| 发表于：2008-01-10 22:51:43 楼主 |
我做了一个登录框的程序,可是有个问题感觉很奇怪!程序如下;
import javax.swing.joptionpane;
import javax.swing.jdialog;
import javax.swing.jlabel;
import javax.swing.jtextfield;
import javax.swing.jpasswordfield;
import javax.swing.jpanel;
import javax.swing.jframe;
import javax.swing.jbutton;
import java.awt.dimension;
import java.awt.borderlayout;
import java.awt.flowlayout;
import java.awt.toolkit;
import java.awt.event.windowlistener;
import java.awt.event.windowevent;
import java.awt.event.windowadapter;
import java.awt.event.actionlistener;
import java.awt.event.actionevent;
public class logintest extends jframe
{
private jpanel panel1 = new jpanel（);
private jpanel panel2 = new jpanel（);
private jpanel panel3 = new jpanel（);
private jlabel lblname = new jlabel（"姓名");
private jlabel lblpass = new jlabel（"密码");
private jtextfield field1 = new jtextfield（10);
private jpasswordfield field2 = new jpasswordfield（10);
private jbutton btnlogin = new jbutton（"登录");
private jbutton btnexit = new jbutton（"退出");
public logintest（string title)
{
super（title);
this.add（panel1,borderlayout.north);
this.add（panel2,borderlayout.center);
this.add（panel3,borderlayout.south);
panel1.setlayout（new flowlayout（flowlayout.center,10,15));
panel2.setlayout（new flowlayout（flowlayout.center,10,9));
panel3.setlayout（new flowlayout（flowlayout.center,24,8));
panel1.add（lblname);
panel1.add（field1);
panel2.add（lblpass);
panel2.add（field2);
panel3.add（btnlogin);
panel3.add（btnexit);
addwindowlistener（new windowadapter（)
{
public void windowclosing（windowevent event)
{
system.exit（0);
}
});
jdialog.setdefaultlookandfeeldecorated（true);
btnlogin.addactionlistener（new actionlistener（)
{
public void actionperformed（actionevent event)
{
if（field1.gettext（).tostring（).equals（"") & field2.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"用户名和密码都不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
else if（field1.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"用户名不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
else if（field2.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"密码不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
}
});
this.setresizable（flase);
setsize（400,300);
dimension d = toolkit.getdefaulttoolkit（).getscreensize（);
this.setlocation（（d.width-400)/2,（d.height-300)/2);
setvisible（true);
}
public static void main（string[] args)
{
jframe.setdefaultlookandfeeldecorated（true);
new logintest（"登录");
}
}
上面这段程序没有问题,就是想不通的是那个if...else语句.我如果把第二和第三个else语句全部写成if,如;
btnlogin.addactionlistener（new actionlistener（)
{
public void actionperformed（actionevent event)
{
if（field1.gettext（).tostring（).equals（"") & field2.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"用户名和密码都不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
if（field1.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"用户名不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
if（field2.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"密码不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
}
});
当我两个输入框都为空的时候,先弹出用户名和密码不能为空的对话框,接着另外两个对话框也弹出来了,如过我把第三个else语句直接写成else,如;
btnlogin.addactionlistener（new actionlistener（)
{
public void actionperformed（actionevent event)
{
if（field1.gettext（).tostring（).equals（"") & field2.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"用户名和密码都不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
else if（field1.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"用户名不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
else（field2.gettext（).tostring（).equals（""))
{
joptionpane.showconfirmdialog（logintest.this,"密码不能为空","警告",
joptionpane.yes_no_option,joptionpane_error_message);
}
}
});
程序就抱错,说我 else（field2.gettext（).tostring（).equals（""))不是语句,并且后面还差;非要if后面的两个控制语句
都要用else if才不会错,所以我一直感到很疑惑,望大家指点指点是什么原因.按理说应该是if...else if....else或者if...if...if都可以啊!
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