Monthly Archives: October 2009

Why do you have to be like the others?

Bad news, It seems with the assignment rush and the exam cram time coming I’m finding it hard to find time to study. This means the lowest priority things get pushed off my TODO list. Sadly this means I won’t be blogging until mid November. Thanks for understanding.

To play me out, this commercial. Try to guess what product they’re advertising.

Random Thought: I know this sounds crazy, but if anyone wants to write an article during the hiatus just check me an email. You’ll get full credit.

Using Subversion for Assignments

If you’ve never heard of subversion before then you are in for a pleasant surprise. Subversion is a version control tool, which means it will keep track of several files and all their old versions. Normally subversion is used to help multiple people work together on a single project. It tracks all their changes and combines them all, even flagging when conflicts occur and assists in resolving them. It is also useful when working alone on a school assignment. Here’s a few dot points that capture the essence of why Subversion is useful with assignments:

  • Subversion allows you to work on the same assignment on multiple computers.
  • Subversion can email you with changes you’ve made, allowing to review them.
  • Subversion allows you to show a teacher that you’ve been working on an assignment over the whole time available and not just in the last few days. this gives you greater leverage when asking for an extension.
  • Subversion can help you prove in a disciplinary hearing that you did not plagiarise any code from others showing the natural growth your code had.
  • Subversion can get back that file you just accidentally emptied out of the trash.
  • Subversion can show you all the changes you made between the time you fixed that annoying bug, and now, when you just reintroduced it.

The first step to making an assignment in is to build your repository. If you didn’t do this first that’s okay, you can easily import an existing project into a subversion repository. To create a repository you simply use the ‘svnadmin create’ command. You should then create some folders that should be in every subversion repository (trunk, tags and branches). This next block of commands will show you how to create the initial project. If you’re using these instructions to import an existing project just copy your files into the trunk folder before you run the ‘svn import’ command.

mkdir -p /home/daniel/svn/newproject
svnadmin create /home/daniel/svn/newproject
mkdir -p /tmp/newrepo/{trunk,branches,tags}
svn import /tmp/newrepo file:///home/daniel/svn/newproject -m "Create Initial Structure"
rm -rf /tmp/newproject

The trunk, tags and branches folders aren’t strictly required but can be very useful in certain circumstances. The trunk folder is where you main copy sits, it should be the latest stable version of the software. In an assignment though this is where you will probably be doing all your work, you generally don’t have the need or the time to make and merge branches. Which leads us to branches. Generally you branch software when you are about to make a major change that may break other developers work. You most likely don’t have other developers on your assignment and if you do you’ve probably all decided on what parts you will work on. Finally tags are for labelling certain versions with a specific tag. For example if you have to submit your assignment weekly you could tag each week as you submit, or you could tag as you finish each requirement. To populate these folders you just copy whatever it is you want into them. Subversion will only use a minuscule amount of space as the copy will be stored internally to the repository.

Before you can edit the files in the repository you need to check it out. You can check it out to the same machine, you can use SSH or you could check it out over WebDAV depending how you’ve set it up. The following command checks out the trunk folder into a folder called newproject. This is one of the few times you have to type the full path to the repository. Subversion remembers this for you so that next time you use a subversion command its pre filled.

svn checkout file:///home/daniel/svn/newproject/trunk newproject

What you’ve just checked out is called a ‘working copy’. This is where you make your changes before uploading them again in to the repository. Your working copy also includes copies of the versions you originally checked out so that if you want to revert back to them you can. Because they are stored in the working copy you don’t need access to the repository to revert. To revert back to the version you checked out from the repository you simply run ‘svn revert <filename>’. You can also find the differences between these versions and the current ones by using ‘svn diff <filename>’. The filename is optional and if omitted will print all the changes in the current directories and below.

Part 2 to come…
Random Thought: I’ve just redesigned my website, I’d love to know what my readers think. If you could post your comment on the new design, I’d appreciate it.

Paramaterized Java Classes

One of the biggest features of Java 1.5 was generics. In particular all the collection classes had been extended to use parametrized classes. Normally the collection classes accepted and returned Objects which is the class all other Java classes descend from. Unfortunately this meant that you had to cast everything you got back out of a collection to what you expected it to be. and until you did you would only be able to call methods that were provided by Object. You also had to be ready to catch an exception in case the class could not be cast because it was the wrong object.

Generics and parametrized classes allow Java programmers to place a type on a class and have that type inherited by its methods. For example you can now declare an ArrayList class with a type String. This alters the ArrayList class so that its add method now only accepts objects of type String, the get method now also returns objects of type String. This makes everything type safe which means you don’t have to cast anything and your code won’t compile if you try to put something in the ArrayList that doesn’t match its class.

Java uses parametrized classes to build its collections and you’ll want to use them too if you’re making your own collection class. For example if you were implementing a stack, a queue or a multi-priority FIFO queue are good cases for parametrized classes. Be careful though of the lure parametrized classes can have. They are not a replacement for polymorphism and shouldn’t be used when polymorphism would make more sense. For example if your multi-priority queue gets the priority out of the object itself then you’d need an interface that provides a method to get the priority. Then your class will only be able to accept items that implement that interface, which makes sense in this case as we need to priority to be able to store it.

A parametrized class is really simple to use. Here is an example implementation of a stack collection backed by an ArrayList:

import java.util.ArrayList;
import java.util.Collection;

/**
 * This class acts as a stack. Items can be 'pushed' which adds them to the top
 * of the stack. items can also be 'popped' which removes and returns the top
 * item on the stack and removes it. This means only the most recently added
 * item is available at the current time. To get to older items you need to
 * first remove the others.
 * 
 * Note: Java already has a stack object that should probably be used in
 *       preference to this one. This is only an example implementation.
 * 
 * @author Daniel Hall <daniel@danielhall.me>
 *
 * @param <T> The type of items that can be stored in the Stack.
 */
public class Stack<T> {
	/* Uses the same type as this class to store the items */
	private ArrayList<T> array = new ArrayList<T>();
	
	/**
	 * Creates a Stack containing items already in a collection. The collection
	 * must have the same parameterized type as this class to ensure that we get
	 * the right objects.
	 * @param c The Collection to initialize with
	 */
	public Stack(Collection<T> c) {
		array.addAll(c);
	}
	
	/**
	 * Creates an empty Stack object
	 */
	public Stack() {
		
	}
	
	/**
	 * Adds an item to the top of the stack.
	 * @param item The item which will be added to the top of the stack.
	 */
	public void push(T item) {
		array.add(item);
	}
	
	/**
	 * Removes the first item from the stack
	 * @return The item that was on the top of the stack.
	 */
	public T pop() {
		/* This gets the size so we don't have to do it twice. */
		int count = array.size();
		
		/* If the stack is empty return null, note that the Java implementation
		 * of stack throws an Exception instead.
		 */
		if (count == 0) {
			return null;
		}
		
		/* Remove the last added object (which will have index count - 1) */
		return array.remove(count - 1);
	}
}

Random thought: John Lions wrote a book about the Unix source code, in the seventies, which because it also included some code, was blocked from being published until 1996.