解决方案4】：

抛出一个自定义异常..

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

【讨论】：

【解决方案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);

  }
}

【讨论】：