field.setToolTipText("Enter your Password");

import javax.swing.*;  
public class ToolTipExample {
public static void main(String[] args) {  
 JFrame f=new JFrame("Password Field Example");  
 //Creating PasswordField and label
 JPasswordField value = new JPasswordField(); 
 value.setBounds(100,100,100,30);  
 value.setToolTipText("Enter your Password");
 JLabel l1=new JLabel("Password:");  
 l1.setBounds(20,100, 80,30);  
 //Adding components to frame
 f.add(value);  f.add(l1);
 f.setSize(300,300);  
 f.setLayout(null);  
 f.setVisible(true);   
}
}