Spring bean lifecycle documentation often mention callback methods.
While trying to find the meaning of callback, I came through few links that mention it is passing of one function as an argument to another, which could be achieved through interfaces in Java.
I am confused, if this is callback then what are lambda expression and functional interfaces ? Are they same or different?
Callback is a pattern where you pass a function somewhere and it gets called later.
Functional interfaces are a way of specifying what kind of function you expect.
A lambda is a quick way of implementing a functional interface. Lambdas are useful if you want to use callbacks.
For example:
Suppose I am going to generate a message at some point in the future, and you want to be told when it happens. I have a method that lets you give me a function to call when the message is ready.
public void callThisWithMessage(Consumer<String> messageConsumer);
You give me a message consumer, and I will call it later when the message is ready. This is an example of a callback.
The type of function you can give to me here is specified by the Consumer interface, which is a functional interface. This particular functional interface says that it has a method that accepts a parameter (in this case a string).
If you want to use my callback service, you can implement a consumer using a lambda function.
callThisWithMessage(msg -> System.out.println("Message received: "+msg));
This creates a lambda function that implements the functional interface Consumer<String>, and passes that to my method for subsequent callback.
Lambda expressions are one of several ways to implement functional interfaces.
Functional interfaces are used as callbacks, but not all callbacks are functional interfaces. Interfaces used as callbacks can have multiple abstract methods, while functional interfaces can only have a single abstract method.
Related
Beginning with Java SE 8, if the formal parameter of a method is a functional interface, the argument can be either an object implementing that interface or a reference to some method. It means that the argument can also be a reference to a method that is not logically related to the interface's purpose. Is it possible to force the argument to be only an object implementing the interface, but not to be a method reference? Although it is possible to make the interface non-functional by adding a second abstract method, that additional method nevertheless should be implemented. Is there another way?
Is it possible to force the argument to be only an object implementing the interface, but not to be a method reference?
There is not.
Although it is possible to make the interface non-functional by adding a second abstract method, that additional method nevertheless should be implemented. Is there another way?
Indeed, that's the downside of doing that. You can't provide a default implementation, as an interface that has exactly 1 non-defaulted method is considered a FunctionalInterface, and you can't decree it to be not so.
What you can do, however, is turn that interface into an abstract class, which aren't eligible for being supplied in lambda/methodref form.
More generally, don't fight java features. If someone uses a method ref, they know what they are doing. Or they don't, but if they are just stumbling about without a clue, trust me, you can't stop an idiot from ruining a code base by designing good APIs and adding every linter rule you manage to scrounge together. Idiots are far too inventive to be stopped by mere mortals.
I'm trying to learn Akka (with Java) and understand some code. I have seen something like this, this method signature in a Actor class :
#Override
public void aroundReceive(PartialFunction<Object, BoxedUnit> receive, Object msg)
I've never heard of that method before and don't understand it. What is the purpose of that method ? Where does this PartialFunction<Object, BoxedUnit> receive argument comes from ? I thought it was up to the programmer to implement receive object.
As for PartialFunction<Object, BoxedUnit>, the receive function in an (untyped) actor is an instance of that (this is made abundantly clear in the Scala API, and somewhat less-so in the Java API).
An Actor implementing this (typically via a Scala mixin or extending an abstract class which overrides it) would take the receive function from the actor implemented by the programmer and intercept calls to certain messages (e.g. timing messages) or do pre-/post-processing of messages which are passed onto the given receive.
PartialFunction<Object, BoxedUnit> basically means:
this is a function which doesn't promise to have a result for any particular input; it's the responsibility of the caller to check beforehand (isDefinedAt) if the function will have a result or to accept that the function will throw an exception. (PartialFunction: a function which is not defined over the entirety of its domain)
Object (or in Scala terms Any (technically AnyRef, but autoboxing lets us forget that for a moment)): the function can theoretically accept anything
BoxedUnit indicates that the function returns no useful result (Unit in Scala is like void in Java, but is actually an object (a singleton to be precise)).
(to some extent PartialFunction<Object, BoxedUnit> is the type which tells us and the compiler the least information possible).
First, I am using Akka in Scala, and not in Java. I hope this answer will give you a lead.
In Scala, this method is #InternalApi. The docs of that in Scala is:
Marks APIs that are considered internal to Akka and may change at any point in time without any
warning.
For example, this annotation should be used when the Scala {#code private[akka]} access
restriction is used, as Java has no way of representing this package restricted access and such
methods and classes are represented as {#code public} in byte-code
One purpose of this method is to wrap the receive method with custom behaviour. You can see an example for that in the Timers trait.
Java 8 gave us many fun ways to use functional interfaces and with them a new annotation: #FunctionalInterface. Its job is to tell the compiler to yell at us if we fail to stick to the rules of a functional interface (only one abstract method that needs overriding please).
There are 43 interfaces in the java.util.function package with this annotation. A search of jdk.1.8.0/src for #FunctionalInterface only turns up 57 hits. Why are the other interfaces (such as AutoCloseable) that could have added #FunctionalInterface still missing it?
There is a bit of a vague hint in the annotations documentation:
"An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface"
Is there any good reason NOT to intend that an interface I've designed (that may simply happen to be a functional interface) not be used as one? Is leaving it off an indication of anything besides not realizing it could have been added?
Isn't adding abstract methods to any published interface going to screw anyone implementing it, functional or not? I feel cynical assuming they just didn't bother to hunt them all down but what other explanation is there?
Update: After looking over "Should 'Comparable' be a 'Functional interface'?" I find I still have nagging questions. When a Single Method Interface and a Functional Interface are structurally identical what's left to be different? Is the difference simply the names? Comparable and Comparator are close enough to the same semantically. Turns out they are different structurally though so still not the best example...
Is there a case when an SMI is structurally fine to use as a Functional Interface but still discouraged over the semantic meaning of the name of the interface and the method? Or perhaps the contract implied by the Javadocs?
Well, an annotation documenting an intention would be useless if you assume that there is always that intention given.
You named the example AutoCloseable which is obviously not intended to be implemented as a function as there’s Runnable which is much more convenient for a function with a ()->void signature. It’s intended that a class implementing AutoCloseable manages an external resource which anonymous classes implemented via lambda expression don’t do.
A clearer example is Comparable, an interface not only not intended to be implemented as a lambda expression, it’s impossible to implement it correctly using a lambda expression.
Possible reasons for not marking an interface with #FunctionalInterface by example:
The interface has programming language semantics, e.g. AutoClosable or Iterable (that’s unlikely to happen for your own interfaces)
It’s not expected that the interface has arbitrary implementations and/or is more an identifier than the actual implementation, e.g. java.net.ProtocolFamily, or java.lang.reflect.GenericArrayType (Note that the latter would also inherit a default implementation for getTypeName() being useless for lambda implementations as relying on toString())
The instances of this interface should have an identity, e.g. java.net.ProtocolFamily, java.nio.file.WatchEvent.Modifier, etc. Note that these are typically implemented by an enum
Another example is java.time.chrono.Era which happens to have only a single abstract method but its specification says “Instances of Era may be compared using the == operator.”
The interface is intended to alter the behavior of an operation for which an implementation of the interface without inheriting/implementing anything else makes no sense, e.g. java.rmi.server.Unreferenced
It’s an abstraction of common operations of classes which should have more than just these operations, e.g. java.io.Closeable, java.io.Flushable, java.lang.Readable
The expected inheritance is part of the contract and forbids lambda expression implementations, e.g. in java.awt: ActiveEvent should be implemented by an AWTEvent, PrinterGraphics by a Graphics, the same applies to java.awt.print.PrinterGraphics (hey, two interfaces for exactly the same thing…), wheras javax.print.FlavorException should be implemented by a javax.print.PrintException subclass
I don’t know whether the various event listener interfaces aren’t marked with #FunctionalInterface for symmetry with other multi-method event listener that can’t be functional interfaces, but actually event listeners are good candidates for lambda expressions. If you want remove a listener at a later time, you have to store the instance but that’s not different to, e.g. inner class listener implementations.
The library maintainer has a large codebase with more than 200 candidate types and not the resources to discuss for every interface whether it should be annotated and hence focuses on the primary candidates for being used in a functional context. I’m sure, that, e.g. java.io.ObjectInputValidation, java.lang.reflect.InvocationHandler, juc RejectedExecutionHandler & ThreadFactory wouldn’t be bad as #FunctionalInterface but I have no idea whether, e.g. java.security.spec.ECField makes a good candidate. The more general the library is, the more likely users of the library will be able to answer that question for a particular interface they are interested in but it would be unfair to insist on the library maintainer to answer it for all interfaces.
In this context it makes more sense to see the presence of a #FunctionalInterface as a message that an interface is definitely intended to be usable together with lambda expressions than to treat the absence of the annotation as an indicator for it’s being not intended to be used this way. This is exactly like the compiler handles it, you can implement every single abstract method interface using a lambda expression, but when the annotation is present it will ensure that you can use this interface in this way.
Planned expansion. Just because an interface matches the requirements of an SMI now doesn't mean that expansion isn't needed later.
In java 8, functional interface is an interface having exactly one abstract method called functional method to which the lambda expression’s parameter and return types are matched.
The java.util.function contains general purpose functional interfaces used by JDK and also available for end users. While they are not the complete set of funtional interfaces to which lambda expressions might be applicable, but they provide enough to cover common requirements. You are free to create your own functional interfaces whenever existing set are not enough.
There are many such interfaces available which deserves to be designated as functional interface but java.util.function package already provides functional interfaces for our almost all purposes.
For example look into following code.
public interface Comparable<T> {
public int compareTo(T o);
}
#FunctionalInterface
public interface ToIntFunction<T> {
int applyAsInt(T value);
}
public static void main(String[] args){
ToIntFunction<String> f = str -> Integer.parseInt(str);
Comparable<String> c = str -> Integer.parseInt(str);
}
Comparable can also take an object and derive some int type value but there is a more general dedicated interface ToIntFunction is provided to perform this task. There is no such hard rule that all the deserving interfaces should be annotated with #FunctionalInterface but to gain the advantage of lambda feature, the interface should fulfill all criterias defined by FunctionalInterface.
I am working in java from some time. I know their are some thing knows as interface in java. While reading about them I come to know their is marker interface. Recently when i started reading about java 8 I come to know about an other interface Functional Interface.
I am just wondering what are the different kind of Interfaces available in java?
The Java language specification doesn't itself define the term marker interface and the term has been coined by authors, developers and designers. One common question asked is if we can create a marker interface or not and the answer is yes because of following reason:
We can't create marker interface similar to Serializable or Cloneable but we can simulate the functionality by writing extra code around the custom marker interface.
An empty interface is known as tag or marker interface. For example Serializable, EventListener, Remote(java.rmi.Remote) are tag interfaces. These interfaces do not have any field and methods in it.
Read more here: http://beginnersbook.com/2016/03/tag-or-marker-interfaces-in-java/
Functional Interface is the new addition in Java 8, An interface with exactly one abstract method is called Functional Interface. Read more here.
There are no other types of Interfaces in Java.
There's no special meaning for each.
Marker interface is kind of "design pattern", you attach a label/tag to a set of objects in order to indicates that they have something in common, they're OK for some kind of process or operations. Serializable is a typical example, it marks objects that they can be serialized/deserialized.
On the other hand for FunctionalInterface, it's just an interface with restriction that can only have one abstract method, and thus represents a single function contract. Java 8 add lambda expression for functional programming, for FP we need to pass function back and forth so often. Say we have an interface like:
public interface StringTrasformer {
String transform(Object obj);
}
Traditionally we can only create instance of asynchronous class like:
someObj.doTransform(new StringTransformer() {
#Override
public String transform(Object object) {
return "result";
}
});
But there's only one method to be implemented, so it's no need to make code so verbose, with lambda expression it could be as short as:
abc.doTransform(object -> "result");
Annotation FunctionalInterface is used for compiler to check whether the interface you have annotated is a valid one. Even functional interface is for lambda expressions, method referencesand constructor references, but nothing prevents you to use it the traditional way. Because essentially it is just an normal interface.
I've read today about the Java 8 release. But I don't understand fully the concept of reference methods in Java 8. Does this mean that Java now has the support of functions as first class objects? I have seen, how to construct a reference to function. But it seems to me, that the Converter object they provide has quite limited functionality. Is it now possible in Java:
to pass the function as the argument to another function?
to return the function as the return value from another function?
and what about closures? Are they implemented fully like in functional languages, or they do have some limitations? It seems to me that there are some limitations (you cannot change the value of the variable you reference in a closure, it must be marked as final and etc).
The most important aspects of first-class functions have been blended into the existing Java type system. No true function type has been introduced; any single-method interface is its own "function type". So, as for your first two questions, you can freely pass around instances of such functional interfaces.
There are many subtle changes to the semantics, which allow one to use lambda syntax/method references to implement any such interface. You can even use higher-order functions such as compose, which returns a generic Function type, and pass it to a method which expects a compatible functional interface type.
you cannot change the value of the variable you reference in a closure
This is not a limitation specific to Java. In fact, most FP languages don't support mutable variables of any kind. Note that there is no requirement to use the final keyword on the variable; the concept of effectively final takes care of that.
It is possible. How do you do it?
First construct a "Functional Interface" (or use one of the provided ones). A functional interface is an interface with a single method. java.lang.Runnable is an example.
Second, write a method that takes a functional interface as a parameter.
public void doAThing(Runnable r) {
r.run();
}
Third, write a method with the correct signature.
public class MyClass {
public void runAThing() {
System.out.println("I executed!");
}
}
Fourth, call the function passing in a method reference.
MyClass mc = new MyClass();
doAThing(mc::runAThing);
You'll note that none of the classes you've wrote ever explicitly implements Runnable. This is handled for you by the compiler.
You can do something similar using a lamdba expression:
doAThing(() -> System.out.println("I executed as a lamdba expression!"));
To return the function as a value from another function, just return an instance of Runnable.
Methods are not first class objecs in Java, apart from the already existing usage in reflection, to answer your questions:
Yes, you can pass it on, but it needs to satisfy the signature.
For a void method() you use a Runnable, like this:
Runnable method = this::method if it is in the same class, and then run the actual method with method.run().
However for a double method() you need to use a DoubleSupplier, like this:
DoubleSupplier method = this::method, and then use it as double d = method.get().
And many more signatures, and you can even define your own with Functional Interfaces.
Yes it is possible, but only specific signatures as shown in Point 1.
Lambdas behave exactly as anonymous inner classes, which are closures by itself, Java has had support for closures since they introduced anonymous inner classes. The only thing that is added now is that the syntax is much prettier.
No, not first class functions. Lambda expression are wrapped in a interface, and some syntatic sugar applied for brevity.
But you can;t create a function on its own and pass it around different methods, whether thats a key point or not is a different question.