Develop Reference

Find all calls to interface implementers?

There are a large number of classes in this codebase which use a specific interface. However, picking a few at random, I've been unable to find one which is actually called anywhere; as such, I don't have a great idea of how to use it.
Is there a way in Eclipse to find every instance of any class which implements this interface?
In other words, suppose there exists an interface Interface, and classes ClassA, ClassB, ClassC, ..., ClassX, which all implement it. I want to see every point in the code where something like `ClassX obj = new ClassX(). Most of the classes I'm finding that implement this interface don't have any point where they're actually used; I assume they're for future use.

Open the interface class, hold Control and move your mouse to interface's name, select open implementation. That's the simplest and easiest way to do.

Yes, highlight the interface name and hit F4 or right click -> Open type hierarchy.
Update after OP's edit:
If you are using a framework that uses dependency injection like spring probably you don't find any reference because some of the implementations are defined in a xml file.
Also consider if some implementations are created and invoked via reflection.

Some classes might be loaded during runtime e.g. using reflection. To catch-them-all you can set a method entry breakpoint on the interface method. This is explained in this answer. That way all calls to implementation methods will suspend the JVM regardless of what is the object type.
Do note that unlike the line breakpoints the method breakpoints will really slow down the performance of the JVM.

Extending Default Access 3rd Party Method in Java Play Framework

So I'm using a third party library in a Play application (namely the Echo Nest Java API). There are some oversights in how some methods are exposed by the library, and I need to modify one method in particular, which involves using a constructor with default access.
I had initially thought that creating a class in the same package that extends the class that I need to modify would do the trick, but it seems Java's runtime package handling is thwarting that attempt (that is, since different classloaders are being used for the two different classes, their being in the same package is not enough). I have some notion of potentially modifying the classloader for either my subclass or the Echo Nest library, but have just about no idea how to go about doing that or whether there's a better solution available.
Any pointers in the right direction would be appreciated!

So it turns out the solution was to load the library class using Play's default classloader, set the constructor to be accessible, and then use newInstance() to instantiate the library class. Some code, in case any one else runs into a similar problem:
// Use the default application classloader to load the library class
Class artistClazz = Play.application().classloader().loadClass("com.echonest.api.v4.Artist");
// Get the package private constructor that we need
Constructor<Artist> constructor = artistClazz.getDeclaredConstructor(EchoNestAPI.class, Map.class);
// Make sure it's accessible to this class
constructor.setAccessible(true);
return constructor.newInstance(this, (Map) mq.getObject("artist"));
I'm not by any means convinced that this was the best or cleanest solution to this problem, but it minimally affected the code outside of this modified subclass so I'll probably stick to it for now.

Make a Java class have higher compile priority than one inside a library in IDEA

We have some portion of functionality packed in an external library and it is attached to our project. That library can't be changed in any way. Amongst others there are two classes lying inside it: com.myorg.Grandpa and com.myorg.Dad that extends com.myorg.Grandpa. Also there are com.myorg.Grandson extending com.myorg.Dad and a few other classes outside of the library extending com.myorg.Grandpa.
I decompile com.myorg.Grandpa class and add a new method new_method() to it.
Then I try to use new_method() in com.myorg.Grandson but IDEA won't let me do it cause Grandson extends Dad which extends library's Grandpa which doesn't contain new_method().
I tried to delete Grandpa from library and surprisingly IDEA didn't say a word and successfully compiled a project despite of the fact that in the boundaries of a library Dad extends non existing class.
The question is how to force Dad to extend a new Grandpa without deleting the one inside a library?

You could
Add an abstract class between Dad and GrandSon: Extend Dad, and add your method in the sub class. Then derive GrandSon from that sub class.
Put an instance of Dad in a new class, and let your IDE create delegate methods to the aggregated Dad instance. Again add your new method to the new class.
There is another possibility:
If you have to modify classes in place, use aspectj to weave in code: aspectj changes the byte-code (it does not need source code) at run-time. This way you can add methods or fields.

The fact is that you are duplicating classes with full package signature, so you will get the one that the classloader loads first. I know that in Websphere you can tweak classloader priorities, but couldn't say in your case.
Anyway, why not just do it without decompiling? You are causing yourself hard coupling to an external library and bad practices (maybe copyright issues) by decompiling/customizing. Besides, if the library gets updated, you will run into trouble having to reconstruct your customized classes.
Options:
Create your own implementation, for instance:
Create an Interface that replicates all methods in Grandpa plus the one you need.
Extend Grandpa class and implement the added method from your interface, all other methods will be left intact.
Extend all other extending classes from your own class hierarchie.
Instead of using the libraries common class, use your Interface as naming
This way you are kind of creating your own interface to the library, if it changes, you know where to make changes.
You could even do it without the interface, it's kind of wrapping the functionality, it would depend on what you need to achieve.
Anyway, I would try to solve it by own code and not by messing up with the library, it is just not worth it to do such tricks, if a new Programmer takes the project, they will need a lot of time to find out why and how it behaves.
Now, there might be variations in how to structure the class hierarchie, but it would depend on the specific implementation you need, so you would have to post more detailed data on what the library is and what you're trying to add to it if you expect some more specific answer...
Regards

It has to appear first to the class loader.
IDEA should load your class first if is in your project. You may also try to create a separate library for your class and include it in your project.
See also: http://www.jetbrains.com/idea/webhelp/configuring-module-dependencies-and-libraries.html

How can I discover which classes implement methods of a class using Eclipse?

I'm working with Eclipse and 2 API. Is there any way I can get information about which classes use a method of a class? For example, I have gameStarted() method in Game class, I would like to know other classes who use gameStarted() method. I also want to know which classes extend a class and which classes create an instance of a class.
I think I need to know this in order to understand how a part of an API works.

Call Hierarchy will tell you what calls a method or constructor.
Type Hierarchy will tell you what extends a class / what a class extends.

Select the method name in the source code and choose from the context menu
Call hierarchy to explore all callers and their callers,
Declarations for all places where this method is defined.

Make a class extends another class at runtime

There is a library have a base class (let's call it CBase) that performs some tasks and one can create classes that extends this CBase class.
The behavior of the CBase is not enough for me, so I would like to create my own CBase class (let's call it MyCBase) that have the same methods and members but these methods don't do the same thing.
Until now everything is ok. But what blocks me is that I would like to replace CBase by MyCBase. However, I have a lot of classes that extend CBase and I don't want to change them all.
Is it possible to replace CBase by MyCBase at runtime ?
So that
public class A extends CBase {}
becomes
public class A extends MyCBase {}
Can I perform this using code enhancement ? (like we do to add methods to a class at runtime. Is it also possible to change inheritance this way ?)
Thank you for your help !
EDIT
I would like to write a plugin for a framework, this is why I would like to change inheritance at runtime. This way users of the framework can use my plugin without changing their source code (changing the inheritance of their classes from CBase to MyCBase)
EDIT 2
Is it possible to do like this: ?
CtClass cc = CtClass.forName("pkg.AClass");
cc.setSuperclass(CtClass.forName("mylib.MyCBase"));
cc.compile();

I'm not expert. Probably you could extend ClassLoader. But I highly recommend don't do it. The replacement will touch many of your classes but it will be clear in code reading and app execution.
I think there is also room for architecture improvement since you have so many classes extend CBase. People are trying to remove dependencies from other libraries or keep it really small. Because in this case you could easily switch to another library or add your own functionality.

I dont think you can change the extends of a class at runtime. I would suggest to change the extends of the objects or build an interface, which contains all the things your need

Changing all derived classes is a simple matter, provided you control their source code:
Create a new class in your project. Call it CBase, and put it in the same package as the library class.
Use the rename/move refactoring of your IDE to rename CBase to MyBase. This will have the IDE rename all references to the renamed/moved class ...
Write the code for MyBase, extending from CBase.
If you can not do this (for instance because some derived classes are in a library you do not control), you replace the implementation of CBase with your own. Simply create a class of the same package and name in your project (the classloader searches the classpath in order, and uses the first class of the proper package and name it finds). This approach however is very brittle, as the compiler can not check binary compability between the old and new version of CBase. The JVM will check this compatibility when classes are loaded, but since classes are only loaded when needed, its hard to test your changes. (Which is why I do not recommend this approach if there are other options).
You could also change the classes as they are loaded my manipulating the class file, that that's going to be even more brittle, and the compiler would allow you to use any additional features MyBase might have. ==> Definitely not a good idea.