if(!strl[0].equals("")&&!strl[1].equals("")&&!strl[2].equals("")&&!strl[3].equals("")&&!strl[4].equals("")&&!strl[5].equals("")) {
            strl[6] = jcb.getSelectedItem().toString();
            sql = "insert into student (StuNO, StuName , StuSex, Class, Department, Password, Permitted) values('" +
            strl[0]+"','" + strl[1]+"','" + strl[2]+"','" +strl[3]+"','" + strl[4] +"','"+strl[5]+"','" +strl[6]+"')";
            db = new DataBase();
            int i=db.updateDb(sql);
         if(i==1) {
              JOptionPane.showMessageDialog(this,"添加成功！","提示",JOptionPane.INFORMATION_MESSAGE);              
                       return;        }
            else {
               JOptionPane.showMessageDialog(this,"添加失败！","提示",JOptionPane.INFORMATION_MESSAGE);
               return;
           }
            Vector<String> v = new Vector<String>();
            for(int j=0; j<6; j++) {
                v.add(strl[j]);
                if(j<6) {
                    jf[j].setText("");
                }
            }
这么写的时候，就会在Vector<String> v = new Vector<String>();显示有错误
但是这样写的时候：
if(!strl[0].equals("")&&!strl[1].equals("")&&!strl[2].equals("")&&!strl[3].equals("")&&!strl[4].equals("")&&!strl[5].equals("")) {
            strl[6] = jcb.getSelectedItem().toString();
            sql = "insert into student (StuNO, StuName , StuSex, Class, Department, Password, Permitted) values('" +
            strl[0]+"','" + strl[1]+"','" + strl[2]+"','" +strl[3]+"','" + strl[4] +"','"+strl[5]+"','" +strl[6]+"')";
            db = new DataBase();
            db.updateDb(sql);
            Vector<String> v = new Vector<String>();
            for(int j=0; j<6; j++) {
                v.add(strl[j]);
                if(j<6) {
                    jf[j].setText("");
                }
            }
就没有显示错误了，请问这是怎么回事？

应该是 Unreachable code 错误吧
你的if、else代码块中都有return，后面的代码执行不下去

十分感谢