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Is there a way to tell if there is code not being used in java?

Whenever I program, I seem to accumulate a lot of "trash" code, code that is not in use anymore. Just to keep my code neat, and to avoid making any expensive and unnecessary computations, Is there an easy way to tell if there is code that is not being used?

One of the basic principles which will help you in this regard is to reduce visibility of everything as much as possible. If a class can be private don't make it default, protected or public. Same applies for methods and variables. It is much easier when you can say for sure if something is not being used outside a class. In cases like this even IDEs like Eclipse and IntelliJ Idea will suggest you about unused code.
Using this practice while developing and refactoring code is the best way to clean unused code confidently without the possibility of breaking the application. This will help in scenarios even when reflection is being used.

It's difficult to do in Java since it's a reflective language. (You can't simply hunt for calls to a certain class or function, for example, since reflection can be used to call a function using strings that can only be resolved at runtime.)
So in full generality, you cannot be certain.
If you have adequate unit tests for your code base then the possibility of redundant code should not be a cause for concern.

I think "unused code" means the code that is always not executed at runtime. I hope I interpreted you correctly.
The way to do a simple check on this is very easy. Just use IntelliJ IDEA to write your code. It will tell you that parts of your code that will never be executed and also the parts where the code can be simplified. For example,
if (x == 5) {
}
And then it will tell you that this if statement is redundant. Or if you have this:
return;
someMethod();
The IDE will tell you that someMethod() can never be reached. And it also provides a lot of other cool features.
But sometimes this isn't enough. What if you have
if (x == 5) {
someMethod();
}
But actually in your code, x can only be in the range of 1 to 4? The IDE won't tell you about this. You can use a tool that shows your code coverage by running lots of tests. Then you can see which part of your code is not executed.
If you don't want to use such a tool, you can put breakpoints in your methods. Then run some tests by hand. When the debugger steps through your code, you can see exactly where the code goes and exactly which piece(s) of code is not executed.
Another method to do this is to use the Find/Replace function of the IDE. Check if some of your public/private methods are not being called anywhere. For example, to check whether someMethod() is called, search for someMethod in the whole project and see if there are occurrences other than the declaration.
But the most effective way would be,
Stop writing this kind of code in the first place!

i think the best way to check that is to install a plugin of coverage like eclemma and create unit and integration tests to get 100% of coverage of the code that accomplish the use code/task you have.
The code that don't need to be tested or don't pass over it after the tests are completed and run, is code that you are not using

Try to avoid accumulating trash in the first place. Remove stuff you don't need anymore. (You could make a backup or better use a source code management system.)
You should also write unit tests for your functions. So you know if it still works after you remove something.
Aside from that, most IDEs will show you unused local variables and private methods.

I do imagine situation when you have app developed by years and some part of your functions doesn't used anymore even they still working. Example: Let's assume you make some changes on internal systems when specific event occured but it is not occurs anymore.
I would say you could use AspectJ to obtain such data / log and then analyze after some time.

Java annotation-based code injection without altering the annotated code

There is a Java application and I have no permission to alter the Java code apart from annotating classes or methods (with custom or existing annotations).
Using annotations and annotations only I have to invoke code which means that every time an instance of an annotated class is created or an annotated method is called, some extra Java code must be executed (e.g a call to a REST Webservice). So my question is: how can I do this?
In order to prevent answers that I have already checked I will give you some solutions that seem to work but are not satisfying enough.
Aspect Oriented Programming (e.g AspectJ) can do this (execute code before and after the call of an annotated method) but I don't really want the runtime overhead.
Use the solution provided here which actually uses reflection. This is exactly what I need only that it alters the initial code further than just annotating and so I cannot use it.
Use annotation processor for source code generation as suggested here by the last answer. However, still this means that I will alter the source code which I don't want.
What I would really like is a way to simply include a Java file that somehow will execute some Java lines every time the annotated element will be triggered.

Why not skip annotations completely and use byteman to inject code at runtime into the entry points of your code.
I have to agree with the comment above though, that this sort of restriction is ridiculous and should be challenged.

How to convince Eclipse to offer non-requisite methods?

(Eclipse v4.2.2/JDT v3.8.2)
How can I convince Eclipse to offer eg, Serializable's readObject(...) and writeObject(...) when using content assist? These methods never appear in the list of method names, even when the surrounding class implements Serializable.
Additionally, if this is possible, how would I customize the skeleton to contain eg, in.defaultReadObject();?
I understand why they don't appear, but is there any way to make them do so?

What you are looking for is called a Code Template. These are helpful for all kinds of repetitive code tasks. You'll never have to type a for loop again.

java code template

I'm trying to make a code template that will generate tostring, constructor from field, and a default constructor.
I already looked at Useful Eclipse Java Code Templates and in http://help.eclipse.org/galileo/index.jsp?topic=/org.eclipse.jdt.doc.user/reference/ref-tostring-templates.htm but it was not what I was looking for.
I tried this plugin http://eclipse-jutils.sourceforge.net/ but I still need to manually select an option in the menu (and it doesn't have a "constructor from fields" option).
I need to generate these methods and constructors for more then 100 classes so this the best way i found coz eclipse dont give tool to do it for more then one class and for this one class that he give this tool i need to do it one by one the (generate tostring ,constructor from field and also default constructor)
i will love to some help or some advice on a way to create these methods for all my classes, automatically.
thanks in advance.

I don't know of a plugin that will do this for multiple classes.
I'd just do it manually, even though it'd take time.
You could also use reflection and a scripting language like Groovy/JRuby/etc. to create the constructors, and rely on something like Commons' ToStringBuilder to create a toString, or just use reflection again.
(One problem is if you don't want a property in the constructor or toString you need to have a mechanism to tell the generator as much.)

I have just used Practically Macros, within a few minutes of install from the market place, I could generate *constructors*, getters / setters, toString, hashcode and equals (basically chaining the standard eclipse commands) in a single command. Just what I was looking for and saved me loads of time. I can also see a lot more uses for it, well done to Earnst (the creator).

Junit and messageboxes

I am new to JUnit and I got a sample java project in which I need to write unit tests for all the methods.
Unfortunately the code is poorly designed and some of the methods are done from the UI. Furthermore, some of the methods pop up a messagebox and do not return a return value.
I have two questions: First, without modifying the existing code, is there a way I can suppress the message boxes and not press enter every time I run the unit tests?
Second question: can a test function expect a message box and assert failure\success upon it's string content?
I appreciate any help, I know the best solution is to fix the code itself - separate the BusinessLogic completely from the UI and to test expected result, or even if message boxes are somehow mandatory use modal message boxes (like humble dialog boxes) but unfortunately I am not allowed to change anything in the code.
Thanks :)
Nili

There are all sorts of ways you could get started if only you were allowed to edit the code, so my first approach would be to see if you can get this restriction relaxed, and to read Working Effectively With Legacy Code.
Failing that you could try using a GUI testing framework like FEST-Swing to check the contents of the message boxes are as expected.

Not allowed to change the code, you say? First thought it to have a look at JMockit which really opens up a lot of possibilities when you are severely constrained by code that was not written with much concern about how it should be tested. It should enable you to, without modifying any code, substitute your preferred implementation of bothersome parts while your test is running--so only in the context of testing would you have altered the test subject (be careful to write a meaningful test!) or its dependencies. Other mock object frameworks can be useful, too, but the investment to learn JMockit is really time well-spent.

unfortunately I am not allowed to change anything in the code.
There's all sorts of stuff on Google about how to automate Swing testing with JUnit. Unfortunately, there's no way to get around this problem when testing.