小问题，可就是不明白，呵呵，菜鸟不要见怪啊。

import javax.swing.JFrame;
import javax.swing.JOptionPane;
public class Test_ extends JFrame{
     public static void main(String arg[]){
         Test B=new Test();
         B.Fn();
         
     }
     void Fn(){
         String str=JOptionPane.showInputDialog("请输入你的成绩：");
         int a=Integer.parseInt(str);

         if (a>=60&&a<80)
              JOptionPane.showMessageDialog(this,"恭喜你及格了！,继续加油！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if (a>=80&&a<100);
          JOptionPane.showMessageDialog(this,"very good！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if (a<60&&a>40||a==40)
             JOptionPane.showMessageDialog(this,"you need to study hard！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if(a<40||a==40)
             JOptionPane.showMessageDialog(this,"hope you to have a good attitude","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if(a>100)
             JOptionPane.showMessageDialog(this,"error","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         
     }     
         

}



当我输入12时，它为什么还会弹出very good 啊？
----------------解决方案--------------------------------------------------------
你把Fn()里的代码换成下面的代码试试：
我只改了判断条件

----------------解决方案--------------------------------------------------------
import javax.swing.JFrame;
import javax.swing.JOptionPane;
public class Test_ extends JFrame{//应该是Test吧？？
     public static void main(String arg[]){
         Test B=new Test();
         B.Fn();
         
     }
     void Fn(){
         String str=JOptionPane.showInputDialog("请输入你的成绩：");
         int a=Integer.parseInt(str);
         if (a>=60&&a<80)
              JOptionPane.showMessageDialog(this,"恭喜你及格了！,继续加油！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if (a>=80&&a<100);//多加了一小分号
          JOptionPane.showMessageDialog(this,"very good！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if (a<60&&a>40||a==40)
             JOptionPane.showMessageDialog(this,"you need to study hard！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if(a<40||a==40)
             JOptionPane.showMessageDialog(this,"hope you to have a good attitude","成绩情况",JOptionPane.INFORMATION_MESSAGE);
         if(a>100)
             JOptionPane.showMessageDialog(this,"error","成绩情况",JOptionPane.INFORMATION_MESSAGE);         
     }            
}
//以下我的建议，请见谅哦哦
import javax.swing.JFrame;
import javax.swing.JOptionPane;
public class Test extends JFrame{
     public static void main(String arg[]){
         Test B=new Test();
         B.Fn();
         
     }
     void Fn(){
         String str=JOptionPane.showInputDialog("请输入你的成绩：");
         int a=Integer.parseInt(str);

         if (a>=60&&a<80){
            JOptionPane.showMessageDialog(this,"恭喜你及格了！,继续加油！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
            }
         else if (a>=80&&a<100){
        JOptionPane.showMessageDialog(this,"very good！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
        }
         else if (a<60&&a>40){
           JOptionPane.showMessageDialog(this,"you need to study hard！","成绩情况",JOptionPane.INFORMATION_MESSAGE);
           }
         else if(a<=40&&a>=0){
           JOptionPane.showMessageDialog(this,"hope you to have a good attitude","成绩情况",JOptionPane.INFORMATION_MESSAGE);
           }
         else{
           JOptionPane.showMessageDialog(this,"error","成绩情况",JOptionPane.INFORMATION_MESSAGE);
           }
         
     }
----------------解决方案--------------------------------------------------------