Question

这是我班级的小程序。为什么我的最终g.drawString为我所有的变量打印0？

import javax.swing.*;
import java.awt.*;

public class Days extends JApplet
{
private int month;

private int  day; 

private int  year_string,

private int total_days;

private int month_days;

public final int minMonth = 1; 

public final int maxMonth = 12; 

public final int minDay = 1; 

public final int maxDay = 31; 

public final int minYear = 1000; 

public final int maxYear = 2999;

public void init()
  {
    String month_string,
           day_string,
           year_string;

    month_string = JOptionPane.showInputDialog("Enter the month (" + minMonth +
                                               "-" + maxMonth + ")");
    day_string   = JOptionPane.showInputDialog("Enter the day (" + minDay +
                                               "-" + maxDay + ")");
    year_string  = JOptionPane.showInputDialog("Enter the year (" + minYear +
                                               "-" + maxYear + ")");
     int month = Integer.parseInt( month_string );
     int day = Integer.parseInt( day_string );
     int year = Integer.parseInt( year_string );



    switch (month){
      case 1: 
              month_days = 0;
                 break;
      case 2: 
              month_days = 31;
                 break;
      case 3: 
              month_days = 59;
                 break;
      case 4: ;
              month_days = 90;
                 break;
      case 5: 
              month_days = 120;
                 break;
      case 6: 
              month_days = 151;
                 break;
      case 7: 
              month_days = 181;
                 break;
      case 8: 
              month_days = 212;
                 break;
      case 9: 
              month_days = 243;
                 break;
      case 10: 
              month_days = 273;
                 break;
      case 11:
               month_days = 304;
                  break;
      case 12: 
               month_days = 334;
                  break;        

        }

     int total_days = month_days + day;
  }

  public void paint(Graphics g)
  {


     g.drawString("The number of days from the beginning of " + year_string + " to " + month + "/" + day + "/" + year_string +
                  "= " + total_days, 40, 30);

    showStatus("Alex Collins");
  }

}

Answer 1

发布的来源不在这里编译。当修复编译错误时，似乎问题是变量被声明为类属性，以及方法的本地。本地声明的属性是“遮蔽”我们需要的另一个属性，但在声明它的方法的末尾消失（超出范围）。

请注意：

Applet比（基于框架的）应用程序更难开发和调试。
应用程序比命令行应用程序更难编码。
变量作用域是Java 101，应该在命令行应用程序中进行整理。

这是applet的工作版本。

import javax.swing.*;
import java.awt.*;

public class Days extends JApplet
{
private int month;
private int  day;
private String  year_string;
private String  month_string;
private String  day_string;
private int total_days;
private int month_days;
public final int minMonth = 1;
public final int maxMonth = 12;
public final int minDay = 1;
public final int maxDay = 31;
public final int minYear = 1000;
public final int maxYear = 2999;

public void init()
  {
    month_string = JOptionPane.showInputDialog("Enter the month (" + minMonth +
                                               "-" + maxMonth + ")");
    day_string   = JOptionPane.showInputDialog("Enter the day (" + minDay +
                                               "-" + maxDay + ")");
    year_string  = JOptionPane.showInputDialog("Enter the year (" + minYear +
                                               "-" + maxYear + ")");
     int month = Integer.parseInt( month_string );
     int day = Integer.parseInt( day_string );
     int year = Integer.parseInt( year_string );



    switch (month){
      case 1:
              month_days = 0;
                 break;
      case 2:
              month_days = 31;
                 break;
      case 3:
              month_days = 59;
                 break;
      case 4: ;
              month_days = 90;
                 break;
      case 5:
              month_days = 120;
                 break;
      case 6:
              month_days = 151;
                 break;
      case 7:
              month_days = 181;
                 break;
      case 8:
              month_days = 212;
                 break;
      case 9:
              month_days = 243;
                 break;
      case 10:
              month_days = 273;
                 break;
      case 11:
               month_days = 304;
                  break;
      case 12:
               month_days = 334;
                  break;

        }
     total_days = month_days + day;
  }

  public void paint(Graphics g)
  {
     g.drawString("The number of days from the beginning of " + year_string + " to " + month + "/" + day + "/" + year_string +
                  "= " + total_days, 10, 30);
    showStatus("Alex Collins");
  }
}

Applet不携带从init方法到paint方法的变量

1 个答案: