问题描述
- 关于java中if 和showMessageDialog(）错误的问题。
-
这段代码好奇怪。
1.它能编译通过。
2.但是这一段有点奇怪
if((psw1==psw2))
{
JOptionPane.showMessageDialog(frame, "alert");
System.out.println(psw1);
}什么问题呢？ 1.不论psw1==psw2是否成立，shouwMessageDialog 永远没有执行（或许执行了，但没弹出对话框）.我试过把 if（）去了，或把 showMessageDialog 放到 Main（）里，打还是没反应。 2.不论psw1==psw2是否成立，、System.out.println(psw1);都会被执行。。 这是怎么回事？是bug吗？还是我犯了其他 隐秘的错误? 欲知真相， 敬请关注以下源码。import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.FlowLayout;
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.JPasswordField;
import javax.swing.JTextField;public class winReg implements ActionListener{
private JFrame frame = new JFrame("register"); private Container c = frame.getContentPane(); private JTextField username = new JTextField(); private JPasswordField password = new JPasswordField(); private JPasswordField password2= new JPasswordField(); private JButton confirm = new JButton("confirm"); private JButton cancel = new JButton("cancel"); public winReg(){ frame.setSize(600,400); c.setLayout(new BorderLayout()); initFrame(); confirm.addActionListener(this); cancel.addActionListener(this); frame.setVisible(true); } private void initFrame() { //顶部 JPanel titlePanel = new JPanel(); titlePanel.setLayout(new FlowLayout()); titlePanel.add(new JLabel("new register")); c.add(titlePanel,"North"); //中部表单 JPanel fieldPanel = new JPanel(); fieldPanel.setLayout(null); JLabel l1 = new JLabel("user name:"); l1.setBounds(50, 20, 50, 20); JLabel l2 = new JLabel("password:"); JLabel l3 = new JLabel("confirm password:"); l2.setBounds(50, 60, 100, 20); l3.setBounds(50, 90, 150, 20); fieldPanel.add(l1); fieldPanel.add(l2); fieldPanel.add(l3); username.setBounds(230,20,120,20); password.setBounds(230,60,120,20); password2.setBounds(230,90,120,20); fieldPanel.add(username); fieldPanel.add(password); fieldPanel.add(password2); c.add(fieldPanel,"Center"); //底部按钮 JPanel buttonPanel = new JPanel(); buttonPanel.setLayout(new FlowLayout()); buttonPanel.add(confirm); buttonPanel.add(cancel); c.add(buttonPanel,"South"); } public static void main(String[] args){ JOptionPane.showMessageDialog(frame, "alert", "alert", JOptionPane.ERROR_MESSAGE); new winReg(); } @Override public void actionPerformed(ActionEvent e) { // TODO Auto-generated method stub if((e.getSource()==confirm)){ String usn=username.getText(); String psw1=password.getPassword(); String psw2=password.getPassword(); //check wether psw same if((psw1==psw2)) { JOptionPane.showMessageDialog(frame, "alert"); System.out.println(psw1); } } }}
解决方案
首先，你在main方法中用了frame，main方法是静态的，你的frame也要声明成static的
``` private JFrame frame = new JFrame("register");
要改成：
```private static JFrame frame = new JFrame("register");
其次，你的JOptionPane类要导入，加代码
```import javax.swing.JOptionPane;
最后，password.getPassword();返回值类型是char[]
String psw1=password.getPassword();
String psw2=password.getPassword();
要换成：
String psw1=String.valueOf(password.getPassword());
String psw2=String.valueOf(password.getPassword());
修改后的完整代码：
（你的类名首字母要大写，等号左右两端要空开，花括号要层层嵌入，这些编写代码的基本风格要注意，这些都没帮你改，需要注意）
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.FlowLayout;
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.JPasswordField;
import javax.swing.JTextField;
import javax.swing.JOptionPane;
public class winReg implements ActionListener{
private static JFrame frame = new JFrame("register");
private Container c = frame.getContentPane();
private JTextField username = new JTextField();
private JPasswordField password = new JPasswordField();
private JPasswordField password2= new JPasswordField();
private JButton confirm = new JButton("confirm");
private JButton cancel = new JButton("cancel");
public winReg(){
frame.setSize(600,400);
c.setLayout(new BorderLayout());
initFrame();
confirm.addActionListener(this);
cancel.addActionListener(this);
frame.setVisible(true);
}
private void initFrame() {
//顶部
JPanel titlePanel = new JPanel();
titlePanel.setLayout(new FlowLayout());
titlePanel.add(new JLabel("new register"));
c.add(titlePanel,"North");
//中部表单
JPanel fieldPanel = new JPanel();
fieldPanel.setLayout(null);
JLabel l1 = new JLabel("user name:");
l1.setBounds(50, 20, 50, 20);
JLabel l2 = new JLabel("password:");
JLabel l3 = new JLabel("confirm password:");
l2.setBounds(50, 60, 100, 20);
l3.setBounds(50, 90, 150, 20);
fieldPanel.add(l1);
fieldPanel.add(l2);
fieldPanel.add(l3);
username.setBounds(230,20,120,20);
password.setBounds(230,60,120,20);
password2.setBounds(230,90,120,20);
fieldPanel.add(username);
fieldPanel.add(password);
fieldPanel.add(password2);
c.add(fieldPanel,"Center");
//底部按钮
JPanel buttonPanel = new JPanel();
buttonPanel.setLayout(new FlowLayout());
buttonPanel.add(confirm);
buttonPanel.add(cancel);
c.add(buttonPanel,"South");
}
public static void main(String[] args){
JOptionPane.showMessageDialog(frame, "alert", "alert", JOptionPane.ERROR_MESSAGE);
new winReg();
}
@Override
public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub
if((e.getSource()==confirm)){
String usn=username.getText();
String psw1=String.valueOf(password.getPassword());
String psw2=String.valueOf(password.getPassword());
//check wether psw same
if((psw1==psw2))
{
JOptionPane.showMessageDialog(frame, "alert");
System.out.println(psw1);
}
}
}
}
解决方案二：
代码写错了。string psw1=password.getpassword() string psw2=password2.getPassword()
如果按照你的写法，psw1永远等于psw2.至于showMessageDialog为什么没出现 对awt不熟悉所以无法解答
解决方案三：
==比较的是地址，用equals来比较字符字符串
解决方案四：
非常非常感谢各位的回答。此问题已找到答案。无奈近来项目进展缓慢，一直没时间上csdn，因此未能及时结贴。实在抱歉
解决方案五：
if中用equals比较看看，psw可是字符串