i have change this line to the following line
if (button instanceof JButton)..
and true enough,i faced these problems...
below is the error when i click  the button..

java.lang.NullPointerException
at PasswordBox.actionPerformed(PasswordBox.java:80)
at javax.swing.AbstractButton.fireActionPerformed
(AbstractButton.java:1764)
at
javax.swing.AbstractButton$ForwardActionEvents.actionPerformed
(AbstractButton.java:1817)
at javax.swing.DefaultButtonModel.fireActionPerformed
(DefaultButtonModel.java:419)
at javax.swing.DefaultButtonModel.setPressed
(DefaultButtonModel.java:257)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased
(BasicButtonListener.java:245)
at java.awt.Component.processMouseEvent(Component.java:5093)
at java.awt.Component.processEvent(Component.java:4890)
at java.awt.Container.processEvent(Container.java:1566)
at java.awt.Component.dispatchEventImpl(Component.java:3598)
at java.awt.Container.dispatchEventImpl(Container.java:1623)
at java.awt.Component.dispatchEvent(Component.java:3439)
at java.awt.LightweightDispatcher.retargetMouseEvent
(Container.java:3450)
at java.awt.LightweightDispatcher.processMouseEvent
(Container.java:3165)
at java.awt.LightweightDispatcher.dispatchEvent
(Container.java:3095)
at java.awt.Container.dispatchEventImpl(Container.java:1609)
at java.awt.Window.dispatchEventImpl(Window.java:1585)
at java.awt.Component.dispatchEvent(Component.java:3439)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:450)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy
(EventDispatchThread.java:197)
at java.awt.EventDispatchThread.pumpEventsForHierarchy
(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents
(EventDispatchThread.java:144)
at java.awt.EventDispatchThread.pumpEvents
(EventDispatchThread.java:136)
at java.awt.EventDispatchThread.run
(EventDispatchThread.java:99)