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!

Do-it-yourself equals and hash code methods

Usually you can depend on the Java Object uberclass to provide your class with adequate .equals (to determine two objects's equality) and hashCode() (to index your object within a hash).
But sometimes, you just need a little more. You can override these methods and give your own classes more specific instructions about what to do when they are being tested for equality or being indexed. I still have much to learn about this, but found a tasty looking link for further, later perusal.
http://www.geocities.com/technofundo/tech/java/equalhash.html

Data Facades

One of the signs you're moving from basic programming to more advanced stuff:
the use of Data Facades. I noticed this in some of the newer parts of our code base, where it's being refactored by a more competent programmer than I! :)
Here's a little blurb on it from the MSDN site:

"Consider using a data facade to wrap the most relevant data needed by the client. You can develop a wrapper object, with a coarse-grained interface, to encapsulate and coordinate the functionality of one or more objects that have not been designed for efficient remote access. It provides clients with single interface functionality for multiple business objects."

What does that mean? Say my server-side code has a few queries that have very little difference between them. Instead of making if/else statements to differentiate them, create a *DataFacade class for each of these states. These in turn should all implement an interface that has methods for getting a part of the query. The *DataFacade classes will in turn have the specifics for their method bodies.

GWT - UIServiceAsync timing

I've been working a lot with GWT (google web toolkit), and have been using the user interface 'client side' , tying in to the back end. (server side). This introduces a few problems, especially with database calls. GWT translates Java into Javascript for you, but it doesn't try to generate javascript for the code you keep hidden away on the server side (thank goodness, especially since GWT currently only supports Java 1.4!)
So how does it know when the calls to the server side are complete?

Using AsyncCallbacks. These have onSuccess and onFailure methods you write yourself, which tell your UI side what to do when your server side calls have finished.

I have found that the timing of these prove to be a pain. Your client side code, like the tide and time, waits for no man. It will merrily march on, after you have made a call to get some data from the database for instance, and not care whether or not it has completed to run the next line. So you can't really depend on the feedback from the server side on the next line, your data might very well be null.
..or the line after that. or the next one.

So how do you make sure that darn query's output can be properly shown to the user?

the answer lies in the onSuccess method.
You make a structure to contain whatever you need on the UI side, and draw itself (as empty for now, or with a 'loading, please wait' message) to the screen.
Then, pass that container to the Async call. In the onSuccess method, you tell it to write the output to that container and draw itself again.

This way the user will always see something, and you won't get null Pointer exceptions.

comparing objects with nulls

When trying to test an object to see if it's null or not (for good error handling purposes), it's good to know: a null object's .equal method doesn't measure up to a null. ie, you can't do this:



String s = null;
String r = null;

possiblySetMyString(r);

if(s.equals(null)){
System.out.println("string not set");
throw new RuntimeException();
}else{
System.out.println(s);
}

public void possiblySetMyString(String t){
s = t;
}



Nope! That won't work. you can't check if a string 'equals' null with the object's .equals method , because it's going to try to compare various String parameters to a null. Instead, replace the s.equals(null) with s==null.
Note: use == to check for primatives (int, char, bool, null etc), and use .equals to compare two objects. If you're comparing two different types, better convert before comparing.

Assignment does not give a copy.

In my naive assuption, I believed that the statement


myMethod Report(Report report){
Report copyofreport = report;
return copyofreport;
}


would give me a new object, an instance of the class Report.

It kind of worked, but introduced a subtle bug. I tried calling this method multiple times, thinking I could get an array of identical reports- but when I did something to my first report, all of the 'copies' I had made were similarly affected, even though I hadn't touched them.

What this actually did was make an array of references that all pointed to my original object, report. when I changed one, the references all reflected the same change.

solution: to make a new, identical object, there's a handy interface called cloneable. implement cloneable, override the method if necessary, and then call .clone().



// public class Report implements Cloneable;
ReportmyMethod Report(Report report){
Report copyofreport = report.clone();
return copyofreport;
}



Much better.
This only creates a 'shallow copy' though, so if there are any inner objects in the cloneable class, the objects will have the same reference. This really only matters if you're going to change the inner objects, but it's good to be aware of.