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06/25/2017 2:48 pm
In your typical white-collar work environment, each employee has a computer at their desk. The computers are connected with each other over a LAN (local area network). If you have internet access at home, the computers in your house are most likely organized into a LAN as well. A LAN is a group of computers that are physically close to one another and that can communicate with each other over a network.  All of the computers in a LAN are said to belong to a broadcast domain, which means that if one computer sends out a broadcast message, then all the other computers can hear it.


One of the most common ways to join computers into a LAN is to use Ethernet. There are four properties of an Ethernet cable: EMI resistance, heat resistance, flexibility, and speed.

EMI resistance:STP (shielded twisted pair) cables are designed to protected again EMI (electromagnetic interference). A shop floor is a good example of a place where STP cables should be used because it has lots of electrical motors and other machinery. However, the vast majority of environments do not require significant protection from EMI, so they use UTP (unshielded twisted pair) cables, which are less expensive.

Heat resistance: Ethernet cabling is often run through the walls and ceilings of a building (called plenum space) in order to keep the cables out of the way. These areas of the building can get very hot. The rubbery outside of an Ethernet cable is typically made with a material called PVC. If PVC starts to melt due to high heat, it can give off poisonous fumes. A plenum-grade cable, however, will not melt in the heat because it is made out of material that is designed for heat-intensive environments. Plenum-grade cabling is much more expensive than PVC, so you should only buy it for cabling that you intend to use in plenum space.

Flexibility: You also need to think about the kind of physical wear-and-tear the cable will be getting. Will the cable sit in plenum space, untouched for most of its existence? Or will the cable spend most of its time in your office drawer, being used for various purposes around the office? Standard core cabling is made out of material that is flexible, which means you can bend it, step on it, and twist it (to a reasonable extent) without breaking it. Solid core cabling, on the other hand, is not so flexible. But its advantage is that it is a better conductor and will transfer data more effectively.

Speed rating: Every Ethernet cable has a speed rating, which defines its max data transfer speed. A cable’s speed rating is usually stamped on the outside of the cable itself. It’s sometimes referred to as a “CAT rating”, since the speed rating begins with the letters “CAT”. The maximum cable length varies between speed ratings, but for most speed ratings it is 100 meters. The ratings are measured in Mbps (megabits per second) or Gbps (gigabytes per second). To get a better feel for how fast this is, I like to divide this number by 8, which tells me how many bytes per second it supports. Note that, in order to take advantage of the full speed a cable offers, all other parts of your network infrastructure must support that speed rating, such as the network cards in the computers and the switches.

Standard Max speed/notes
CAT 1 This is the technical name for a telephone cable! Telephone cables use a RJ-11 connector, whereas Ethernet cables use a RJ-45 connector.
CAT 3 10 Mbps, some variants support 100 Mbps
CAT 5 100 Mbps
CAT 5e 1000 Mbps
CAT 6 1000 Mbps, 10 Gbps (55 meter max cable length)
CAT 6a/e 10 Gbps
CAT 7 10 Gbps with better shielding


All the computers in an Ethernet network connect to a central device called a switch, which routes the various network data to where it needs to go. This is called a star bus topology—“star” refers to the fact that the computers connect to a central switch (instead of to each other) and “bus” refers to the central device that routes all traffic.

Note that a device called a “bus” can serve as the central device as well, but buses are much more inefficient than switches because they broadcast all messages they receive to all computers, whereas switches only send out messages to the computer that the messages are intended for. Switches used to be more expensive than buses, but not anymore.

To prevent unauthorized computers from connecting to the network, you can disable unused ports on a switch.

Structured Cabling

Larger companies have the money and talent to organize their networks using structured cabling system. The aim of such a system is to create an organized, secure (both from an information safety perspective and a physical safety perspective), and reliable way of connecting all of your company’s computers to each other.

A typical structured cabling system is organized as follows. All cabling, including Ethernet cables and telephone cables, are run from each work area(the office space that an employee occupies) to a central room called the telecommunications room. This cabling is referred to as the horizontal cabling. Each piece of horizontal cabling is referred to as a run. Vocabulary rocks!

In an ideal environment, the horizontal Ethernet cabling would run through plenum space and be of plenum-grade, solid core construction. Each work area would then contain wall outlets that connect to the horizontal cabling. It’s interesting to note that Ethernet wall outlets have CAT ratings as well! Therefore, it’s important to make sure the outlet matches the CAT rating of your horizontal cabling.

The telecommunications room is the central destination for all the horizontal cabling. It contains specially designed equipment rackswhich are used to store its computer equipment. All rack-mounted equipment adheres to a measurement standard, simply referred to as U, which defines the height of the equipment. 1U equals 1.75 inches. Most rack-mounted equipment is either 1U, 2U, or 4U.

One piece of equipment you’re likely to find in a telecommunications room is a patch panel. A patch panel makes it easy to rearrange your network without having to mess with the horizontal cabling (which often uses fragile, solid core cables). The horizontal cabling is plugged into the back of the patch panel using a connector called a 110 punchdown block. This kind of connector connects the individual wiresinside of the Ethernet cable to the patch panel. A punchdown tool is used to attach the cable in this way. Connecting the horizontal cabling to the patch panel is a time consuming process and is meant to be more or less permanent. The other side of the patch panel contains much more flexible RJ-45 ports, which are easy to plug and unplug (kind of like the telephone switches of old). Patch cables are plugged into these ports. Patch cables are short (typically 2-5 feet long), standard core, UTP Ethernet cables. You then use the patch cables to rearrange your network as you like, as often as you like.

SOHO, Bro!

SOHO environments (small office/home office) do not always have the luxury of implementing a structured cabling solution. But there are a number of technologies that you can use to form a LAN without this.

What’s the Wi-Fi password?

The most common and quickest way to create a LAN is to go wireless. Wireless networks are not as fast as wired networks, but for most purposes, they are fast enough. Various wireless standards have been released over the years, each of which varies in speed. In general, they are backwards compatible with each other. Most wireless routers support multiple standards anyway, so you don’t have to worry too much about compatibility most of the time.

Standard Max speed Frequency Range
802.11b 12 Mbps 2.4Ghz 300 feet
802.11a (came out after b) 54 Mbps 5 GHz 150 feet
802.11g 54 Mbps 5 GHz 300 feet
802.11n 100+ Mbps 2.4 & 5 GHz 300+ feet
802.11ac 1 Gbps 5 GHz 300+ feet

Since Wi-Fi transmits its data over the air, securing your Wi-Fi network is of the utmost importance. The latest wireless security protocol is WPA2—all the other standards are vulnerable to security flaws, so you should never use them. Your wireless network should also be password protected, otherwise anyone can connect to it. Another way to secure your network is to configure your router to disable its SSID broadcast, which is what causes your network to appear on a device’s list of available networks. You can also enable MAC address filtering, which only gives pre-approved devices access to the network. Lastly, you should change the router’s administrator password because routers are often all configured with the same administrator password when they leave the factory.

One downside to Wi-Fi is that the wireless signal can be disrupted in many ways. Thick or metallic walls in your building can weaken or stop a wireless signal. Any devices that use the same parts of the wireless spectrum can cause interference as well, such as baby monitors and garage door openers (this is called radio frequency interference or RFI). If you have neighbors that have their own wireless networks, they can interfere with your network too. The parts of your building that get weak or no signal are called dead zones.

Ethernet over Power

If Wi-Fi isn’t an option for your particular environment, you can buy special devices that plug into your electrical outlets which allow you to create an Ethernet network using the electrical wiring of your house! This is called Ethernet over Power (not to be confused with Power over Ethernet, which supplies electrical power through an Ethernet network). This is an example of a bridge because it connects two dissimilar network technologies. Ethernet over Power only supports speeds at around 100Mbps however, so it’s not very fast.
06/11/2017 10:45 pm
Just like your have a home address that uniquely identifies your residence out of all the residences in the world, computers have IP addresses, which serve the same purpose.  They uniquely identify a computer in a network so that it can receive messages from other computers.


IPv4 was created when the internet was born in 1981 and is still used today.  It is the network communication protocol that computers use to talk to each other over the internet.  An IPv4 address is a unique identifier that is used to identify an individual computer that is connected to the internet.  It is 32-bits long and is commonly represented in dotted-decimal notation.  This notation divides the bits into four, 8-bit chunks and displays each chunk as a number ranging from 0 to 255.  Each number is separated with a dot.  For example:

At its inception, the set of all possible IPv4 addresses, called the address space, was divided into “classes”.  Each class contained a finite number of “chunks” of addresses.  The number of addresses in each chunk varied depending on the class.  The idea was that institutions, such as companies and schools, could purchase one of these chunks, and then dole out the addresses in the chunk to all the computers on their network.  Larger institutions with lots of computers could purchase a more expensive, higher class chunk that had lots of addresses, while smaller institutions that had fewer computers could purchase a cheaper, lower class chunk that had fewer sub addresses.

The classes are summarized below.  If you want to learn more about the logic behind how they were organized, I suggest you read this Wikipedia page.

Number of chunks
Number of addresses in each chunk
Class A
Class B
Class C
Class D
Class E

Do you see a problem here?

The problem with this scheme was that companies were unlikely to use every address that was available to them.  The choices for the number of addresses you could have varied wildly—you could have 16,777,216, 65,536, or 256!  You couldn't have anything in between!  If a company needed, say, 1,000 addresses, they had no choice but to purchase a Class B address and put all the rest to waste.  To top it off, some of the organizations that were involved in the early development of the internet possessed Class A chunks, which they were hardly making any use of.

This started to become a pressing issue as the internet grew.  The risk that all IP addresses would be used up, called IP address exhaustion, became a real possibility.


As shown, the way the class system divided up its chunks of addresses was very coarse-grained, which resulted in lots of wasted addresses.  To combat this, the class system was done away with in 1993 and replaced with a system called CIDR (Classless Inter-Domain Routing).  This system gives organizations many more choices regarding how many addresses they are assigned, which results in less wasted addresses.

CIDR uses something called variable-length subnet masking (VLSM), which allows the address's subnet mask (the part that identifies which organization an address belongs to) to be of any size.  The class system, on the other hand, only permitted the subnet mask to be 8 bits (Class A), 16 bits (Class B), or 24 bits (Class C) long.  With CIDR, if your company only needed 1,000 addresses, you could purchase a 1,024 chunk (22-bit subnet mask, leaving 10-bits for the address, 2^10=1,024).

CIDR notation consists of an IP address, followed by the number of bits the address uses for its subnet mask.  For example, represents the IP address with the first 22 bits of that address being the subnet mask.

But CIDR is only a stop-gap measure.  The IPv4 address space consists of about 4.3 billion addresses, which seems like a lot.  But on a global scale, it is not.  If the internet continues to grow, the IPv4 address space will soon run out.  A more permanent solution would be to increase the length of the IP address.  Enter IPv6.


Created in 1998, IPv6 addresses are a whopping 128 bits long, resulting in an incredibly large address space of 3.4 x 10^38 (the number of grains of sand on Earth...or something?).

IPv6 addresses are represented as eight, four character, hexadecimal strings separated by colons.


Because they are so long, there are tricks you can employ to make them shorter.  If a segment contains all zeroes, you can replace the segment with a single zero:


If an address contains consecutive segments which consist of all zeroes, you can replace them with a double colon (but you can only use this trick once):


And if a segment begins with zeroes, you can leave the zeroes out (unless the segment contains all zeroes, in which case you must leave one zero in):


IPv6 and IPv4 are not compatible with each other, which complicates the migration process.  While it is likely that the network card in your computer supports both IPv4 and IPv6, the infrastructure around the globe that makes the internet work cannot switch over so easily.  It will be a long and piecemeal process.  But if all goes well, you won't even know it happened.
06/06/2017 10:41 pm
There are many different kinds of printers on the market. In office environments, laser printers are by far the most numerous. Not only do they produce good quality printouts, but they are fast, which is important when you have people to please and deadlines to meet. Therefore, computer technicians have to be very familiar with how laser printers work so breakdowns can be fixed and Janet can get her TPS reports on time.

A laser printer follows a specific process when printing a sheet of paper. The process can be divided into seven steps.

1. Processing

In order to start printing, the printer has to first receive print data from a computer. The program the user is printing from (say, a word processor) has to first convert the document to some kind of format the printer understands. Many Windows applications use a system called GDI (graphical device interface), which is used in conjunction with the specific printer driver, to generate this print data.

The application then sends the print data to the print spooler, which is responsible for queuing up print jobs and sending them one at a time to the printer. Once the print job has been completely sent to the printer, it disappears from the print spooler (whether the printer is done printing it or not).

Note that, while it is possible to cancel a print job from the print spooler, this only stops the flow of information from the computer to the printer. For example, if the spooler sends half of the print job before you cancel it, the printer will print exactly that, even if the job is canceled before any pages came out of the printer. Therefore, you should also press the “stop” button on the printer itself to be sure the printing truly stops (do not pull out the paper tray, as this could jam the printer).

2. Charging

The rest of this process is centered around an important part of the printer called the drum. The drum is a cylinder shaped component which is used to transfer images onto the sheets of paper.  It does this using positive and negative electrical charges.

In the Charging step, the primary corona wire (or primary charge roller) gives the drum's surface a uniform negative electrical charge.

3. Exposing

A laser draws a positively charged image into the drum (hence the name, “laser printer”).

4. Developing

Negatively-charged toner particles attach themselves to the positively-charged parts of the drum the laser drew from the last step. "Toner" is the stuff that makes up the image on the piece of paper (it is a laser printer's “ink”).

5. Transferring

Here is where the actual piece of paper comes into play. The transfer corona (or transfer roller) applies a positive charge to a sheet of paper. Then, the negatively-charged toner particles on the drum attach themselves to the positively-charged paper. Voila! The toner has been “transferred” to the page.

6. Fusing

At this point, the toner is simply resting on top of the page like a layer of dust. In the Fusing step, the toner is melted onto the page using a heating element called the fuser (toner is mostly made of plastic). Hot Pockets! The page is now done!

7. Cleaning

Now that the page is done, the printer has to be “reset” for the next page. Notably, the drum must be cleaned. First, any residual toner is scraped off using a rubber cleaning blade. Then, erase lamps give the drum a neutral charge. Go back to step 2.
05/15/2014 12:25 pm
Git is a version control system that has gained a lot of popularity over the past few years. It started out as a custom-designed VCS (version control system) for the Linux kernel and has since ballooned in popularity, arguably thanks to Github, a free source code hosting site that is powered by Git. Below are five facts about Git that you might not know:

1. Stashing

This command saves all the uncommitted modifications you've made to your working copy, and then reverts your working copy back to its original state. This is useful if you want to switch branches, but your changes are in an incomplete state and you don't want to commit anything yet (git will not allow you to switch branches if you have uncommitted changes).

The command to run a stash operation is git stash. When you're ready to re-apply your stashed changes, run git stash apply.

You can also create multiple stashes. To see a list of all stashes, run git stash list. By default, when you run git stash apply, it will apply the stash at the top of the list. To apply a different stash, run git stash apply stash@{2} where stash@{2} is the name of the stash as shown in the stash list.

Also note that when you apply a stash, it will remain in the stash list. To delete a stash, run git stash drop. Or, you can run git stash pop to apply a stash and then delete it.

Stashes are only stored in your local repository. They cannot be pushed to remote repositories.

For more information, see: http://git-scm.com/book/en/Git-Tools-Stashing

2. Amending commits

With most other version control systems, if you forgot to include a file in a commit, you have to make a second commit. This is annoying because it makes the commit history longer than it should be. With Git, instead of making a second commit, you can "amend" the previous commit. This will merge your commit in with the previous one.

> git commit --amend

For more information, see: http://git-scm.com/book/en/Git-Basics-Undoing-Things

3. Git is a file system

At its core, Git is actually a key/value data store. The commands that you use on a daily basis, like push and commit, are tools that are built on top of the data store. To demonstrate, I'll show you how to add and retrieve files from a Git repository, without using any of the typical Git commands.

First, initialize an empty repository:

> git init
Initialized empty Git repository in /home/michael/git-blog/.git/

Next, add a file to the repository. This command will return a SHA-1 hash, which we will need to retrieve the file again.

> echo 'file data' | git hash-object -w --stdin 

Then, retrieve the file like so:

> git cat-file -p 987721052266a93a2509c3a8ac9e8c86341d0835
file data

For more information, see: http://git-scm.com/book/en/Git-Internals-Git-Objects

4. Commit message templates

If you work in a corporate environment, your team might have a policy on how commit messages have to be formatted. Git allows you to define a file that contains the default commit message to use for all commits.

> git config --global commit.template path/to/commit-message.txt

For more information, see: http://git-scm.com/book/en/Customizing-Git-Git-Configuration

5. Ignoring files during export

Git includes a command that lets you easily generate an archive file (.zip, .tar, etc) of your project. By default, all files in your project are included, but there may be some files which you want to exclude. To do this, use the export-ignore attribute:

> echo "test/ export-ignore" > .gitattributes
> git add .gitattributes
> git commit -m "Added .gitattributes file."
> git archive -o latest.zip HEAD

For more information, see: http://git-scm.com/book/en/Customizing-Git-Git-Attributes

05/01/2014 2:53 pm

Writing GUI applications can be difficult. It's easier to understand the flow of a command-line program--you start at the top and go to the bottom. But the flow of a GUI application, with its listeners, event handlers, and callbacks, goes all over the place. Add database queries and network calls to the mix, and things get even more complicated.

Enter MVP

The Model-View-Presenter (MVP) pattern helps to manage this complexity. MVP belongs to that family of design patterns that separates the application data and logic from the way in which the information is displayed to the user. To summarize:

  • The Model is responsible for maintaining the application's raw data (typically by persisting it in a database).
  • The View is responsible for presenting the data to the user (for example, in the form of a webpage or dialog box).
  • And the Presenter is responsible for tying the model and view together. In MVP, the model and view know nothing of each other!

Presenter First

The idea with "Presenter First" is that, using the MVP pattern, you start by writing the presenter class before anything else. This forces you to think abstractly about how your dialog window is going to behave. And, in the process of writing the presenter, you naturally figure out what functionalities the model and view will need to support. So, writing the model and view becomes just a matter of implementing an interface.

Another benefit to Presenter First is that it allows you to unit test your dialog's application logic. This is because the model and view are represented as interfaces, which can be easily mocked-out in the unit tests.

To summarize, the three benefits of Presenter First are:

  1. By using MVP, the view is cleanly separated from the application data and logic. In other words, your JFrame and JDialog classes become truly "dumb"--they contain no database calls or application logic.
  2. In the process of writing the presenter, the APIs for the model and view are essentially written automatically.
  3. The application logic of your dialog is finally unit-testable!


As an example, let's create a simple login dialog. This dialog will ask the user for a username and password. If the credentials are valid, then a session token will be returned and the dialog will close. The user can also choose to have the application remember his username and password.

We start by writing the presenter class.


import java.awt.event.*;

public class LoginPresenter{
    private final ILoginView view;
    private final ILoginModel model;

    public LoginPresenter(ILoginView view, ILoginModel model){
        this.view = view;
        this.model = model;

        //invoked when the user clicks "Login"
        view.addLoginListener(new ActionListener(){
            public void actionPerformed(ActionEvent event){

        //invoked when the user clicks "Cancel" or closes the window
        view.addCancelListener(new ActionListener(){
            public void actionPerformed(ActionEvent event){

        //populate the dialog with its initial data

        //finally, display the dialog

    private void onLogin(){
        //get the data that the user entered
        String username = view.getUsername();
        String password = view.getPassword();
        boolean rememberMe = view.getRememberMe();

        //send the network call to log the user in
        String session = model.login(username, password);

        if (session == null){
            //credentials were bad, so show an error dialog to the user

        //persist the login credentials if "remember me" is checked
        if (rememberMe){
        } else {



    private void onCancel(){

The constructor adds event handlers which will fire with the user presses the "Login" and "Cancel" buttons. Then, it populates the view with data from the model (in this case, the saved username and password). The "onLogin()" method contains logic which determines if the login was successful or not and acts accordingly.

Now that our presenter is written, we can write the model and view interfaces, which allows the presenter class to compile.


public interface ILoginModel{
    String login(String username, String password);

    String getCachedUsername();
    void setCachedUsername(String username);
    String getCachedPassword();
    void setCachedPassword(String password);
    boolean getCachedRememberMe();
    void setCachedRememberMe(boolean rememberMe);
    String getSession();
    void setSession(String session);


import java.awt.event.*;

public interface ILoginView{
    void addLoginListener(ActionListener listener);
    void addCancelListener(ActionListener listener);

    String getUsername();
    void setUsername(String username);
    String getPassword();
    void setPassword(String password);
    boolean getRememberMe();
    void setRememberMe(boolean rememberMe);

    void onBadLogin();
    void onSuccessfulLogin();

    void display();
    void close();

Next, we write our tests! Using a stubbing framework like Mockito helps, but it's not required (you could always create your own test implementations of the model and view interfaces).


import java.awt.event.*;
import java.util.*;
import org.junit.*;
import org.mockito.invocation.*;
import org.mockito.stubbing.*;
import static org.mockito.Mockito.*;

public class LoginPresenterTest{
    public void init(){
        ILoginView view = mock(ILoginView.class);

        ILoginModel model = mock(ILoginModel.class);

        LoginPresenter presenter = new LoginPresenter(view, model);


    public void bad_login(){
        ILoginView view = mock(ILoginView.class);
        ListenerAnswer loginAnswer = new ListenerAnswer();

        ILoginModel model = mock(ILoginModel.class);
        when(model.login("user", "password")).thenReturn(null); //"null" = bad login

        LoginPresenter presenter = new LoginPresenter(view, model);

        //click "login"

        verify(model, never()).setSession(anyString());
        verify(view, never()).onSuccessfulLogin();
        verify(view, never()).close();

    public void valid_login(){
        ILoginView view = mock(ILoginView.class);
        ListenerAnswer loginAnswer = new ListenerAnswer();

        ILoginModel model = mock(ILoginModel.class);
        when(model.login("user", "password")).thenReturn("abc123"); //non-null token = good login

        LoginPresenter presenter = new LoginPresenter(view, model);

        //click "login"

        verify(view, never()).onBadLogin();

    public void rememberMe_true(){
        ILoginView view = mock(ILoginView.class);
        ListenerAnswer loginAnswer = new ListenerAnswer();

        ILoginModel model = mock(ILoginModel.class);
        when(model.login("user", "password")).thenReturn("abc123");

        LoginPresenter presenter = new LoginPresenter(view, model);

        //click "login"


    public void rememberMe_false(){
        ILoginView view = mock(ILoginView.class);
        ListenerAnswer loginAnswer = new ListenerAnswer();

        ILoginModel model = mock(ILoginModel.class);
        when(model.login("user", "password")).thenReturn("abc123");

        LoginPresenter presenter = new LoginPresenter(view, model);

        //click "login"


    public void cancel(){
        ILoginView view = mock(ILoginView.class);
        ListenerAnswer cancelAnswer = new ListenerAnswer();

        ILoginModel model = mock(ILoginModel.class);

        LoginPresenter presenter = new LoginPresenter(view, model);

        //click "cancel"

        verify(model, never()).setSession(anyString());

    private class ListenerAnswer implements Answer<Object>{
        private final List<ActionListener> listeners = new ArrayList<ActionListener>();

        public Object answer(InvocationOnMock invocation) {
            ActionListener listener = (ActionListener)invocation.getArguments()[0];
            return null;

        public void fire(){
            for (ActionListener listener : listeners){

Once our tests pass, we can write the real implementations of the model and view interfaces. Again, this is basically just a matter of creating a new class and having that class implement the interface.


import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import static javax.swing.SpringLayout.*;

public class LoginViewImpl extends JFrame implements ILoginView {
    private final JButton login, cancel;
    private final JTextField username;
    private final JPasswordField password;
    private final JCheckBox rememberMe;

    public LoginViewImpl() {

        login = new JButton("Login");
        cancel = new JButton("Cancel");
        username = new JTextField();
        password = new JPasswordField();
        rememberMe = new JCheckBox("Remember me");

        JLabel title = new JLabel("Please enter your login credentials.");
        JLabel usernameLabel = new JLabel("Username:");
        JLabel passwordLabel = new JLabel("Password:");


        Container contentPane = getContentPane();
        SpringLayout layout = new SpringLayout();


        layout.putConstraint(WEST, title, 5, WEST, contentPane);
        layout.putConstraint(NORTH, title, 5, NORTH, contentPane);

        layout.putConstraint(WEST, usernameLabel, 5, WEST, contentPane);
        layout.putConstraint(NORTH, usernameLabel, 10, SOUTH, title);
        layout.putConstraint(WEST, username, 10, EAST, usernameLabel);
        layout.putConstraint(NORTH, username, 0, NORTH, usernameLabel);
        layout.putConstraint(EAST, username, 100, WEST, username);

        layout.putConstraint(WEST, passwordLabel, 5, WEST, contentPane);
        layout.putConstraint(NORTH, passwordLabel, 5, SOUTH, usernameLabel);
        layout.putConstraint(WEST, password, 0, WEST, username);
        layout.putConstraint(NORTH, password, 0, NORTH, passwordLabel);
        layout.putConstraint(EAST, password, 100, WEST, password);

        layout.putConstraint(WEST, rememberMe, 5, WEST, contentPane);
        layout.putConstraint(NORTH, rememberMe, 5, SOUTH, passwordLabel);

        layout.putConstraint(WEST, login, 5, WEST, contentPane);
        layout.putConstraint(NORTH, login, 10, SOUTH, rememberMe);
        layout.putConstraint(WEST, cancel, 5, EAST, login);
        layout.putConstraint(NORTH, cancel, 0, NORTH, login);


    public void addLoginListener(ActionListener listener) {

    public void addCancelListener(final ActionListener listener) {
        addWindowListener(new WindowAdapter(){
            public void windowClosing(WindowEvent event){

    public String getUsername() {
        return username.getText();

    public void setUsername(String username) {

    public String getPassword() {
        return new String(password.getPassword());

    public void setPassword(String password){

    public boolean getRememberMe() {
        return rememberMe.isSelected();

    public void setRememberMe(boolean rememberMe) {

    public void onBadLogin() {
        JOptionPane.showMessageDialog(this, "Invalid login credentials.");

    public void onSuccessfulLogin() {
        JOptionPane.showMessageDialog(this, "Login successful.");

    public void display() {

    public void close() {


import java.io.*;
import java.util.*;

public class LoginModelImpl implements ILoginModel{
    private final File file;
    private final Properties properties;
    private String session;

    public LoginModelImpl(File file) throws IOException{
        this.file = file;
        this.properties = new Properties();

        if (file.exists()){
            this.properties.load(new FileReader(file));

    public String login(String username, String password){
        //normally, a network or database call would be made here
        if ("test".equals(username) && "test".equals(password)){
            return "abc123";
        return null;

    public String getCachedUsername(){
        return properties.getProperty("username");

    public void setCachedUsername(String username){
        properties.setProperty("username", username);

    public String getCachedPassword(){
        return properties.getProperty("password");

    public void setCachedPassword(String password){
        properties.setProperty("password", password);

    public boolean getCachedRememberMe(){
        String value = properties.getProperty("rememberMe");
        return (value == null) ? false : Boolean.parseBoolean(value);

    public void setCachedRememberMe(boolean rememberMe){
        properties.setProperty("rememberMe", rememberMe + "");

    public String getSession(){
        return session;

    public void setSession(String session){
        this.session = session;

    private void save() {
        try {
            properties.store(new FileWriter(file), "");
        } catch (IOException e){
            throw new RuntimeException(e);

To run our program, we simply create a new instance of LoginPresenter, passing in the model and view implementations that we created above.


import java.io.*;

public class Main{
    public static void main(String args[]) throws Throwable {
        File cache = new File("cache.properties");

        ILoginModel model = new LoginModelImpl(cache);
        ILoginView view = new LoginViewImpl();
        new LoginPresenter(view, model);

And that's all there is to it!

Download the source code


More blog entries >>

How this page works

Last Updated: 1/3/2012

My blog is actually hosted on blogger.com. The way I'm able to display my blog posts here is by parsing the blog's RSS feed. RSS feeds are used by blogs to help alert their avid readers whenever a new post is created. They are just XML files that contain data on the most recent blog posts. They include things like the title and publish date of each post, as well as the actual blog post text. I can use most of the data from my RSS feed without any trouble, but there are a few things I need to tweak in order to display everything properly.

View the source

Fixing the code samples

One tweak is fixing the code samples I often include in my posts. Blogger replaces all newlines in the blog post with <br /> tags. This is a problem because, due to the syntax highlighting library I use, the <br /> tags themselves show up in the code samples. So, I need to replace all of these tags with newline characters. However, I can't just replace all <br /> tags in the entire blog post because I only want to replace the tags that are within code samples. This means that I have to use something a little more complex than a simple search-and-replace operation:

$content = //the blog post
$contentFixed = preg_replace_callback('~(<pre\\s+class="brush:.*?">)(.*?)(</pre>)~', function($matches){
	$code = $matches[2];
	$code = str_replace('<br />', "\n", $code);
	return $matches[1] . $code . $matches[3];
}, $content);

Here, I'm using the preg_replace_callback PHP function, which will execute a function that I define every time the regular expression finds a match in the subject string. I know that each code sample is wrapped in a <pre> tag and that the tag has a class attribute whose value starts with "brush:", so I use that information to find the code samples. Then, for each match the regular expression finds, it calls my custom function, where I have it replace the <br /> tags with newlines.

Fixing the dates

Because the publish dates of each blog post in the RSS feed are relative to the UTC timezone, I also have to make sure to apply my local timezone to each date. Otherwise, the dates will not be displayed correctly (like saying that I made a post at 2am in the morning).

$dateFromRss = 'Tue, 20 Dec 2011 02:30:00 +0000';
$dateFixed = new DateTime($dateFromRss);
$dateFixed->setTimezone(new DateTimeZone('America/New_York'));

Adding Highslide support to images

One extra feature that I included is adding Highslide support to each image (Highslide is a "lightbox" library which lets you view images in special popup windows). To do this, I load the blog post into a DOM, use XPath to query for all links that have images inside of them, and then add the appropriate attributes to the link tag.

$content = //the blog post

//XML doesn't like "&nbsp;", so replace it with the proper XML equivalent
//see: http://techtrouts.com/webkit-entity-nbsp-not-defined-convert-html-entities-to-xml/
$content = str_replace("&nbsp;", "&#160;", $content);

//load the text into a DOM
//add a root tag incase there isn't one
$xml = simplexml_load_string('<div>' . $content . '</div>');

//if there's a problem loading the XML, skip the highslide stuff
if ($xml !== false){
	//get all links that contain an image
	$links = $xml->xpath('//a[img]');
	//add the highslide stuff to each link
	foreach ($links as $link){
		$link->addAttribute('class', 'highslide');
		$link->addAttribute('onclick', 'return hs.expand(this)');

	//marshal XML to a string
	$content = $xml->asXML();
	//remove the XML declaration at the top
	$content = preg_replace('~^<\\?xml.*?\\?>~', '', $content);
	//trim whitespace
	$content = trim($content);
	//remove the root tag that we added
	$content = preg_replace('~(^<div>)|(</div>$)~', '', $content);

As you can see, the blog post text has to be awkwardly manipulated in order to be read into a DOM and written back out as a string. That's why I have a lot of comments here--when I have to revisit this code in 6 months, I won't be totally confused.

Caching the RSS file

One last thing to mention is that I cache the RSS file so that my website doesn't have to contact Blogger every time someone loads this page. When the cached file gets to be more than an hour old, a fresh copy of the file is downloaded from Blogger.

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