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I am reading couple of blogs and answers as well on lambda expression provided in Java 8.
I am not able to figure out, is there any run-time benefit of unit lambda expression or not?
I have copied following text from different sources which are too much confusing to me.
One answer is saying -
"lambdas do NOT create a new scope, they share the same scope as the
enclosing block/environment"
In the one blog -
"There are no runtime benefits of using lambda expressions, so I will
use it cautiously because I don’t mind writing few extra lines of
code."
From one other blog -
"one more benefit lambda expressions Sequential and Parallel Execution
Support by passing behavior in methods"
So there is lots off confusion for me.
Please help me to clear this so I can avoid keeping wrong approach in mind before digging more in to this stuff.
I have run following code and I can say the lambda expression in the following code is just replacement for the anonymous inner class and its running on main thread.
List<Integer> list = new ArrayList<>();
list.add(12);
list.forEach(V -> {
System.out.println(V);
});
Are we reducing any time complexity or space complexity?
“lambdas do NOT create a new scope, they share the same scope as the enclosing block/ environment” is an almost correct statement (they do create a new scope, but not the way inner classes do), but doesn’t have anything to do with runtime performance. This has to do with correctness of the code.
Within an anonymous inner class, identifiers may get resolved through the lexical scope, finding a match in the surrounding scope, or by inheritance, finding a match in the class hierarchy of the anonymous inner class. The rules for resolving identifiers in this scenario are complex and easy to confuse.
Further, the body of an anonymous class creates a new scope that allows to create variables having the same name as local variables of the surrounding context, shadowing these variables.
In contrast, a lambda expression works like other expressions within the context they are written in. They do not inherit any members from the functional interface they will get converted to, they can not create new variables shadowing existing local variables and even this and super have the same meaning as within the surrounding context:
JLS§15.27.2. Lambda Body
Unlike code appearing in anonymous class declarations, the meaning of names and the this and super keywords appearing in a lambda body, along with the accessibility of referenced declarations, are the same as in the surrounding context (except that lambda parameters introduce new names).
So when you have the expressions x.foo(y) and () -> x.foo(y) within the same block, it will be obvious whether x and y will be the same x and y for both expressions and hence, it will be the same foo method in each case, which you can not say that simple for anonymous inner classes, as you have to analyze the entire inner class and its type hierarchy first.
This makes lambda expressions ideal for scenarios where you want to define a local function and, e.g. pass it to a method as a parameter, without even thinking about the actual interface being used. The interface itself does not influence the lambda expression beyond defining the functional signature.
But this also implies that there might be use cases of anonymous classes that can’t be covered by lambda expressions. But the purpose of lambda expressions isn’t to be a general replacement for anonymous inner classes.
When it comes to performance or easy of parallel processing, shmosel’s answer says it already. We can’t make such general statements without knowing which operation/problem we are looking at and which solutions we are actually comparing.
lambdas do NOT create a new scope, they share the same scope as the enclosing block/environment
I'm not sure what your point is with this quote.
There are no runtime benefits of using lambda expressions
Relative to what? Lambdas may perform better than anonymous classes, but neither will outperform simple control flow statements.
one more benefit lambda expressions Sequential and Parallel Execution Support by passing behavior in methods
Streams and lambdas enable declarative programming, which makes parallelization more accessible. So using lambdas may indirectly improve performance.
anonymous inner class is compiled into extra .class file, but lambda is translated dynamically. Here is a detailed post: How will Java lambda functions be compiled?
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As we now Google announced in 2017 first-class support for Kotlin.
But what does it change? Is there more documentation for new features written in Kotlin language?
How does it affect Java?
My another question:
Are Kotlin written aps generally faster?(on Android device)
EDIT: Guys I though this question is properly asked.. Don't vote me down
Java Issues Addressed by Kotlin
NullSafety — The Billion Dollar Mistake is the name given to the danger of null references in code. Kotlin’s type system is aimed at eliminating the danger of these null references. This has been one of the most common pitfalls in Java — and many other programming languages as well.
No more Raw Types — Kotlin is designed with Java interoperability in mind. So now, existing Java code can be called from Kotlin in an effective way. This allows the calling of Java code from Kotlin. Alternatively, Kotlin code can be used in Java rather smoothly.
Invariant Arrays — The basic types used in Kotlin are Numbers, Arrays, Characters, and Strings. Unlike Java, the arrays in this programming language are invariant, meaning that Kotlin does not let a user assign an Array to an Array. This prevents a possible Run time Failure, which is one of the issues faced in Java.
Function Types — In Kotlin, a lambda expression or an anonymous function can access the variables declared in the outer scope. That is opposed to Java’s SAM-conversions — Kotlin has proper function types.
Use-site Variance — Wildcard Types are one of the trickiest parts of Java’s Type System. This issue does not occur in Kotlin — as it does not have any Wildcard Types, just Type Projections and declaration-site variances.
Exceptions — Kotlin does not have any checked exceptions, as all exception classes in this language are the descendants of the class Throwable. And every exception has a message, stack trace, and an optional cause.
Why Choose Kotlin
Smart Casts
Working with the mixed types requires knowing the type of an object at the Run time in order to safely cast the object to the desired type — and, further, to call methods or access properties on it. For class casting in Java, we first check the type of the variable using the ‘instance of’ operator and then cast it to the target type.
Whereas in Kotlin, when we perform an ‘!is’ or ‘is’ check on a variable, the compiler tracks this information and will automatically cast the variable to the target type where is the ‘!is’ or ‘is’ check is true in the scope.
Singletons
Once in a while, a user needs to create an object of a slight modification of some class but without explicitly declaring a new subclass for it. Java handles this case with anonymous inner classes, but Kotlin generalizes the same concept by using object expressions and declarations. Just like the anonymous inner classes in Java, the code in object expressions can access variables from the enclosing scope. But in Kotlin, this is not restricted to final variables like in Java.
Data Classes
The whole purpose of creating classes is to hold data and in some classes — standard functionality with utility functions can be mechanically derived from that data. This is known as a Data Class in Kotlin. These classes generally contain some old boilerplate code in the form of toString(), hashcode(), equals(), setters, and getters.
Basically, Kotlin’s Data Classes are like regular classes but with some additional functionality.
NOTE: There are more things that have in Kotlin which help developers to write faster, consize and clean code
This question already has answers here:
Closure in Java 7 [closed]
(7 answers)
Closed 9 years ago.
I haven't worked with closures in java 7 as yet and was wondering how they work and what is their main advantage or best use case in utilizing them?
Update:
I should have done my homework better. Here is the Project Lambda site for JSR 335 : Lambda Expressions for the Java Programming Language. They are claiming clousures will be in Java 8. I'll have to look into it more to see if that is really the case.
Java 7 has no closures. They've been rumored for a long time, and they are apparently set to appear in Java 8. Of course, I have been promised a Ghostbusters reboot that has been rumored too.
However, you can fake closures with anonymous inner classes. But make no mistake, these aren't closures.
As for the benefits of closures, I can't put it any better than Stack Overflow legend #jaif from this post:
"You can see it as a generalization of a class.
Your class holds some state. It has some member variables that its methods can use.
A closure is simply a more convenient way to give a function access to local state.
Rather than having to create a class which knows about the local variable you want the function to use, you can simply define the function on the spot, and it can implicitly access every variable that is currently visible.
When you define a member method in a traditional OOP language, its closure is "all the members visible in this class".
Languages with "proper" closure support simply generalize this, so a function's closure is "all the variables visible here". If "here" is a class, then you have a traditional class method.
If "here" is inside another function, then you have what functional programmers think of as a closure. Your function can now access anything that was visible in the parent function.
So it's just a generalization, removing the silly restriction that "functions can only be defined inside classes", but keeping the idea that "functions can see whatever variables are visible at the point where they're declared". "
Are Java 8 closures really first-class values or are they only a syntactic sugar?
I would say that Java 8 closures ("Lambdas") are neither mere syntactic sugar nor are they first-class values.
I've addressed the issue of syntactic sugar in an answer to another StackExchange question.
As for whether lambdas are "first class" it really depends on your definition, but I'll make a case that lambdas aren't really first class.
In some sense a lambda wants to be a function, but Java 8 is not adding function types. Instead, a lambda expression is converted into an instance of a functional interface. This has allowed lambdas to be added to Java 8 with only minor changes to Java's type system. After conversion, the result is a reference just like that of any other reference type. In fact, using a Lambda -- for example, in a method that was passed a lambda expression as parameter -- is indistinguishable from calling a method through an interface. A method that receives a parameter of a functional interface type can't tell whether it was passed a lambda expression or an instance of some class that happens to implement that functional interface.
For more information about whether lambdas are objects, see the Lambda FAQ Answer to this question.
Given that lambdas are converted into objects, they inherit (literally) all the characteristics of objects. In particular, objects:
have various methods like equals, getClass, hashCode, notify, toString, and wait
have an identity hash code
can be locked by a synchronized block
can be compared using the == and != and instanceof operators
and so forth. In fact, all of these are irrelevant to the intended usage of lambdas. Their behavior is essentially undefined. You can write a program that uses any of these, and you will get some result, but the result may differ from release to release (or even run to run!).
Restating this more concisely, in Java, objects have identity, but values (particularly function values, if they were to exist) should not have any notion of identity. Java 8 does not have function types. Instead, lambda expressions are converted to objects, so they have a lot baggage that's irrelevant to functions, particularly identity. That doesn't seem like "first class" to me.
Update 2013-10-24
I've been thinking further on this topic since having posted my answer several months ago. From a technical standpoint everything I wrote above is correct. The conclusion is probably expressed more precisely as Java 8 lambdas not being pure (as opposed to first-class) values, because they carry a lot of object baggage along. However, just because they're impure doesn't mean they aren't first-class. Consider the Wikipedia definition of first-class function. Briefly, the criteria listed there for considering functions first-class are the abilities to:
pass functions as arguments to other functions
return functions from other functions
assign functions to variables
store functions in data structures
have functions be anonymous
Java 8 lambdas meet all of these criteria. So that does make them seem first-class.
The article also mentions function names not having special status, instead a function's name is simply a variable whose type is a function type. Java 8 lambdas do not meet this last criterion. Java 8 doesn't have function types; it has functional interfaces. These are used effectively like function types, but they aren't function types at all. If you have a reference whose type is a functional interface, you have no idea whether it's a lambda, an instance of an anonymous inner class, or an instance of a concrete class that happens to implement that interface.
In summary, Java 8 lambdas are more first-class functions than I had originally thought. They just aren't pure first-class functions.
Yes, they are first class values (or will be, once Java 8 is released...)
In the sense that you can pass them as arguments, compose them to make higher order functions, store them in data structures etc. You will be able to use them for a broad range of functional programming techniques.
See also for a bit more definition of what "first class" means in this context:
http://en.wikipedia.org/wiki/First-class_citizen
As I see it, it is syntactic sugar, but in addition with the type inference, a new package java.util.functions and semantic of inner classes it does appear as a first-class value.
A real closure with variable binding to the outside context has some overhead. I would consider the implementation of Java 8 optimal, sufficiently pure.
It is not merely syntactical sugar at least.
And I wouldn't know of any more optimal implementation.
For me Lambdas in Java 8 is just syntax sugar because you cannot use it as First class Citizen (http://en.wikipedia.org/wiki/First-class_function) each function should be wrapped into object it imposes many limitation when comparing to language with pure first class function like SCALA. Java 8 closures can only capture immutable ("effectively final") non-local variables.
Here is better explanation why it is syntax-sugar Java Lambdas and Closures
I hear lambdas are coming soon to a Java near you (J8). I found an example of what they will look like on some blog:
SoccerService soccerService = (teamA, teamB) -> {
SoccerResult result = null;
if (teamA == teamB) {
result = SoccerResult.DRAW;
}
else if(teamA < teamB) {
result = SoccerResult.LOST;
}
else {
result = SoccerResult.WON;
}
return result;
};
So right off the bat:
Where are teamA and teamB typed? Or aren't they (like some weird form of generics)?
Is a lambda a type of closure, or is it the other way around?
What benefits will this give me over a typical anonymous function?
The Lambda expression is just syntactic sugar to implement a target interface, this means that you will be implementing a particular method in the interface through a lambda expression. The compiler can infer the types of the parameters in the interface and that's why you do not need to explicitly define them in the lambda expression.
For instance:
Comparator<String> c = (s1, s2) -> s1.compareToIgnoreCase(s2);
In this expression, the lambda expression evidently implements a Comparator of strings, therefore, this implies the lambda expression is syntactic sugar for implementing compare(String, String).
Thus, the compiler can safely assume the type of s1 and s2 is String.
Your target interface type provides all the information the compiler needs to determine what are the actual types of the lambda parameters.
Briant Goetz, Java Language Architect at Oracle Corportion has published a couple of articles of the work in progress in JDK 8 Lambdas. I believe the answers to your questions are there:
State of Lambda.
State of Lambda Libraries Edition.
Translation of Lambda Expressions
JVMLS 2012: Implementing Lambda Expressions in Java
This second article explains how the lambda expressions are implemented at the bytecode level and may help you delve into the details of your second question.
See this page for a full version of that example (however, the relevant parts are shown below).
The types are inferred from the SoccerService interface and SoccerResult enum, not shown in your snippet:
enum SoccerResult{
WON, LOST, DRAW
}
interface SoccerService {
SoccerResult getSoccerResult(Integer teamA, Integer teamB);
}
The benefit (of lambdas versus standard anonymous classes) is just reduced verbosity:
(x, y) => x + y
versus something like:
new Adder()
{
public int add(int x, int y)
{
return x + y;
}
}
For the difference between a closure and a lambda, see this question.
Where are teamA and teamB typed? Or aren't they (like some weird form of generics)?
Lambda's use target typing, much like generic method calls (since 1.5) and the diamond [not an] operator (since 1.7). Roughly, where the type the result applied to is stated (or can be inferred) that is used to supply the type of the Single Abstract Method (SAM) base type and hence the method parameter types.
As an example of generic method inference in 1.5:
Set<Team> noTeams = Collections.emptySet();
And diamond operator in 1.7:
Set<Team> aTeams = new HashSet<>();
Team, team, team, team, team, team. I even love saying the word team.
Is a lambda a type of closure, or is it the other way around?
A lambda is a limited form of closure in almost exactly the same way as anonymous inner classes, but with some random differences to catch you out:
The outer this is not hidden by an inner this. This means that the same text in a lambda and an anonymous inner class can mean subtly but completely different things. That should keep Stack Overflow busy with odd questions.
To make up for the lack of inner this, if assigned directly to a local variable, then that value is accessible within the lambda. IIRC (I could check, but wont), in an anonymous inner class the local will be in scope and hide variables in an outer scope but you can't use it. I believe the lack of an instance initialiser makes this much easier to specify.
Local fields that do not happen to be marked final but could be, are treated as if they are final. So not are they in scope, but you can actually read (though not write) them.
What benefits will this give me over a typical anonymous function?
A briefer syntax. That's about it.
Of course the rest of the Java syntax is just as hopelessly bad as ever.
I don't believe this is in the initial implementation, but instead of being implemented as [inner] classes, lambdas can use method handles. The performance of method handles falls somewhat short of earlier predictions. Doing away with classes, should reduce bytecode footprint, possibly runtime footprint and certainly class loading time. There could be an implementation where most anonymous inner classes (not Serializable, trivial static initialiser) didn't go through the poorly conceived class loading mechanism without any particularly noticeable incompatibilities.
(Hope I've got the terminology wrt hiding correct.)
Is my interpretation of lambda expression in the context of c++ and Java is correct?
They are not quite as same. Both create unnamed classes, but their similarity ends at that point.
In C++ you create a closure catching your local variables, optionally by reference. In Java, you just get a snapshot of the current local variable's values (and those variables need to be "final").
The purpose of anonymous inner classes is to extend another class or implement another interface ad-hoc. For that reason, anonymous inner classes can simulate the job of lambda expressions to some extend, for example by implementing the Runnable interface. Lambda expressions are specifically designed to be called and possibly modify the local variables in their environment.
A Java anonymous inner class can refer to final data in the enclosing method, and to all data (including mutable) in the enclosing class. So methods in anonymous classes cannot change the values of variables in the enclosing method, but they can change values of members in the enclosing class.
A C++ lambda can refer to all data (including mutable) in the enclosing function, and if it is nested inside a member function then it can do the same for the data of the enclosing class. The precise degree of dependency on the enclosing scope(s) is declared by the programmer, so it is explicit rather than implicit.
This makes them quite similar but the Java feature treats local variables/parameters in methods differently, on the principle that they should not be mutable from outside the method, especially in a language which has traditionally employed threading so casually.
Compare with C# lambdas, which have no restrictions and all dependencies are implicit. This makes them by far the least verbose of these features (also helped by them having the best type inference). But on the downside, they invalidate all the simple rules about threading, i.e. it is no longer necessarily true that local variables are "on the thread stack" and hence never require locking before access.
C++0x lambda expressions are unnamed methods, Javas anonymous classes are unnamed classes. So they share not having a name but the concepts are different.
Starting with the most obvious fact, that lambda functions (may) return a value and anonymous classes can be used to create instances (objects).
BTW - wikipedia mentions, that only lambda functions are proposed for C++0x.