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Where are the rules for annotation classes written for java

I am trying to understand how the annotation understands what to do when we use the annotation. I am not talking about the behaviors like when to execute
that is covered by Retention, values etc. I want to understand how annotations understand the rules for that annotation. For example how does #Override annotation knows how to check if the function overrides a method in super class and so on. I tried digging a lot and I reached here but I don't get where the rules for the annotations are written. It feels like magic to me.

As already commented, processors (e.g. the compiler) must interpret the annotations, but the running program can also read/use the annotations (e.g by reflection).
For example the #Override annotation is used by the compiler, see it's documentation:
Indicates that a method declaration is intended to override a method declaration in a supertype. If a method is annotated with this annotation type compilers are required to generate an error message unless at least one of the following conditions hold:
The method does override or implement a method declared in a supertype.
The method has a signature that is override-equivalent to that of any public method declared in Object.
The #Override annotation is part of the standard API, other annotations may be part of some framework (e.g .JUnit #Test) or additional annotation processors, see Annotation Processing in javac. The developer can also declare annotations, see Annotation Interfaces.
In other words:
an Annotation is just that, an annotation. It is like a tag or mark that can be added to some elements (e.g. to a method, class, ...). There is no rule or anything similar directly attached to it. But some tools, like the compiler, or even normal Java code (some framework/library or written by you) can read and handle that annotation as desired.
There are a couple of annotations in the Java Language Specification (JLS) which compilers are required to handle. The action for the #Override (as example) is coded in the compiler, to do as specified in the JLS. Same for #Deprecated.

Why java annotation syntax has () [parentheses]?

Why in "java" when you declare a "parameter" of an "annotation" you have to put "pair of parentheses" after the parameter, annotation are anyway "very different" form "interface" syntactically, so why this weird syntax...I know it has something to do with, that annotation are managed using interface behind the scenes or something, but what exactly?

This is what a normal interface declaration would look like:
public interface Example {
String method();
}
Note that when annotations were released, the java feature 'default methods for interfaces' wasn't around yet. The default keyword already existed (and has existed since java 1.0), solely as a thing you can put in a switch block, as the 'case' for 'it didnt match any of the cases'.
This is what an annotation interface definition looks like:
public #interface Example {
String method();
}
or, if defaults are involved:
public #interface Example {
String method() default "";
}
Note how parsing wise there is no difference between these two, other than the '#' symbol. In the 'default' case; yeah that's entirely new, but looking solely at that bit, it's not weird looking. The parentheses are, but not that bit.
The reason it's done this way is to not 'explode' parsing. Since the introduction of module-info in java9, if you want to write a parser for java, you mostly DO need an 'on the fly switching modes' parser; the "language specification" for a module file is so very different.
But that is a big step; the vast majority of parser libraries out there don't deal with this well, they can't switch grammars in the middle of parsing a source file.
Even if I can snap my fingers and break java (which, to be clear, java does not generally do: updating the language so that existing code now no longer compiles or means something else is a big step that is very rarely taken for obvious reasons. It restricts language design, though. That's the cost of being the world's most popular language*)... there are advantages here.
The way annotations work is that, if you at runtime obtain one, it acts like an object that is an instance of the annotation interface:
Example foo = Class.forName("foo.bar.Baz").getAnnotation(Example.class);
System.out.println(foo.method());
Note how it's not foo.method. It's foo.method(). And that has different reasons: fields, in java, are second-rate citizens. You can't put them in lambda method references (ClassName::methodName is valid java; there's no such thing for fields), they don't inherit, you can't put them in interfaces (fields in interfaces are automatically public, final, and static, i.e. constants. They don't decree a requirement to any implementing class, unlike methods in interfaces). That means fields as a general point of principle aren't used in public APIs in java. It'd be weird if in this instance they would be.
So, given that the params act like args-less method calls, it's convenient in that sense that you declare them that way as well in an #interface definition.
*) Give or take a few spots.

Why can methods in Java 8 interfaces not be static and final? [duplicate]

One of the most useful features of Java 8 are the new default methods on interfaces. There are essentially two reasons (there may be others) why they have been introduced:
Providing actual default implementations. Example: Iterator.remove()
Allowing for JDK API evolution. Example: Iterable.forEach()
From an API designer's perspective, I would have liked to be able to use other modifiers on interface methods, e.g. final. This would be useful when adding convenience methods, preventing "accidental" overrides in implementing classes:
interface Sender {
// Convenience method to send an empty message
default final void send() {
send(null);
}
// Implementations should only implement this method
void send(String message);
}
The above is already common practice if Sender were a class:
abstract class Sender {
// Convenience method to send an empty message
final void send() {
send(null);
}
// Implementations should only implement this method
abstract void send(String message);
}
Now, default and final are obviously contradicting keywords, but the default keyword itself would not have been strictly required, so I'm assuming that this contradiction is deliberate, to reflect the subtle differences between "class methods with body" (just methods) and "interface methods with body" (default methods), i.e. differences which I have not yet understood.
At some point of time, support for modifiers like static and final on interface methods was not yet fully explored, citing Brian Goetz:
The other part is how far we're going to go to support class-building
tools in interfaces, such as final methods, private methods, protected
methods, static methods, etc. The answer is: we don't know yet
Since that time in late 2011, obviously, support for static methods in interfaces was added. Clearly, this added a lot of value to the JDK libraries themselves, such as with Comparator.comparing().
Question:
What is the reason final (and also static final) never made it to Java 8 interfaces?

This question is, to some degree, related to What is the reason why “synchronized” is not allowed in Java 8 interface methods?
The key thing to understand about default methods is that the primary design goal is interface evolution, not "turn interfaces into (mediocre) traits". While there's some overlap between the two, and we tried to be accommodating to the latter where it didn't get in the way of the former, these questions are best understood when viewed in this light. (Note too that class methods are going to be different from interface methods, no matter what the intent, by virtue of the fact that interface methods can be multiply inherited.)
The basic idea of a default method is: it is an interface method with a default implementation, and a derived class can provide a more specific implementation. And because the design center was interface evolution, it was a critical design goal that default methods be able to be added to interfaces after the fact in a source-compatible and binary-compatible manner.
The too-simple answer to "why not final default methods" is that then the body would then not simply be the default implementation, it would be the only implementation. While that's a little too simple an answer, it gives us a clue that the question is already heading in a questionable direction.
Another reason why final interface methods are questionable is that they create impossible problems for implementors. For example, suppose you have:
interface A {
default void foo() { ... }
}
interface B {
}
class C implements A, B {
}
Here, everything is good; C inherits foo() from A. Now supposing B is changed to have a foo method, with a default:
interface B {
default void foo() { ... }
}
Now, when we go to recompile C, the compiler will tell us that it doesn't know what behavior to inherit for foo(), so C has to override it (and could choose to delegate to A.super.foo() if it wanted to retain the same behavior.) But what if B had made its default final, and A is not under the control of the author of C? Now C is irretrievably broken; it can't compile without overriding foo(), but it can't override foo() if it was final in B.
This is just one example, but the point is that finality for methods is really a tool that makes more sense in the world of single-inheritance classes (generally which couple state to behavior), than to interfaces which merely contribute behavior and can be multiply inherited. It's too hard to reason about "what other interfaces might be mixed into the eventual implementor", and allowing an interface method to be final would likely cause these problems (and they would blow up not on the person who wrote the interface, but on the poor user who tries to implement it.)
Another reason to disallow them is that they wouldn't mean what you think they mean. A default implementation is only considered if the class (or its superclasses) don't provide a declaration (concrete or abstract) of the method. If a default method were final, but a superclass already implemented the method, the default would be ignored, which is probably not what the default author was expecting when declaring it final. (This inheritance behavior is a reflection of the design center for default methods -- interface evolution. It should be possible to add a default method (or a default implementation to an existing interface method) to existing interfaces that already have implementations, without changing the behavior of existing classes that implement the interface, guaranteeing that classes that already worked before default methods were added will work the same way in the presence of default methods.)

In the lambda mailing list there are plenty of discussions about it. One of those that seems to contain a lot of discussion about all that stuff is the following: On Varied interface method visibility (was Final defenders).
In this discussion, Talden, the author of the original question asks something very similar to your question:
The decision to make all interface members public was indeed an
unfortunate decision. That any use of interface in internal design
exposes implementation private details is a big one.
It's a tough one to fix without adding some obscure or compatibility
breaking nuances to the language. A compatibility break of that
magnitude and potential subtlety would seen unconscionable so a
solution has to exist that doesn't break existing code.
Could reintroducing the 'package' keyword as an access-specifier be
viable. It's absence of a specifier in an interface would imply
public-access and the absence of a specifier in a class implies
package-access. Which specifiers make sense in an interface is unclear
- especially if, to minimise the knowledge burden on developers, we have to ensure that access-specifiers mean the same thing in both
class and interface if they're present.
In the absence of default methods I'd have speculated that the
specifier of a member in an interface has to be at least as visible as
the interface itself (so the interface can actually be implemented in
all visible contexts) - with default methods that's not so certain.
Has there been any clear communication as to whether this is even a
possible in-scope discussion? If not, should it be held elsewhere.
Eventually Brian Goetz's answer was:
Yes, this is already being explored.
However, let me set some realistic expectations -- language / VM
features have a long lead time, even trivial-seeming ones like this.
The time for proposing new language feature ideas for Java SE 8 has
pretty much passed.
So, most likely it was never implemented because it was never part of the scope. It was never proposed in time to be considered.
In another heated discussion about final defender methods on the subject, Brian said again:
And you have gotten exactly what you wished for. That's exactly what
this feature adds -- multiple inheritance of behavior. Of course we
understand that people will use them as traits. And we've worked hard
to ensure that the the model of inheritance they offer is simple and
clean enough that people can get good results doing so in a broad
variety of situations. We have, at the same time, chosen not to push
them beyond the boundary of what works simply and cleanly, and that
leads to "aw, you didn't go far enough" reactions in some case. But
really, most of this thread seems to be grumbling that the glass is
merely 98% full. I'll take that 98% and get on with it!
So this reinforces my theory that it simply was not part of the scope or part of their design. What they did was to provide enough functionality to deal with the issues of API evolution.

It will be hard to find and identify "THE" answer, for the resons mentioned in the comments from #EJP : There are roughly 2 (+/- 2) people in the world who can give the definite answer at all. And in doubt, the answer might just be something like "Supporting final default methods did not seem to be worth the effort of restructuring the internal call resolution mechanisms". This is speculation, of course, but it is at least backed by subtle evidences, like this Statement (by one of the two persons) in the OpenJDK mailing list:
"I suppose if "final default" methods were allowed, they might need rewriting from internal invokespecial to user-visible invokeinterface."
and trivial facts like that a method is simply not considered to be a (really) final method when it is a default method, as currently implemented in the Method::is_final_method method in the OpenJDK.
Further really "authorative" information is indeed hard to find, even with excessive websearches and by reading commit logs. I thought that it might be related to potential ambiguities during the resolution of interface method calls with the invokeinterface instruction and and class method calls, corresponding to the invokevirtual instruction: For the invokevirtual instruction, there may be a simple vtable lookup, because the method must either be inherited from a superclass, or implemented by the class directly. In contrast to that, an invokeinterface call must examine the respective call site to find out which interface this call actually refers to (this is explained in more detail in the InterfaceCalls page of the HotSpot Wiki). However, final methods do either not get inserted into the vtable at all, or replace existing entries in the vtable (see klassVtable.cpp. Line 333), and similarly, default methods are replacing existing entries in the vtable (see klassVtable.cpp, Line 202). So the actual reason (and thus, the answer) must be hidden deeper inside the (rather complex) method call resolution mechanisms, but maybe these references will nevertheless be considered as being helpful, be it only for others that manage to derive the actual answer from that.

I wouldn't think it is neccessary to specify final on a convienience interface method, I can agree though that it may be helpful, but seemingly the costs have outweight the benefits.
What you are supposed to do, either way, is to write proper javadoc for the default method, showing exactly what the method is and is not allowed to do. In that way the classes implementing the interface "are not allowed" to change the implementation, though there are no guarantees.
Anyone could write a Collection that adheres to the interface and then does things in the methods that are absolutely counter intuitive, there is no way to shield yourself from that, other than writing extensive unit tests.

We add default keyword to our method inside an interface when we know that the class extending the interface may or may not override our implementation. But what if we want to add a method that we don't want any implementing class to override? Well, two options were available to us:
Add a default final method.
Add a static method.
Now, Java says that if we have a class implementing two or more interfaces such that they have a default method with exactly same method name and signature i.e. they are duplicate, then we need to provide an implementation of that method in our class. Now in case of default final methods, we can't provide an implementation and we are stuck. And that's why final keyword isn't used in interfaces.

Reduce visibility of classes and methods

TL;DR: Given bytecode, how can I find out what classes and what methods get used in a given method?
In my code, I'd like to programmatically find all classes and methods having too generous access qualifiers. This should be done based on an analysis of inheritance, static usage and also hints I provide (e.g., using some home-brew annotation like #KeepPublic). As a special case, unused classes and methods will get found.
I just did something similar though much simpler, namely adding the final keyword to all classes where it makes sense (i.e., it's allowed and the class won't get proxied by e.g., Hibernate). I did it in the form of a test, which knows about classes to be ignored (e.g., entities) and complains about all needlessly non-final classes.
For all classes of mine, I want to find all methods and classes it uses. Concerning classes, there's this answer using ASM's Remapper. Concerning methods, I've found an answer proposing instrumentation, which isn't what I want just now. I'm also not looking for a tool like ucdetector which works with Eclipse AST. How can I inspect method bodies based on bytecode? I'd like to do it myself so I can programmatically eliminate unwanted warnings (which are plentiful with ucdetector when using Lombok).

Looking at the usage on a per-method basis, i.e. by analyzing all instructions, has some pitfalls. Besides method invocations, there might be method references, which will be encoded using an invokedynamic instruction, having a handle to the target method in its bsm arguments. If the byte code hasn’t been generated from ordinary Java code (or stems from a future version), you have to be prepared to possibly encounter ldc instructions pointing to a handle which would yield a MethodHandle at runtime.
Since you already mentioned “analysis of inheritance”, I just want to point out the corner cases, i.e. for
package foo;
class A {
public void method() {}
}
class B implements bar.If {
}
package bar;
public interface If {
void method();
}
it’s easy to overlook that A.method() has to stay public.
If you stay conservative, i.e. when you can’t find out whether B instances will ever end up as targets of the If.method() invocations at other places in your application, you have to assume that it is possible, you won’t find much to optimize. I think that you need at least inlining of bridge methods and the synthetic inner/outer class accessors to identify unused members across inheritance relationships.
When it comes class references, there are indeed even more possibilities, to make a per-instruction analysis error prone. They may not only occur as owner of member access instructions, but also for new, checkcast, instanceof and array specific instructions, annotations, exception handlers and, even worse, within signatures which may occur at member references, annotations, local variable debugging hints, etc. The ldc instruction may refer to classes, producing a Class instance, which is actually used in ordinary Java code, e.g. for class literals, but as said, there’s also the theoretical possibility to produce MethodHandles which may refer to an owner class, but also have a signature bearing parameter types and a return type, or to produce a MethodType representing a signature.
You are better off analyzing the constant pool, however, that’s not offered by ASM. To be precise, a ClassReader has methods to access the pool, but they are actually not intended to be used by client code (as their documentation states). Even there, you have to be aware of pitfalls. Basically, the contents of a CONSTANT_Utf8_info bears a class or signature reference if a CONSTANT_Class_info resp. the descriptor index of a CONSTANT_NameAndType_info or a CONSTANT_MethodType_info points to it. However, declared members of a class have direct references to CONSTANT_Utf8_info pool entries to describe their signatures, see Methods and Fields. Likewise, annotations don’t follow the pattern and have direct references to CONSTANT_Utf8_info entries of the pool assigning a type or signature semantic to it, see enum_const_value and class_info_index…

why MyClass.class exists in java and MyField.field isn't?

Let's say I have:
class A {
Integer b;
void c() {}
}
Why does Java have this syntax: A.class, and doesn't have a syntax like this: b.field, c.method?
Is there any use that is so common for class literals?

The A.class syntax looks like a field access, but in fact it is a result of a special syntax rule in a context where normal field access is simply not allowed; i.e. where A is a class name.
Here is what the grammar in the JLS says:
Primary:
ParExpression
NonWildcardTypeArguments (
ExplicitGenericInvocationSuffix | this Arguments)
this [Arguments]
super SuperSuffix
Literal
new Creator
Identifier { . Identifier }[ IdentifierSuffix]
BasicType {[]} .class
void.class
Note that there is no equivalent syntax for field or method.
(Aside: The grammar allows b.field, but the JLS states that b.field means the contents of a field named "field" ... and it is a compilation error if no such field exists. Ditto for c.method, with the addition that a field c must exist. So neither of these constructs mean what you want them to mean ... )
Why does this limitation exist? Well, I guess because the Java language designers did not see the need to clutter up the language syntax / semantics to support convenient access to the Field and Method objects. (See * below for some of the problems of changing Java to allow what you want.)
Java reflection is not designed to be easy to use. In Java, it is best practice use static typing where possible. It is more efficient, and less fragile. Limit your use of reflection to the few cases where static typing simply won't work.
This may irk you if you are used to programming to a language where everything is dynamic. But you are better off not fighting it.
Is there any use that is so common for class literals?
I guess, the main reason they supported this for classes is that it avoids programs calling Class.forName("some horrible string") each time you need to do something reflectively. You could call it a compromise / small concession to usability for reflection.
I guess the other reason is that the <type>.class syntax didn't break anything, because class was already a keyword. (IIRC, the syntax was added in Java 1.1.)
* If the language designers tried to retrofit support for this kind of thing there would be all sorts of problems:
The changes would introduce ambiguities into the language, making compilation and other parser-dependent tasks harder.
The changes would undoubtedly break existing code, whether or not method and field were turned into keywords.
You cannot treat b.field as an implicit object attribute, because it doesn't apply to objects. Rather b.field would need to apply to field / attribute identifiers. But unless we make field a reserved word, we have the anomalous situation that you can create a field called field but you cannot refer to it in Java sourcecode.
For c.method, there is the problem that there can be multiple visible methods called c. A second issue that if there is a field called c and a method called c, then c.method could be a reference to an field called method on the object referred to by the c field.

I take it you want this info for logging and such. It is most unfortunate that such information is not available although the compiler has full access to such information.
One with a little creativity you can get the information using reflection. I can't provide any examples for asthere are little requirements to follow and I'm not in the mood to completely waste my time :)

I'm not sure if I fully understand your question. You are being unclear in what you mean by A.class syntax. You can use the reflections API to get the class from a given object by:
A a = new A()
Class c = a.getClass()
or
Class c = A.class;
Then do some things using c.
The reflections API is mostly used for debugging tools, since Java has support for polymorphism, you can always know the actual Class of an object at runtime, so the reflections API was developed to help debug problems (sub-class given, when super-class behavior is expected, etc.).
The reason there is no b.field or c.method, is because they have no meaning and no functional purpose in Java. You cannot create a reference to a method, and a field cannot change its type at runtime, these things are set at compile-time. Java is a very rigid language, without much in the way of runtime-flexibility (unless you use dynamic class loading, but even then you need some information on the loaded objects). If you have come from a flexible language like Ruby or Javascript, then you might find Java a little controlling for your tastes.
However, having the compiler help you figure our potential problems in your code is very helpful.

In java, Not everything is an object.
You can have
A a = new A()
Class cls = a.getClass()
or directly from the class
A.class
With this you get the object for the class.
With reflection you can get methods and fields but this gets complicated. Since not everything is an object. This is not a language like Scala or Ruby where everything is an object.
Reflection tutorial : http://download.oracle.com/javase/tutorial/reflect/index.html
BTW: You did not specify the public/private/protected , so by default your things are declared package private. This is package level protected access http://download.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html