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logging in interface methods

I have been working on java 7 so far and recently moved to java-8, one thing which was surprising is that you can add methods in java-8 interfaces.
So far so good....loved this new stuff!
Now, my problem is that logging is an essential part of any development but seems lombok.extern.slf4j won't let you add log stuffs in by interface methods as it is only allowed on classes and enums.
How do you log your interface methods (if by lombok or is this the only way?? ) ? Or is interface methods not supposed to be logged? what am i missing here?
P.S : At present i am working with System.out.println.... yeah...thats noob :)

Currently Lombok #Slf4j annotation is not supported on interfaces,
but it can be circumvented like this
public interface MyInterface
{
#Slf4j
final class LogHolder
{}
default void action() {
LogHolder.log.error("Error TEST");
}
}

you can add logger to your interface manually, but your logger will be public:
public interface SomeInterface {
Logger log = LoggerFactory.getLogger(SomIface.class);
default void action() {
log.info("TEST");
}
}

Logging is an implementation detail, so an interface shouldn't deal with it. If logging is considered as the responsibility of the interface that will lead to several problems. For example:
If there are more than one class implementations, you don't know which is used because they log with the same name. It's also not possible to fine-tune their log levels by their names in config.
Default methods are public which means they can be called from the outside. This is not very desirable for logging methods.
Logging methods would just pollute the interface. What would one say if 'Map' interface would contain such kind of default methods? 'logDebug' or so. It's just confusing and leads to unnecessary questions. An interface should be a clean API for the intended purpose.
What you could use instead:
Some kind of delegation which Lombok also has support for. (Composition over inheritance)
Some kind of Aspect Oriented Programming technique. There are frameworks for that but it's also possible to achieve the same by "Dynamic Proxies". This is also in connection with interfaces.

We can add an abstract method getLogger in interface. And ask the implementer to pass the logger object.
public interface CustomRepo{
Logger getLogger();
default void processData(Data data){
// Do something
getLogger().info("Processing");
// Do something
}
}
#Slf4j
public RepoImplementer implements CustomRepo {
#Override
public Logger getLogger(){
return log;
}
}

Can interfaces evolve with time?

Interfaces are great from a flexibility standpoint. But in case, where an interface is used by a large number of clients. Adding new methods to the interface while keeping the old mehtods intact will break all clients' code as new methods won't be present in clients. As shown below:
public interface CustomInterface {
public void method1();
}
public class CustomImplementation implements CustomInterface {
#Override
public void method1() {
System.out.println("This is method1");
}
}
If at some point later in time, we add another method to this interface all clients' code will break.
public interface CustomInterface {
public void method1();
public void method2();
}
To avoid this we have to explicitly implement new methods in all clients' code.
So I think of interfaces and this scenario as following:
Interfaces once written are like carving in stone. They are rarely supposed, and expected to change. And if they do, they come with a huge cost(rewriting the whole code) which programmers should be ready for.
In continuation with the point above, Is it possible to write interfaces that can stand the test of time?
How such a scenario is handled in interfaces where you expect additional functionality in future? That is anticipating change in the contract by which all clients are binded.
EDIT: Default method is indeed a nice addition to Java Interfaces which a lot of people have mentioned in their answers. But my question was more in the context of code design. And how forcing method implementation on the client is an intrinsic character of an interface. But this contract between an interface and a client seems fragile as functionality will eventually evolve.

One solution to this problem was introduced in Java 8 in the form of default methods in interfaces. It allowed to add new methods to existing Java SE interfaces without breaking existing code, since it supplied default implementation to all the new methods.
For example, the Iterable interface, which is widely used (it's a super interface of the Collection interface) was added two new default methods - default void forEach(Consumer<? super T> action) and default Spliterator<T> spliterator().

public interface CustomInterface {
public void method1();
}
public interface CustomInterface2 extends CustomInterface {
public void meathod2();
}
Other than default method you can use inheritance property as show above by which new interface will have all previous method along with new methods and use this interface in your required situation.

Java 8 has introduced default implementation for methods. These implementations reside in the interface. If a new method with a default implementation is created in an interface that is already implemented by many classes, there is no need to modify all the classes, but only the ones that we want to have a different implementation for the newly defined method than the default one.
Now, what about older Java versions? Here we can have another interface that extends the first one. After that, classes that we want to implement the newly-declared method will be changed to implement the new interface. As shown below.
public interface IFirst {
void method1();
}
public class ClassOne implements IFirst() {
public void method1();
}
public class ClassTwo implements IFirst() {
public void method1();
}
Now, we want method2() declared, but it should only be implemented by ClassOne.
public interface ISecond extends iFirst {
void method2();
}
public class ClassOne implements ISecond() {
public void method1();
public void method2();
}
public class ClassTwo implements IFirst() {
public void method1();
}
This approach will be ok in most cases, but it does have downsides as well. For example, we want method3() (and only that one) for ClassTwo. We will need a new interface IThird. If later we will want to add method4() that should be implemented by both ClassOne and ClassTwo, we will need to modify (but not ClassThree that also implemented IFirst) we will need to change both ISecond and IThird.
There rarely is a "magic bullet" when it comes to programming. In the case of interfaces, it is best if they don't change. This isn't always the case in real-life situations. That is why it is advised that interfaces offer just "the contract" (must-have functionality) and when possible use abstract classes.

A future interface change shouldn't break anything that has been working -- if it does, it's a different interface. (It may deprecate things, though, and a full cycle after deprecation it may be acceptable to say that throwing an Unimplemented exception is acceptable.)
To add things to an interface, the cleanest answer is to derive a new interface from it. That will allow using objects implementing the new behaviors with code expecting the old ones, while letting the user declare appropriately and/or typecast to get access to the new features. It's a bit annoying since it may require instanceof tests, but it's the most robust approach, and it's the one you'll see in many industry standards.

Interfaces are contracts between the developer and clients, so you're right - they are carved in stone and should not be changed. Therefore, an interface should expose (= demand) only the basic functionality that's absolutely required from a class.
Take the List interface for example. There are many implementations of lists in Java, many of which evolve over time (better under-the-hood algorithms, improved memory storage), but the basic "concept" of a list - add an item, search for an item, remove an item - should not and will not ever change.
So, to your question: Instead of writing interfaces which classes implement, you can use abstract classes. Interfaces are basically purely-abstract classes, in the sense that they do not provide any built-in functionality. However, one can add new, non-abstract methods to an abstract class that clients will not be required to implement (override).
Take this abstract class (= interface) for example:
abstract class BaseQueue {
abstract public Object pop();
abstract public void push(Object o);
abstract public int length();
public void clearEven() {};
}
public class MyQueue extends BaseQueue {
#Override
public Object pop() { ... }
...
}
Just like in interfaces, every class that extends BaseQueue is contractually bound to implement the abstract methods. The clearEven() method, however, is not an abstract method (and already comes with an empty implementation), so the client is not forced to implement it, or even use it.
That means that you can leverage the power of abstract classes in Java in order to create non-contractually-binding methods. You can add other methods to the base class in the future as much as you like, provided that they are not abstract methods.

I think your question is more about design and techniques, so java8 answers are a bit misleading. This problem was known long before java8, so there are some other solutions for it.
First, there are no absolutely chargeless ways to solve a problem. The size of inconviniences that come from interface evolving depends on how the library is used and how deliberate your design is.
1) No techniques will help, if you designed an interface and forgot to include a mandatory method in it. Plan your design better and try to anticipate how clients will use your interfaces.
Example: Imagine Machine interface that has turnOn() method but misses turnOff() method. Introducing a new method with default empty implementation in java8 will prevent compilation errors but will not really help, because calling a method will have no effect. Providing working implementation is sometimes impossible because interface has no fields and state.
2) Different implementations usually have things in common. Don't be afraid to keep common logic in parent class. Inherit your library classes from this parent class. This will enforce library clients to inherit their own implementations from your parent class as well. Now you can make small changes to the interface without breaking everything.
Example: You decided to include isTurnedOn() method to your interface. With a basic class, you can write a default method implementation that would make sence. Classes that were not inherited from parent class still need to provide their own method implementations, but since method is not mandatory, it will be easy for them.
3) Upgrading the functionality is usually achieved by extending the interfaces. There's no reason to force library clients to implement a bunch of new methods because they may not need them.
Example: You decided to add stayIdle() method to your interface. It makes sence for classes in your library, but not for custom client classes. Since this functionality is new, it's better to create a new interface that will extend Machine and use it when it's needed.

Why doesn't interfaces implement methods and override them?

I already read the post of research effort required to post a SO question. I am ashamed again to post this question to a pile of million questions. But I still don't get the idea of interfaces in java. They have unimplemented methods and then defined for every class in which they are implemented. I searched about it. Interfaces were used to support multiple inheritance in java and also to avoid (Deadly) Diamond Death of inheritance. I also came across Composition vs Inheritance and that inheritance is not for code reuse and its for polymorphism. So when I have a common code as a class to extend it will not be supported due to multiple inheritance which gives the option to use Interfaces(Correct me if I am wrong). I also came across that its not possible in most cases to define a generic implementation. So what is the problem in having a common definition (not a perfect generic implementation) of the interface method and then Override it wherever necessary and why doesn't java support it. Eg. When I have 100 classes that implements an interface 70 of them have a common implementation while others have different implementation. Why do I have to define the common method in interface over 70 classes and why can't I define them in Interface and then override them in other 30 classes which saves me from using same code in 70 classes. Is my understanding of interfaces wrong?

First, an interface in Java (as of Java 7) has no code. It's a mere definition, a contract a class must fulfill.
So what is the problem in having a common definition (not a perfect
generic implementation) of the interface method and then Override it
wherever necessary and why doesn't java support it
Yes you can do that in Java, just not with interfaces only. Let's suppose I want from this Example interface to have a default implementation for method1 but leave method2 unimplemented:
interface Example {
public void method1();
public String method2(final int parameter);
}
abstract class AbstractExampleImpl implements Example {
#Override
public void method1() {
// Implement
}
}
Now classes that want to use this method1 default implementation can just extend AbstractExampleImpl. This is more flexible than implementing code in the interface because if you do so, then all classes are bound to that implementation which you might not want. This is the advantage of interfaces: being able to reference a certain behavior (contract) without having to know how the class actually implements this, for example:
List<String> aList = MyListFactory.getNewList();
MyListFactory.getNewList() can return any object implementing List, our code manipulating aList doesn't care at all because it's based on the interface.
What if the class that uses interface already is a Sub-class. Then we
can't use Abstract class as multiple inheritance is not supported
I guess you mean this situation:
class AnotherClass extends AnotherBaseClass
and you want to extend AbstractExampleImpl as well. Yes, in this case, it's not possible to make AnotherClass extend AbstractExampleImpl, but you can write a wrapped inner-class that does this, for example:
class AnotherClass extends AnotherBaseClass implements Example {
private class InnerExampleImpl extends AbstractExampleImpl {
// Here you have AbstractExampleImpl's implementation of method1
}
}
Then you can just internally make all Example methods being actually implemented by InnerExampleImpl by calling its methods.
Is it necessary to have the interface in AnotherClass?
I guess you mean AnotherClass implements Example. Well, this is what you wanted: have AnotherClass implement Example with some default implementation as well as extend another class, or I understood you wrong. Since you cannot extend more than one class, you have to implement the interface so you can do
final Example anotherClass = new AnotherClass();
Otherwise this will not be possible.
Also for every class that implements an interface do I have to design
an inner class?
No, it doesn't have to be an inner class, that was just an example. If you want multiple other classes have this default Example implementation, you can just write a separate class and wrap it inside all the classes you want.
class DefaultExampleImpl implements Example {
// Implements the methods
}
class YourClass extends YetAnotherClass implements Example {
private Example example = new DefaultClassImpl();
#Override
public void method1() {
this.example.method1();
}
#Override
public String method2(final int parameter) {
return this.example.method2(parameter);
}
}

You can create an abstract class to implement that interface, and make your those classes inherit that abstract class, that should be what you want.

A non abstract class that implements and interface needs to implement all the methods from the interface. A abstract class doesn't have to implement all the methods but cannot initiated. If you create abstract class in your example that implements all the interface methods except one. The classes that extend from these abstract class just have to implement the one not already implemented method.

The Java interfaces could have been called contracts instead to better convey their intent. The declarer promise to provide some functionality, and the using code is guaranteed that the object provides that functionality.
This is a powerful concept and is decoupled from how that functionality is provided where Java is a bit limited and you are not the first to notice that. I have personally found that it is hard to provide "perfect" implementations which just need a subclass or two to be usable in a given situation. Swing uses adapters to provide empty implementations which can then be overrides as needed and that may be the technique you are looking for.

The idea of the interface is to create a series of methods that are abstract enough to be used by different classes that implement them. The concept is based on the DRY principle (Don't repeat yourself) the interface allows you to have methods like run() that are abstract enough to be usuable for a game loop, a players ability to run,

You should understand the funda of interface first. Which is
It is use to provide tight coupling means tight encapsulation
It helps us to hide our code from the external environment i.e. from other class
Interface should have only definition and data which is constant
It provide facility to class open for extension. Hence it cannot be replace by the any other class in java otherwise that class will become close for extension. which means class will not be able to extend any other class.

I think you are struggling with the concept of Object Oriented Design more than anything. In your example above where you state you have 100 classes and 70 of them have the same method implementation (which I would be stunned by). So given an interface like this:
public interface Printable
{
void print();
}
and two classes that have the "same" implementation of print
public class First implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
public class Second implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
you would instead want to do this:
public abstract class DefaultPrinter implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
now for First and Second
public class First extends DefaultPrinter
{
}
public class Second extends DefaultPrinter
{
}
Now both of these are still Printable . Now this is where it gets very important to understand how to properly design object hierarchies. If something IS NOT a DefaultPrinter YOU CANNOT AND SHOULD NOT make the new class extend DefaultPrinter

Spring Autowired Class

When we annotate a class as #Autowired, does it HAVE to be a interface or can it be a class?
All the examples of using Spring I have seen, use an interface and then implement it on a class. The interface type is then used to call a function on the Concrete class. Can we not just simply add #Autowired to a concrete class rather than an interface.
I know of the Programme to a interface analogy in JAVA, but if u are not depending on Polymorphism, then why to write an interface?

No, you don't have to use interfaces, this is completely fine as far as Spring is concerned:
#Service
public class FooService {
#Autowired
private FooDao fooDao;
}
or you can even go for construction injection:
#Service
public class FooService {
private final FooDao fooDao;
public FooService(FooDao fooDao) {
this.fooDao = fooDao;
}
}
Often interfaces are anachronic practice repeated by every subsequent generation. Don't use them if they are not required. And they aren't required if they will always have just one implementation or if you want to mock such a class (modern mocking frameworks mock classes without any problem).
There is also nothing wrong in injecting concrete classes, like FooDao in example above. It has some technical implications wrt. proxying, but nothing that can't be comprehended.

Technically #Autowired could be used for an implementation or an interface. Spring doesn't care about it. Injecting an interface is a design strategy.

#Autowired can also be used with a class instead of interface.
But, using interfaces would be a better practice , since it reduces hard coupling between the components.

Confusion in regarding implementing interface methods

HI
I have a question If interface has got four methods,and I like to implement only two methods, how this could be achieved?
Can any explain is that possible or should I go for implementing all the methods.

You can't "partially" implement an interface without declaring the implementing class abstract, thereby requiring that some subclass provide the remaining implementation. The reason for this is that an interface is a contract, and implementing it declares "I provide the behavior specified by the interface". Some other code is going to use your class via the declared interface and will expect the methods to be there.
If you know the use case does not use the other two methods you can implement them by throwing OperationNotSupported. Whether this is valid or not very much depends on the interface and the user. If the interface can legitimately be partially implemented this way it would be a code smell that the interface is poorly designed and perhaps should have been two interfaces.
You may also be able "implement" the interface by doing nothing, though this is usually only proper for "listener" or "callback" implementations.
In short, it all depends.

If you control the design of the interface, simply split it in two. One interface specifies the two only some implementations implement, and one interface specifies the other two (or inherits the first two and adds more, your choice)

You can make the implementing class abstract and implement two of the 4 methods from the interface.

I think #sateesh 's answer is the one closer to solving your problem. Let me elaborate on it. First of all, it is imperative that any class implementing an interface should provide definitions for all of its methods. But in some cases the user may find no use for a majority of the methods in the interface save for one or two. Consider the following interface having 6 abstract methods:
public interface HugeInterface {
void a();
void b();
void c();
void d();
void e();
void f();
}
Suppose your code finds use for the method 'c()' only and you wish to provide implementation details for only the method 'c()'. You can create a new class HugeInterfaceAdapter in a separate file which implements all the methods of the interface HugeInterface like shown below:
public class HugeInterfaceAdapter implements HugeInterface {
public void a() {}
public void b() {}
public void c() {}
public void d() {}
public void e() {}
public void f() {}
}
Note that you need not provide any actual implementation code for any of the methods. Now comes the interesting part. Yes, your class in which the need to implement a huge interface arose in the first place.
public class MyClass {
HugeInterfaceAdapter mySmallInterface = new HugeInterfaceAdapter() {
#Override
public void c() {
//Your class-specific interface implementation code here.
}
};
}
Now you can use the reference variable mySmallInterface in all the places where a HugeInterface is expected. This may seem a little hackish but I may say that it is endorsed officially by Java and classes like MouseAdapter bears testimony to this fact.

It's not possible.
You can implement all four methods, but the two you don't need should throw an UnsupportedOperationException.

If you want a concrete class which is implementing this interface, then it is not possible to have unimplemented methods, but if you make have abstract class implementing this interface then you can leave any number of methods as you want to be unimplemented.

As other answers mention you cannot have a concrete class implementing only some of the methods of the interface it implements. If you have no control over the interface your class is extending, you can think of having Adapter classes.
The abstract Adapter class can provide dummy implementation for the methods of an interface and the client classes can
extend the Adapter class. (Of course the disadvantage is that you cannot extend more than one class)
This is common practice with GUI programming (using Swing) where the event listener class
might not be interested in implementing all methods specified by the EventListener interface. For example
take a look at the java.awt.event.MouseListener interface and and the corresponding adapter class java.awt.event.MouseAdapter.