Spring Beginner Tutorial: Dependency Injection Step 2 – What is a Dependency?

What is Dependency Injection? — Step 2

This is the second in a series of blog posts that I’m putting together for developers who are beginning to work with Spring. This post attempts to describe the fundamentals of what a dependency is.

In order to understand Dependency Injection (sometimes people call it Inversion of Control), we need to understand what dependencies are. Take a look at the Java code below. You can find the full example at my Spring Beginner Tutorial github repo.

I’ve tried to give the simplest example I could think of. In this example we have three classes, ExampleRunner, ClassDependency, and InterfaceDependency. We’ll get into the differences below. ExampleRunner depends on both ClassDependency and InterfaceDependency. In the run method of the ExampleRunner class above, we instantiate both ClassDependency and InterfaceDependency, and then call methods on both of these classes.

What does it mean when we say one class depends on another class?

The reason is simple. We say that one class depends on another because one class needs to call methods (or get information from, etc) on the class it depends on. In order for it to complete its job, it needs or depends on the other class. Without that class, it can’t carry out the work that it expects to do.

What is coupling?

Often, you’ll hear the term ‘coupling’ used in discussions of class dependencies. When one class uses another, we say that they are ‘coupled’. In other words, where you have dependencies, you have coupling, at least in a very basic form.

Types of Dependencies

Since I’ve now covered the basics, let’s dive into the different types of dependencies.

  • Class dependencies
  • Interface dependencies
  • Method dependencies
  • Field dependencies
  • Dependency in method

Class dependencies

Class dependencies are when one class depends on another class. In the example above, ExampleRunner depends on ClassDependency. ClassDependency is a concrete type — i.e. it’s not an interface.

Interface dependencies

ExampleRunner also depends on InterfaceDependency, which, as its name implies, is an interface dependency. InterfaceDependency is an interface. In this example, there is a somewhat hidden dependency in that since we instantiate InterfaceDependencyImpl, ExampleRunner also depends on this class. Later in the tutorial, we’ll get into how Spring helps us remove that dependency, but for now the thing to focus on is that InterfaceDependency is an interface, and the ExampleRunner class calls methods on it through this interface.

Field dependencies

In the example above, ExampleRunner also depends on a field. In this case it’s a field called ‘getField()’ (great name, I know :)). This is called a field dependency in that one class (ExampleRunner in our case) depends on the field of another (getField() on the ClassDependency class). This is what we’ll call a ‘field dependency’.

Dependency in method

Take a look at the dependencyInMethod method. Here, we again depend on ClassDependency. It’s just that, this time, the dependency is contained in a method, not as a field to the ExampleRunner class.

Import statements

One way to get a feel for the interface or class dependencies a given class has is to look at its import statements. Here you can find the packages or classes that contain the classes that the class you are examining uses. There’s a drawback to this, however, in that you won’t be able to see at a glance which classes the given class uses if those classes exist in the same package (since you don’t need to import classes that exist in the same package as the current class).


In this tutorial I describe what a dependency is, and give examples of the different types of dependencies. Understanding this is fundamental for understanding DependencyInjection and Spring.

Spring Beginner Tutorial: Dependency Injection Step 1 – ExampleRunner

What is Dependency Injection? – Step 1

This is the first in a series of blog posts that I’m putting together for developers who are beginning to work with Spring. This post sets us up to understand a basic principle of Spring – Dependency Injection.

To start us off, I’ve created a simple class with a main method. As you can see in the snippet below, all the main method does is instantiate an ExampleRunner class and call the run() method on it. This is what we’ll use to kick off the examples that follow.

Find the snippet below. You can see the full version of this example at my Spring Beginner Tutorial github repo:

In my next post we’ll dig into the fundamentals of Dependency Injection. Sometimes people call it Inversion of Control.


This is the introductory blog post that sets us up for the rest of the tutorial. It describes a simple class that allows us to execute code where we can view the examples that follow.

I hope you’ll follow along and find the tutorial helpful. I’ve been doing Spring for several years now, and I’ve felt that a simple tutorial for beginners is more than overdue. As always, send me your feedback and comments.

Sucking less every sprint

After battling the feature for about a week, he could see the light at the end of the tunnel.  His comment to me was, “man, looking back I could have gotten this completed so much faster if I knew what I know now.”

Earlier this month we hired a recent C.S. graduate.  He had been working as an intern for several months, and with the transition to full-time he let me know that he wanted to take on a bigger task.  The timing was right and I assigned him a simple yet slightly more involved task than he was accustomed to, and he went to work.

During our morning standups each day he would indicated progress — and it truly was progress — but the final solution kept eluding him.  After four days he said during standup, “today is the day I will complete this feature.”  But it didn’t get done.  It took him an additional three days.

I don’t fault him at all for taking the amount of time that he did on this feature.  It was more difficult than he was used to, and he wanted to make sure he nailed it.  In the end, he did a great job.

What struck me was his statement about how quickly he could have completed the task had he known what he knew after he was done.  It seems obvious, doesn’t it?  In a way it’s part of becoming self-aware:

Self awareness

OK, maybe not self-awareness. Maybe it’s closer to self-reflection. This helps us suck less every year:

I’ve often thought that sucking less every year is how humble programmers improve. You should be unhappy with code you wrote a year ago. If you aren’t, that means either A) you haven’t learned anything in a year, B) your code can’t be improved, or C) you never revisit old code. All of these are the kiss of death for software developers.

I’d go as far as to say that you should suck less every sprint. If you’re not self-aware, if you’re not continually self-reflecting — you are dying as a programmer. If your team slogs through sprint after sprint, release after release, without any time to reflect on recent work, and how it could have been improved, it slow its progress to a halt. Or at least lose momentum. Which may turn you into a zombie.

How to Start a Django REST Framework project

Lately I’ve been working with Django REST framework.  Here are some notes to get you started:

1. Create a folder for your project:

2. Create a virtualenv to isolate dependencies, and activate it:

3. Install Django and Django REST framework:

4. ‘Start’ (create) your new project:

5. ‘Start’ (create) your initial app:

6. Sync your database:

7. Create a superuser:

That’s it!  Now you can launch your project by invoking the runserver command:

Pull up in a browser window to see your project running from your local box.

Bash: create alias

If you find yourself repeatedly running a wordy command, having to look up how to run a command, or forgetting a username and password combination, I suggest looking into creating a bash alias.

To create a bash alias, do the following:

1. Start up terminal

2. Type cd ~  to go to your home folder

3. Type touch .bash_profile to create the new file

4. Type vim .bash_profile or open this file in your favorite editor.

5. Add your alias by adding a line to this file in the format alias <alias name>='<command>’ . For example alias gw=’./gradlew’.  This will create an alias named gw , which when typed will run the command ./gradlew  from wherever gw  is typed.

6. Type . .bash_profile  to reload your bash profile which loads any newly added aliases.

At this point you can type your alias instead of the wordy command you had been previously typing.

java.net.BindException: Address already in use:8080

Address already in use, BindException

If you’ve ever encountered this exception:
java.net.BindException: Address already in use:8080
there’s a great tool that you can use to determine what application(s) are running on a specific port: lsof

Looking at the man page for lsof, we find the following:

Lsof revision 4.81 lists on its standard output file information about files opened by processes for the following UNIX dialects:

AIX 5.3
FreeBSD 4.9 for x86-based systems
FreeBSD 7.0 and 8.0 for AMD64-based systems
Linux 2.1.72 and above for x86-based systems
Solaris 9 and 10

(See the DISTRIBUTION section of this manual page for information on how to obtain the latest lsof revision.)

An open file may be a regular file, a directory, a block special file, a character special file, an executing text reference, a library, a stream or a network file (Internet socket, NFS file or UNIX domain socket.) A specific file or all the files in a file system may be selected by path.

Instead of a formatted display, lsof will produce output that can be parsed by other programs. See the -F, option description, and the OUT-PUT FOR OTHER PROGRAMS section for more information.

In addition to producing a single output list, lsof will run in repeat mode. In repeat mode it will produce output, delay, then repeat the output operation until stopped with an interrupt or quit signal. See the +|-r [t[m<fmt>]] option description for more information.

To use it, type the following:
lsof -i:<port>
So in our case, we attempt to bind to port 8080, so running:
lsof -i:8080
Gives us an output like the following:

MyApp 50845 dustinkendall … … … … … …

To kill the process, type:
kill <pid>
So in our case:
kill 50845
This will help you get past the java.net.BindException and allow you to bind to that port.

How to Add a Private Key to Your Mac OS X Keychain

Add a Private Key to Your Mac OS X Keychain

If you are using Mac OS X, you can add a private key to the built-in keychain by typing the following:

Oftentimes your ssh key is named id_rsa and is stored at ~/.ssh. To add this file to your keychain do the following:

Whenever you boot/reboot your Mac, all SSH keys in your keychain will be automatically loaded.

You should be able to see the keys from the command line via:

Hope this helps!

6 tips for better interviewing for Software Engineers

A few weeks ago a good friend of mine interviewed for a Software Engineering position on my team. We have a very high bar when it comes to interviewing — our interview sessions usually include a lot of whiteboarding and tough programming problems. We didn’t end up making my friend an offer, and a few days later he texted me asking for advice that I could give him that he could use to improve his interviewing skills. Here’s what I ended up sending him:

1. Pick 5 hard problems, and know them well. I would suggest starting with 5 good programming problems, maybe even problems you heard while interviewing here or others you’ve come across. Make sure you understand the problem very well. Spend time thinking about the problem.

2. Practice in an environment that simulates the interview setting. Grab a friend and have her/him ask you questions that you’ve prepared in advance, and allow them to ask their own questions. Practice in front of a whiteboard, if possible in a meeting room or office, to simulate a real interview. Envision in your mind being at an actual interview, at the company you plan on interviewing with.

3. Restate the problem to the interviewer, and make sure that they know that you understand the problem. Part of this may be repeating or restating the question back to the interviewer. Doing this shows that you can communicate well. It also helps make sure that you are sure of what they are asking. At this point you want to clarify any assumptions you are making before you move forward. Getting an assumption cleared up or removed will help you tremendously — if they aren’t cleared up you can end up in left field real fast, solving a problem that is either not the problem the interviewer had in mind or is considerably harder than what was originally asked. If you clear up assumptions, you make the problem set very clear in your mind, and more often than not the problem becomes easier than it might have been.

4. Don’t ‘sign off’ too early. What I mean by this is many times interviewees tell the interviewer that they are done with their solution, when they haven’t double checked their work. When you feel you are done, take a deep breath and run through your code a couple of times. Test your code (see point 5). Don’t be too eager to be done with your solution. You’ll appear much better if you relax and take a second to revise and thoroughly inspect your code. You most likely have a problem that you’re not seeing. Fix the problem and repeat this process.

5. Test your code. In particular, boundary conditions. For example, if the problem being asked involved operations on a list, I would make sure that test cases I run through included lists with the following properties: null list, empty list, list with 1 item, list with a many items (or maximum depending on implementation/structure), list with mixture of null and non-null items, etc.

6. Think about memory and big O. More often than not, I’ll ask the interviewee, “Is there any way you can make this algorithm more efficient?” What I’m looking for is an understanding from you that you know there are tradeoffs of time and space when designing an algorithm. Can you reduce the amount of memory your solution uses? Can you improve the time complexity. What is the time complexity of your algorithm? Be prepared to answer these questions. Bonus: take a look at Wirth’s law and understand its implications.


These are just a few of the observations I’ve made while interviewing Software Engineers. Do you have any to add? Leave your suggestions in the comments below.