Perly Java

So, if you have even a tiny bit of Java experience you know about the normal format of the if/then/else control flow.

if(something that might be true){
do this;
}else{
do that;
}

or alternatively,
if(something that might be true){
do this;
}else if(something else that might be true){
do that;
}else{
do the alternative.
}

Of course there are a number of ways to make these statements shorter, like ommitting closing curly braces if the 'do this' operation is one line, etc.
These shorter ways are a little harder to read, but make the code a little more compact which can be a good thing.
But there is an even SHORTER way! If you just have a simple if/then/else statement, nothing fancy, just one operation for the if and the else, then you can get it down to one line.

To make this even more cryptic, it is not even necessary to use the words 'if' or 'else' in there at all: in place of if is '?' and in place of else is ':'.
So the above code fragment would look like this:

(something that might be true)?do this:do that;

This is getting suspiciously like Perl, if you ask me. Java heresy! ;)

Singletons: I finally get it

When I was trying to make a map for a servlet that is created long before I can pass stuff to it, I realized I need a way to make a map of user's input from the GUI, give it to the servlet, and read that map from the server side. But it can't just be any old instantiation of a map object. It has to be the ONE instance that I actually put stuff in, so I know where to point on the server side from the servlet's data processing methods.

I had heard about singletons, and in fact had to study about them before. But I studied them in a kind of abstract way and never really had to use them - until now.

Here is a great great understandable explanation from "Using the Singleton Pattern"
by Budi Kurniawan (http://www.onjava.com/pub/a/onjava/2003/08/27/singleton.html).

How do you write a Singleton class? Simple: create a public static method that is solely responsible for creating the single instance of the class. This also means that the client of the class should not be able to invoke the Singleton class's constructor, either. Because the absence of a constructor will make the compiler create a no-argument public constructor, a class applying the Singleton pattern has a private or protected constructor. Because the constructor is private or protected, there is no way a client can create an instance of that class by calling its constructor. The constructor is not accessible from outside of the class!

If the only constructor cannot be accessed, how do we get an instance of that class? The answer lies in a static method in the class. As mentioned above, a Singleton class will have a static method that calls the constructor to create an instance of the class and return this instance to the caller of the static method. But isn't the constructor private? That's right. However, remember that the static method is also in the same class; therefore, it has access to all members of the class, including the private members of the class.

You might ask the following question: "You can't create an instance of a class by calling its constructor, so how do you call the static method (responsible for creating the single instance of the class) without having the class instance?" Note, though, that static members of a class can be invoked without having an instance of that class. To limit the number of instances to one, the static method has to check if an instance has been created before. If it has, it simply returns a reference to the previous created instance. If it has not, it calls the constructor to create one. It's as simple as that.

So to get my map, I create a singleton class to hold my data, create a static getInstance method, and instantiate it from the GUI with my data mapping. Then on the server, I call the getInstance method (which will pass me back the ONE object with my map), and voila!