About static class

Dose Java has Static class?
What is diff between static class and a final class which all method are static in java?

Dose Java has Static class?yes, but only if the class is an inner class
a static inner class does not have a reference to its enclosing class - because of this, a static inner class cannot use this to access any instance variable of the enclosing class
however, a static inner may access all static variables of the enclosing class
What is diff between static class and a final class
which all method are static in java?a static inner class is linke any other class in java
a final class with static methods is like the java.lang.Math class - you cnnot subclass it and do normally not instantiate it
hope that helps
Harry

Similar Messages

  • Releasing static classes in a browser environment

    Hi all, got a problem I can't seem to find a solution for, so hoping someone knows what to do with this!
    I'm running a Java applet in a browser, under Sun's JVM. It has a custom dynamic class loader, which queries our server and downloads patches to the software. However, this doesn't work entirely as planned. The JVM doesn't seem to want to clear the class cache until the browser is restarted. I'm also wondering about static class definitions and whether I'd have to do something different to refresh those too.
    Ideally, when the loader starts and downloads patches to files, it then completely wipes out its previous definitions of cached classes, and reloads them all again from disk. Is this possible, and how would I go about doing it?
    (The reason for this behaviour is my old old software ran on Java 1.1, and the MS Java plugin exhibits this behaviour, which our users were used to...)
    Thanks in advance!
    David

    Thanks for the quick reply :)
    The loader is not static, so it should be using a new
    instance each time already? Or is it not sufficient
    to simply make it not static?Not really sure how to answer that! It's not static, so presumably it's an instance field of some other class. Now, are you using a new instance of that object each time you download a new patch? If not, that'll probably be the problem. If so, then probably what's going on is, your classloader isn't loading the classes at all, but simply delegating to it's parent. If that's the case, you need to make that delegation fail, for example by not giving your classloader a parent

  • Static Class in Java

    Hi All,
    I have gone through en number of web-sites but i don't get a clear definition and a clear cut picture about static class in java
    I had plenty of questions in my mind. Somebody here - Java gurus and experts kindly please help me to understand about these things.
    What is static class ? When to declare a class as Static ? What is the difference between a singleton class and a static class ? It is allowing to create an instance of a static class . why ?
    Thanks,
    With Love,
    J.Kathir

    Sometimes you want to use a class merely as a place to put utility methods and/or constants. Such classes are never instanciated. There's no special syntax, it's simply a class consisting of static methods, and perhaps static fields. You give it a single private constructor so it can't be accidentally instanciated.
    In use it's not that different from a singleton. Generally I'd use a singleton if there was going to be non-constant global data values in it.

  • About static member class?

    I can unstand the technical detail about static member class. But i still have a question about it.The questionis when shall we use it when we design a poject. In another word, in which situation, static member class will show its advantage? Thanks a lot.

    http://search.java.sun.com/search/java/index.jsp?qp=&nh=10&qt=static+nested+class&col=javaforums

  • To ragnic and other about Singleton class

    Hi ragnic. Thanks for your reply. I posted the code wrong. Heres' my correct one.
    I have a GUI first loaded information and the information is stored in a databse, I have some EJB classes..and my singleton class ABC has some method to access to the EJB..
    so my first GUI will gather info using singleton class and later if I click on
    a button in my first GUI class, will pop up another frame of another class , this class also need to class setPassword in my Singleton..
    are my followign codes correctly??
    iS my Class ABC a SINgleton class? thanks
    Is my class ABC use as single correctly. And It is called from other classes is also correct?
    I'm new to java and like to learn about Singleton class.
    But I really dont' understand it clearly after reading many examples of it.
    I have a project to convert my class abc to a singleton.
    But I dont know how to do it.
    In my class(soon will become a singleton) will have few methods that later I need to use it from another class A and class B.
    I have a GUI application that first load my class A..and my class will call
    class abc(singleton) to get some information from it.
    and then in class A has a button, if I click on that button I will call SIngleton class again to update my password, in the singleton class has method calls updatePassword. But I dont know how to call a singleton from other class.
    I have my code for them below:
    1)public class ABC //attempt using a singleton
    private static ABC theABC = null;
    private ABC(){}
    public synchronized static ABC getABC()
    if(theABC == null)
    theABC= new ABC();
    return the ABC;
    public void updateUserInfo(SimpleUser user)
    throws UserNotFoundException, DelegateException
    try
    UserCollectionHome userCollectionHome = (UserCollectionHome)
    EJBHomeFactory.getFactory().lookupHome("vista/UserCollection",
    UserCollectionHome.class);
    UserHome userHome = (UserHome)
    EJBHomeFactory.getFactory().lookupHome("vista/User",UserHome.class);
    UserCollection uc = userCollectionHome.create();
    uc.updateUserInfo(user, userHome);
    } catch(HomeFactoryException hfe) {
    hfe.printStackTrace();
    throw new DelegateException(hfe);
    } catch(RemoteException re) {
    re.printStackTrace();
    throw new DelegateException(re);
    } catch(CreateException ce) {
    ce.printStackTrace();
    throw new DelegateException(ce);
    } catch(FinderException fe) {
    fe.printStackTrace();
    throw new UserNotFoundException();
    public SimpleUser getID(String id)
    throws UserNotFoundException, DelegateException
    try
    UserCollectionHome userCollectionHome = (UserCollectionHome)
    EJBHomeFactory.getFactory().lookupHome("vista/UserCollection",
    UserCollectionHome.class);
    UserHome userHome = (UserHome)
    EJBHomeFactory.getFactory().lookupHome("vista/User",UserHome.class);
    UserCollection uc = userCollectionHome.create();
    SimpleUser su = uc.getID(id, userHome);
    return su;
    } catch(HomeFactoryException hfe) {
    throw new DelegateException(hfe);
    } catch(RemoteException re) {
    throw new DelegateException(re);
    } catch(CreateException ce) {
    throw new DelegateException(ce);
    } catch(FinderException fe) {
    throw new UserNotFoundException();
    public void setPassword(String lname,String pw)
    throws UserNotFoundException, DelegateException
    try
    UserCollectionHome userCollectionHome = (UserCollectionHome)
    EJBHomeFactory.getFactory().lookupHome("vista/UserCollection",
    UserCollectionHome.class);
    UserHome userHome = (UserHome)
    EJBHomeFactory.getFactory().lookupHome("vista/User",UserHome.class);
    UserCollection uc = userCollectionHome.create();
    uc.setPassword(lname,pw, userHome);//assume that all lname are differents.
    } catch(HomeFactoryException hfe) {
    hfe.printStackTrace();
    throw new DelegateException(hfe);
    } catch(RemoteException re) {
    re.printStackTrace();
    throw new DelegateException(re);
    } catch(CreateException ce) {
    ce.printStackTrace();
    throw new DelegateException(ce);
    } catch(FinderException fe) {
    fe.printStackTrace();
    throw new UserNotFoundException();
    }//Do I have my class as a Singleton correctly???
    2)//Here is my First Frame that will call a Singleton to gather user information
    public A(Frame owner)
    super(owner, "User Personal Information",true);
    initScreen();
    loadPersonalInfo();
    * This method instantiates all the GUI widgets and places them into panels and
    * onto the frame.
    private void initScreen()
    txtFname = new JTextField(20);
    txtLname=new JTextField(20);
    btnsave =new JButton("Save");
    btnChange= new JButton("Click here to change PW");//when you click this button there will be a frame pop up for you to enter informaton..this iwll call class B
    JPanel pnlMain=new JPanel();
    JPanel pnlFname= new JPanel();
    pnlFname.setLayout(new BoxLayout(pnlFname, BoxLayout.X_AXIS));
    pnlFname.setBorder(BorderFactory.createEmptyBorder(0,87,0,90));
    pnlFname.add(new JLabel("First Name:"));
    pnlFname.add(Box.createRigidArea(new Dimension(5,0)));
    pnlFname.add(txtFname);
    JPanel pnlLname= new JPanel();
    pnlLname.setLayout(new BoxLayout(pnlLname, BoxLayout.X_AXIS));
    pnlLname.setBorder(BorderFactory.createEmptyBorder(0,87,0,90));
    pnlLname.add(new JLabel("Last Name:"));
    pnlLname.add(Box.createRigidArea(new Dimension(5,0)));
    pnlLname.add(txtLname);
    pnlMain.add(pnlFname);
    pnlMain.add(pnlLname);
    pnlMain.add(btnsave);
    pnlMain.add(btnChange");
    btnSave = new JButton("Save");
    btnSave.setActionCommand("SAVE");
    btnSave.addActionListener(this);
    btnCancel = new JButton("Cancel");
    btnCancel.setActionCommand("CANCEL");
    btnCancel.addActionListener(this);
    JPanel pnlBottom = new JPanel();
    pnlBottom.setLayout(new BoxLayout(pnlBottom, BoxLayout.X_AXIS));
    pnlBottom.setBorder(BorderFactory.createEmptyBorder(25,55,0,0));
    pnlBottom.add(btnSave);
    pnlBottom.add(Box.createRigidArea(new Dimension(25,0)));
    pnlBottom.add(btnCancel);
    pnlMain.add(pnlBottom);
    this.setContentPane( pnlMain);
    setSize(500,500);
    GraphicUtilities.center(this);
    theABC=ABC.getABC();
    //Do I call my ABC singleton class correctly??
    private void loadPersonalInfo()
    String ID= System.getProperty("user.name");
    SimpleUser user = null;
    try {
    user = ABC.getID(ID);
    //I tried to use method in ABC singleton class. IS this correctly call?
    } catch(UserNotFoundException nfe)
    JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered.",
    "User Not Found",JOptionPane.WARNING_MESSAGE);
    System.exit(0);
    } catch(DelegateException de) {
    JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered",JOptionPane.ERROR_MESSAGE);
    System.exit(0);
    currentUser = user;
    txtFname.setText(currentUser.getFirstName());
    txtLname.setText(currentUser.getLastName());
    //This information will be display in my textfields Fname and Lname
    //I can change my first and last name and hit button SAVE to save
    public void actionPerformed(ActionEvent e)
    if(e.getActionCommand().equals("SAVE")) submitChanges();
    if(e.getActionCommand().equals("CHANGE_PASSWORD")) {
    changepassword=new ChangePassword(new Frame(),name,badgeid);
    public void submitChanges(){
    String currentNTUsername = System.getProperty("user.name");
    SimpleUser user =null;
    try {
    user = theABC.getID(ID);
    user.setFirstName(txtFname.getText().trim());
    user.setLastName(txtLname.getText().trim());
    currentUser = user;
    theABC.updateUserInfo(currentUser);
    //IS this correctly if I want to use this method in singleton class ABC??
    } catch(UserNotFoundException nfe)
    JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered",
    "User Not Found",JOptionPane.WARNING_MESSAGE);
    } catch(DelegateException de) {
    JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered",JOptionPane.ERROR_MESSAGE);
    System.exit(0);
    this.setVisible(false);
    3) click on ChangePassword in my above GUI class A..will call this class B..and in this class B
    I need to access method in a Singleton class- ABC class,,DO i need to inititates it agian, if not what should I do? thanks
    package com.lockheed.vista.userinfo;
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
    import javax.swing.event.*;
    import javax.swing.border.*;
    import javax.swing.table.*;
    import javax.swing.tree.*;
    import java.util.StringTokenizer;
    import java.util.Vector;
    import java.io.*;
    import javax.swing.text.*;
    import javax.swing.text.html.*;
    import javax.swing.colorchooser.*;
    import javax.swing.filechooser.*;
    import javax.accessibility.*;
    import java.beans.*;
    import java.applet.*;
    import java.net.*;
    import org.apache.log4j.*;
    import com.lockheed.common.gui.GraphicUtilities;
    import javax.swing.*;
    import javax.swing.event.*;
    import javax.swing.border.*;
    import java.awt.*;
    import java.awt.event.*;
    import java.util.*;
    import vista.user.UserServicesDelegate;
    import vista.user.SimpleUser;
    import vista.user.UserNotFoundException;
    import vista.user.*;
    import com.lockheed.common.ejb.*;
    import com.lockheed.common.gui.*;
    import com.lockheed.vista.publish.*;
    * This program allow users to change their Vista Web Center's password
    public class ChangePassword extends JDialog
    implements ActionListener{
    protected final Logger log = Logger.getLogger(getClass().getName());
    private UserServicesDelegate userServicesDelegate;
    private User currentUser = null;
    private JPasswordField txtPasswd, txtVerifyPW;
    private JButton btnSubmit,btnCancel;
    private JLabel lblName,lblBadgeID;
    private String strBadgeID="";
    * This is the constructor. It creates an instance of the ChangePassword
    * and calls the method to create and build the GUI.
    public ChangePassword(Frame owner,String name,String badgeid)
    super(owner, "Change Password",true);
    initScreen(name,badgeid);//build the GUI
    * This method instantiates all the GUI widgets and places them into panels and
    * onto the frame.
    private void initScreen(String strname,String strBadgeid)
    txtPasswd = new JPasswordField(20);
    txtVerifyPW=new JPasswordField(20);
    txtPasswd.setEchoChar('*');
    txtVerifyPW.setEchoChar('*');
    JPanel pnlMain=new JPanel();
    pnlMain.setLayout(new BoxLayout(pnlMain, BoxLayout.Y_AXIS));
    pnlMain.setBorder(BorderFactory.createEmptyBorder(20,0,20,0));
    JPanel pnlPW=new JPanel();
    pnlPW.setLayout(new BoxLayout(pnlPW, BoxLayout.X_AXIS));
    pnlPW.setBorder(BorderFactory.createEmptyBorder(0,96,0,30));
    pnlPW.add(new JLabel("Password:"));
    pnlPW.add(Box.createRigidArea(new Dimension(5,0)));
    pnlPW.add(txtPasswd);
    JPanel pnlVerifyPW=new JPanel();
    pnlVerifyPW.setLayout(new BoxLayout(pnlVerifyPW, BoxLayout.X_AXIS));
    pnlVerifyPW.setBorder(BorderFactory.createEmptyBorder(0,63,0,30));
    pnlVerifyPW.add(new JLabel("Verify Password:"));
    pnlVerifyPW.add(Box.createRigidArea(new Dimension(5,0)));
    pnlVerifyPW.add(txtVerifyPW);
    JPanel pnlTop= new JPanel();
    pnlTop.add(pnlPW);
    pnlTop.add(Box.createRigidArea(new Dimension(0,10)));
    pnlTop.add(pnlVerifyPW);
    pnlMain.add(pnlTop);
    btnSubmit = new JButton("Submit");
    btnSubmit.setActionCommand("SUBMIT");
    btnSubmit.addActionListener(this);
    btnCancel = new JButton("Cancel");
    btnCancel.setActionCommand("CANCEL");
    btnCancel.addActionListener(this);
    JPanel pnlBottom = new JPanel();
    pnlBottom.setLayout(new BoxLayout(pnlBottom, BoxLayout.X_AXIS));
    pnlBottom.setBorder(BorderFactory.createEmptyBorder(25,55,20,30));
    pnlBottom.add(btnSubmit);
    pnlBottom.add(Box.createRigidArea(new Dimension(25,0)));
    pnlBottom.add(btnCancel);
    pnlMain.add(pnlBottom);
    this.setContentPane( pnlMain);
    setSize(350,230);
    setVisible(true);
    public void actionPerformed(ActionEvent e)
    if(e.getActionCommand().equals("CANCEL")) this.setVisible(false);
    if(e.getActionCommand().equals("SUBMIT")) submitPW();
    * This method is called when the submit button is clicked. It allows user to change
    * their password.
    public void submitPW(){
    myABC= ABC.getABC();//Is this correct?
    char[] pw =txtPasswd.getPassword();
    String strPasswd="";
    for(int i=0;i<pw.length;i++){
    strPasswd=strPasswd+pw;
    char[] vpw =txtVerifyPW.getPassword();
    String strVerifyPW="";
    for(int i=0;i<vpw.length;i++){
    strVerifyPW=strVerifyPW+pw;
    if((strPasswd==null)||(strPasswd.length()==0)) {
    JOptionPane.showMessageDialog(new JDialog(),"You have not enter a password. Please try again.",
    "Invalid Password",JOptionPane.ERROR_MESSAGE);
    if((!strPasswd.equals(strVerifyPW)))
    //password and verify password do not match.
    JOptionPane.showMessageDialog(new JDialog(),"Your passwords do not match. Reenter and try again.",
    "Invalid Password",JOptionPane.ERROR_MESSAGE);
    try
    myABC.setUserPassword(strPasswd);//try to use a method in Singleton class
    txtPasswd.setText("");
    txtVerifyPW.setText("");
    this.setVisible(false);
    } catch(DelegateException e) {
    JOptionPane.showMessageDialog(new Frame(),
    "Error.",
    "Unable to change password information.",JOptionPane.WARNING_MESSAGE);
    } catch(UserNotFoundException e) {
    JOptionPane.showMessageDialog(new Frame(),
    "Error.",
    "Unable to change password information.",JOptionPane.WARNING_MESSAGE);
    And ofcourse I have other EJB classes to work with these classes.
    ***It compiles okey but when I ran, it say "NullPointerException"
    I think I call my Singleton wrong.
    Please help me.thanks

    1. When replying, use <reply>, don't post a new topic.
    2. Implementing a singleton is a frequently asked question. Search before you post.
    3. This is not a question about Swing. A more appropriate forum would be "New To Java Technology" or perhaps "Java Programming", but see point 1.
    4. When posting code, keep it short. It increases the chance of readers looking at it. And in composing your shorter version for the forum, you just may solve your problem.

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    yeah i understand the ?:;
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  • Static Class Function vs. Instance Variables

    I'm making a Wheel class to spin the wheels on some toy
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    Technically yes, realistically for this class no.A
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  • Singleton Vs Static Class

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