We Keep Coding

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: Making own annotation that highlights methods and adds custom messaging

I have some importand methods in code that are used in a wrong way, people don't get the whole context of the process and invokes wrong methods, for example setters. If I had something like #Deprecated it could highlight / strike/ underline methods and show som info when somebody uses it. For example someone set some variables that are even not persisted as he thought that it would persist. Another person changed one method and spoiled dozen of usecases becaouse he didnt know about them..
I use Java7 and IntelliJ Idea 14

Instead of using an annotation, program defensively, check if the parameters you get make sense. Write tests to verify what happens when invalid input is provided.
I think Automated Tests, Good Method Names and such will do more good than some fancy IDE plugin to stop other developers from invoking wrong methods.

Replacing a method in a compiled .class file

Interesting question here. So I have a .jar I received and it is obfuscated, meaning when I decompile the .class files it doesnt show up 100% perfect so I cannot recompile it. However the only method I need to change has been converted perfectly (but the class does not)). Is there a way to somehow change the .java code and inject replace the method within the class file without totally recompiling?
If this fails im going to bytecode.
Thanks!
EDIT: As a follow up question / or a hack around replacing the WHOLE method. I'm really just trying to change a variable that the method generates locally. If there are any better ways to do that.

Depending on what you really want to do, I do not recommand to decompile / modify / recompile the code (be it for legal, maintainance, understandability, testability reasons.)
Bytecode manipulation may not be the best solution either, but if you want to follow this way have a look at the ASM project, it's a widespread bytecode manipulation framework used by many known projects.
If I were you I would first give a try to aspects (AspectJ.) The power of aspects is that you don't touch existing code, but tell the VM what to do when / before / after / in place of calling a specific method. It allows you to point out the exact context and change, enhance the behavior of the code, by writing your own code in a decoupled fashion.
Hope it helps.

Sorry, this is not an answer, but it is too long for a comment...
I am reflecting this code, not using it as a library. So I dont really need to "use" this code (aka I'm just reflecting and calling functions at runtime).
I'd call the "reflecting and calling functions at runtime" as using the code.
There might be the reasons why to do that, but I'd prefer to simply call the function as a library function if possible (which should be possible if you can do the same using reflection).
...and manually overload it.
There is nothing simpler that extending the class and override a "wrong" method. As of my understanding even if you want to "inject" the method, you have to have the code somewhere. How you will test such code? It'd be a lot easier to just extend the class... Can you specify in bigger detail what you want to achieve with a reasoning why you cannot use what I wrote above?

If you want to change just one method you can actually extend the class and then #Override the method!
Don't know if this is the perfect way to do it,but it works!

What is the optimal way to share code between Activities with different base classes?

I have the following problem:
I have an abstract Activity class, lets call it MyAbstractActivity, that contains some code I'd like to reuse (for example: a standard service binder, common menu items, common initialization code, etc. etc.). Normally I would just use it to subclass my concrete activities and be done with it.
However, I occasionally need to use another supertype, such as a ListActivity or a MapActivity.
So the question is: how do I avoid duplicating that support code within an Activity, if I have to use another base class?
I have thought up of a solution based on the decorator pattern, like this one:
.
However, I see a problem with this approach:
What to do with protected methods (like onCreate())? Should I introduce an additional "bridge" class that makes them public for the purpose of the decorator, similarly to the way presented below (starting to look a bit byzantine...)?
Any other way?
I hope I made myself relatively clear. Thanks in advance for any feedback!
PS. Using static utility classes is not a good solution in my opinion, since it introduces a possibility of hard-to-identify programming bugs.

If I understand correctly, neither Fragments nor the Decorator Pattern are clean or appropriate solutions for what you want to accomplish. They were designed to solve other problems.
I find myself moving "support" code, or "framework" code, or "all that verbose, repetitive, boilerplate crap" to static utility methods. This isn't necessarily the approach I'd take on a non-Android project, but in my Android projects, it works pretty darn well.
Also, know that you don't need to subclass ListActivity to have a ListView.

Can eclipse convert/refactor a method to a class?

This seems like it should be fairly straight-forward, but I can't see anything obvious. What I basically want to do it to point at a method and refactor->extract class. This would take the method in question to a new class with that method as top level public API. The refactoring would also drag any required methods and variables along with it to the new class, deleting them from the old class if nothing else in the old class is using it.
This is a repetitive task I often encounter when refactoring legacy code. Anyway, I'm currently using Eclipse 3.0.2, but would still be interested in the answer if its available in a more recent version of eclipse. Thanks!

I don't think this kind of refactoring exists yet.
Bug 225716 has been log for that kind of feature (since early 2008).
Bug 312347 would also be a good implementation of such a refactoring.
"Create a new class and move the relevant fields and methods from the old class into the new class."
I mention a workaround in this SO answer.

In Eclipse 3.7.1 there is an option to move methods and fields out of a class. To do so:
Make sure the destination class exists (empty class is fine, just as long as it exists in the project).
In the source class, select the methods that you want to remove (the outline view works great for this), right click on the selection, and choose Move
Select the destination class in the drop down/Browse
Your members are now extracted. Fix any visibility issues (Source > Generate Getters and Setters is very useful for this) and you are all set.

This seems like it should be fairly
straight-forward...
Actually, Extract Class is one of the more difficult refactorings. Even in your simple example of moving a single method and its dependencies, there are possible complications:
If the moved method might be used in code you don't know about, you need to have a proxy method in the original class that will delegate to (call) the moved method. (If your application is self-contained or if you know all the clients of the moved method, then the refactoring code could update the calling code.)
If the moved method is part of an interface or if the moved method is inherited, then you will also need to have a "proxy method".
Your method may call a private method/field that some other method calls. You need to choose a class for the called member (maybe in the class that uses it the most). You will need to change access from "private" to something more general.
Depending on how much the original class and the extracted class need to know about each other, one or both may need to have fields initialized that point to the other.
Etc.
This is why I encourage everybody to vote for bug 312347 to get fixed.

Have you tried the Move feature of the Refactor group ? You can create a helper class and move there anything you want.