I just started learning java yesterday since i want to make android apps. So far i only know c. I am reading a book called "Head first java" and it keeps talking about object and classes which are pretty new to me. I just have one question and if someone can clarify this for me that'd be very helpful:
What is/are the difference(s) between classes(java) and structures(c)?
ps: I'd also love if you can recommend me a book that really is for absolute beginners because the book i'm reading right now doesn't have enough details for those that are completely new to object oriented programming. Thank you.
No. You don't get inheritance, methods, abstract methods, polymorphism, and many more object oriented concepts with Structs.
You can try to make C conform to object oriented behavior using structs, but that's different from using an object oriented language.
"Aren't classes in Java equivalent to structures in c?"
Short answer, No.
My answer is too short cause the differences is the difference between two completely different paradigms, and it needs a complete book (not even a chapter) like : "Thinking in java".
Java classes perform several roles, only one of which is (implicitly) defining the layout of the data structure called the instance of that class, but you are right in thinking that objects are essentially data structures, just like instances of struct.
The main difference is that C's struct is something much more raw and "close to the metal" than a Java class. Basically, with much additional framework code and strict adherence to many conventions you could reimplement a good part of Java's objects, but the syntax would not be as clean as in Java.
There are many other differences, such as what exactly is covered by compiler's static checking, automatic memory management, etc. In the end, differences outweigh similarities, but as a point of understanding, it is not wrong to compare Java objects with C structs.
I know, that I am contradicting the view of a lot of people here, but you are right, a class really is nothing more or less than a struct with a lot of bells and whistles.
To be precise, the C++ language actually specifies that an instance of a class without virtual functions and/or virtual inheritance has the same memory layout as a C struct with the same data members in the same order.
All that is added to the struct to provide runtime polymorphism is a single v-table pointer, which is used to look up the virtual members. Inheritance is generated by including the base class structures as data members of the child class struct.
Of course, when you think about it for a second, you realize that classes must be implemented with structs, one way or another. C++/Java classes must be implemented somehow, after all. And it helps to remind yourself that object orientation is not primarily a language feature, it is a paradigm that can be used in any imperative programming language. It can be used in Java, in C++, in C, in Pascal, in Fortran, and even in assembler. All which the so-called object oriented languages add, is syntactic sugar to handle all the technical stuff of object orientation.
Nevertheless, it is important to learn all the bells and whistles when you start working in C++ or Java.
I've first asked this question about the use of final with anonymous inner classes in Java:
Why do we use final keyword with anonymous inner classes?
I'm actually reading the Scala book of Martin Odersky. It seems Scala simplifies a lot of Java code, but for Scala closures I could notice a significant difference.
While in Java we "simulate" closures with an anonymous inner class, capturing a final variable (which will be copied to live on the heap instead of the stack) , it seems in Scala we can create a closure which can capture a val, but also a var, and thus update it in the closure call!
So it is like we can use a Java anonymous innerclass without the final keyword!
I've not finished reading the book, but for now i didn't find enough information on this language design choice.
Can someone tell me why Martin Odersky, who really seems to take care of function's side effects, choose closures to be able to capture both val and var, instead of only val?
What are the benefits and drawbacks of Java and Scala implementations?
Thanks
Related question:
With Scala closures, when do captured variables start to live on the JVM heap?
An object can be seen a bag of closures that share access to the same environment and that environment is usually mutable. So why make closures generated from anonymous functions less powerful?
Also, other languages with mutable variables and anonymous functions work the same way. Principle of lease astonishment. Java is actually WEIRD in not allowing mutable variables to be captured by inner classes.
And sometimes they're just darn useful. For example a self modifying thunk to create your own variant of lazy or future processing.
Downsides? They have all the downsides of shared mutable state.
Here are some benefits and drawbacks.
There is a principle in language design that the cost of something should be apparent to the programmer. (I first saw this in Holt's Design and Definition of the Turing Language, but I forget the name he gave it.) Two things that look the same should cost the same. Two local vars should have similar costs. This is in Java's favour. Two local vars in Java are implemented the same and so cost the same regardless of whether one of them is mentioned in an inner class. Another point in Java's favour is that in most cases the captured variable is indeed final, so there is little cost to the programmer to be prevented from capturing nonfinal local vars. Also the insistence on final simplifies the compiler, since it means that all local variables are stack variables.
There is another principle of language design that says be orthogonal. If a val can be captured why not a var. As long as there is a sensible interpretation, why put in restrictions. When languages are not orthogonal enough, they seems perverse. On the other-hand, languages that have too much orthogonality may have complex (and hence buggy and/or late and/or few) implementations. For example Algol 68 had orthogonality in spades, but implementing it was not easy, meaning few implementations and little uptake. Pascal (designed at about the same time) had all sorts of inelegant restrictions that made writing compilers easier. The result was lots of implementations and lots of uptake.
I came across this during learning Java in the beginning, but I am learning top down, so I would like some direction:
This was on a Java tutorial relatively early on:
As one final general note, you should be aware that subroutines in Java are often referred to as methods. Generally, the term "method" means a subroutine that is contained in a class or in an object. Since this is true of every subroutine in Java, every subroutine in Java is a method (with one very technical exception). The same is not true for other programming languages. Nevertheless, the term "method" is mostly used in the context of object-oriented programming, and until we start doing real object-oriented programming in Chapter 5, I will prefer to use the more general term, "subroutine." However, I should note that some people prefer to use the term "method" from the beginning.
My question is what is that "one very technical exception." Since I am learning multiple stack technologies, I would like to know this specific exception he is referring to. It comes from this tutorial :
http://math.hws.edu/javanotes/c2/
Quoting from chapter 5 of the tutorial you linked to:
Constructors are subroutines, but they are subroutines of a special
type. They are certainly not instance methods, since they don't belong
to objects. Since they are responsible for creating objects, they
exist before any objects have been created. They are more like static
member subroutines, but they are not and cannot be declared to be
static. In fact, according to the Java language specification, they
are technically not members of the class at all! In particular,
constructors are not referred to as "methods."
Class constructors are not methods.
I am a long time OO programmer and a functional programming newbie. From my little exposure algebraic data types only look like a special case of inheritance to me where you only have one level hierarchy and the super class cannot be extended outside the module.
So my (potentially dumb) question is: If ADTs are just that, a special case of inheritance (again this assumption may be wrong; please correct me in that case), then why does inheritance gets all the criticism and ADTs get all the praise?
Thank you.
I think that ADTs are complementary to inheritance. Both of them allow you to create extensible code, but the way the extensibility works is different:
ADTs make it easy to add new functionality for working with existing types
You can easily add new function that works with ADT, which has a fixed set of cases
On the other hand, adding new case requires modifying all functions
Inheritance makes it easy to add new types when you have fixed functionality
You can easily create inherited class and implement fixed set of virtual functions
On the other hand, adding a new virtual function requires modifying all inherited classes
Both object-oriented world and functional world developed their ways to allow the other type of extensibility. In Haskell, you can use typeclasses, in ML/OCaml, people would use dictionary of functions or maybe (?) functors to get the inhertiance-style extensibility. On the other hand, in OOP, people use the Visitor pattern, which is essentially a way to get something like ADTs.
The usual programming patterns are different in OOP and FP, so when you're programming in a functional language, you're writing the code in a way that requires the functional-style extensibility more often (and similarly in OOP). In practice, I think it is great to have a language that allows you to use both of the styles depending on the problem you're trying to solve.
Tomas Petricek has got the fundamentals exactly right; you might also want to look at Phil Wadler's writing on the "expression problem".
There are two other reasons some of us prefer algebraic data types over inheritance:
Using algebraic data types, the compiler can (and does) tell you if you have forgotten a case or if a case is redundant. This ability is especially useful when there are many more operations on things than there are kinds of thing. (E.g., many more functions than algebraic datatypes, or many more methods than OO constructors.) In an object-oriented language, if you leave a method out of a subclass, the compiler can't tell whether that's a mistake or whether you intended to inherit the superclass method unchanged.
This one is more subjective: many people have noted that if inheritance is used properly and aggressively, the implementation of an algorithm can easily be smeared out over a half a dozen classes, and even with a nice class browser at can be hard to follow the logic of the program (data flow and control flow). Without a nice class browser, you have no chance. If you want to see a good example, try implementing bignums in Smalltalk, with automatic failover to bignums on overflow. It's a great abstraction, but the language makes the implementation difficult to follow. Using functions on algebraic data types, the logic of your algorithm is usually all in one place, or if it is split up, its split up into functions which have contracts that are easy to understand.
P.S. What are you reading? I don't know of any responsible person who says "ADTs good; OO bad."
In my experience, what people usually consider "bad" about inheritance as implemented by most OO languages is not the idea of inheritance itself but the idea of subclasses modifying the behavior of methods defined in the superclass (method overriding), specifically in the presence of mutable state. It's really the last part that's the kicker. Most OO languages treat objects as "encapsulating state," which amounts to allowing rampant mutation of state inside of objects. So problems arise when, for example, a superclass expects a certain method to modify a private variable, but a subclass overrides the method to do something completely different. This can introduce subtle bugs which the compiler is powerless to prevent.
Note that in Haskell's implementation of subclass polymorphism, mutable state is disallowed, so you don't have such issues.
Also, see this objection to the concept of subtyping.
I am a long time OO programmer and a functional programming newbie. From my little exposure algebraic data types only look like a special case of inheritance to me where you only have one level hierarchy and the super class cannot be extended outside the module.
You are describing closed sum types, the most common form of algebraic data types, as seen in F# and Haskell. Basically, everyone agrees that they are a useful feature to have in the type system, primarily because pattern matching makes it easy to dissect them by shape as well as by content and also because they permit exhaustiveness and redundancy checking.
However, there are other forms of algebraic datatypes. An important limitation of the conventional form is that they are closed, meaning that a previously-defined closed sum type cannot be extended with new type constructors (part of a more general problem known as "the expression problem"). OCaml's polymorphic variants allow both open and closed sum types and, in particular, the inference of sum types. In contrast, Haskell and F# cannot infer sum types. Polymorphic variants solve the expression problem and they are extremely useful. In fact, some languages are built entirely on extensible algebraic data types rather than closed sum types.
In the extreme, you also have languages like Mathematica where "everything is an expression". Thus the only type in the type system forms a trivial "singleton" algebra. This is "extensible" in the sense that it is infinite and, again, it culminates in a completely different style of programming.
So my (potentially dumb) question is: If ADTs are just that, a special case of inheritance (again this assumption may be wrong; please correct me in that case), then why does inheritance gets all the criticism and ADTs get all the praise?
I believe you are referring specifically to implementation inheritance (i.e. overriding functionality from a parent class) as opposed to interface inheritance (i.e. implementing a consistent interface). This is an important distinction. Implementation inheritance is often hated whereas interface inheritance is often loved (e.g. in F# which has a limited form of ADTs).
You really want both ADTs and interface inheritance. Languages like OCaml and F# offer both.
I've been reading a lot lately about the next release of Java possibly supporting closures. I feel like I have a pretty firm grasp on what closures are, but I can't think of a solid example of how they would make an Object-Oriented language "better". Can anyone give me a specific use-case where a closure would be needed (or even preferred)?
As a Lisp programmer I would wish that the Java community understands the following difference: functions as objects vs. closures.
a) functions can be named or anonymous. But they can also be objects of themselves. This allows functions to be passed around as arguments, returned from functions or stored in data structures. This means that functions are first class objects in a programming language.
Anonymous functions don't add much to the language, they just allow you to write functions in a shorter way.
b) A closure is a function plus a binding environment. Closures can be passed downwards (as parameters) or returned upwards (as return values). This allows the function to refer to variables of its environment, even if the surrounding code is no longer active.
If you have a) in some language, then the question comes up what to do about b)? There are languages that have a), but not b). In the functional programming world a) (functions) and b (functions as closures) is nowadays the norm. Smalltalk had a) (blocks are anonymous functions) for a long time, but then some dialects of Smalltalk added support for b) (blocks as closures).
You can imagine that you get a slightly different programming model, if you add functions and closures to the language.
From a pragmatic view, the anonymous function adds some short notation, where you pass or invoke functions. That can be a good thing.
The closure (function plus binding) allows you for example to create a function that has access to some variables (for example to a counter value). Now you can store that function in an object, access it and invoke it. The context for the function object is now not only the objects it has access to, but also the variables it has access to via bindings. This is also useful, but you can see that variable bindings vs. access to object variables now is an issue: when should be something a lexical variable (that can be accessed in a closure) and when should it be a variable of some object (a slot). When should something be a closure or an object? You can use both in the similar ways. A usual programming exercise for students learning Scheme (a Lisp dialect) is to write a simple object system using closures.
The result is a more complicated programming language and a more complicated runtime model. Too complicated?
They don't make an Object-Oriented language better. They make practical languages more practical.
If you're attacking a problem with the OO hammer - represent everything as interactions between objects - then a closure makes no sense. In a class-based OO language, closures are the smoke-filled back rooms where stuff gets done but no one talks about it afterwards. Conceptually, it is abhorrent.
In practice, it's extremely convenient. I don't really want to define a new type of object to hold context, establish the "do stuff" method for it, instantiate it, and populate the context... i just want to tell the compiler, "look, see what i have access to right now? That's the context i want, and here's the code i want to use it for - hold on to this for me 'till i need it".
Fantastic stuff.
The most obvious thing would be a pseudo-replacement for all those classes that just have a single method called run() or actionPerformed() or something like that. So instead of creating a Thread with a Runnable embedded, you'd use a closure instead. Not more powerful than what we've got now, but much more convenient and concise.
So do we need closures? No. Would they be nice to have? Sure, as long as they don't feel bolted on, as I fear they would be.
I suppose for supporting core functional programming concepts, you need closures. Makes the code more elegant and composable with the support for closures. Also, I like the idea of passing around lines of code as parameters to functions.
There are some very useful 'higher order functions' which can do operations on lists using closures. Higher order functions are functions having 'function objects' as parameters.
E.g. it is a very common operation to apply some transformation to every element in a list. This higher order function is commonly called 'map' or 'collect'. (See the *. spread operator of Groovy).
For example to square each element in a list without closures you would probably write:
List<Integer> squareInts(List<Integer> is){
List<Integer> result = new ArrayList<Integer>(is.size());
for (Integer i:is)
result.add(i*i);
return result;
}
Using closures and map and the proposed syntax, you could write it like that:
is.map({Integer i => i*i})
(There is a possible performance problem here regarding boxing of primitive types.)
As explained by Pop Catalin there is another higher order function called 'select' or 'filter': It can be used to get all the elements in a list complying to some condition. For example:
Instead of:
void onlyStringsWithMoreThan4Chars(List<String> strings){
List<String> result = new ArrayList<String>(str.size()); // should be enough
for (String str:strings)
if (str.length() > 4) result.add(str);
return result;
}
Instead you could write something like
strings.select({String str => str.length() > 4});
using the proposal.
You might look at the Groovy syntax, which is an extension of the Java language to support closures right now. See the chapter on collections of the Groovy User Guide for more examples what to do with closures.
A remark:
There is perhaps some clarification needed regarding the term 'closure'. What I've shown above are strictly spoken no closures. They are just 'function objects'.
A closure is everything which can capture - or 'close over' - the (lexical) context of the code surrounding it. In that sense there are closures in Java right now, i.e. anonymous classes:
Runnable createStringPrintingRunnable(final String str){
return new Runnable(){
public void run(){
System.out.println(str); // this accesses a variable from an outer scope
}
};
}
Java doesn't need closures, an Object oriented language can do everything a closure does using intermediate objects to store state or do actions (in Java's case inner classes).
But closures are desirable as a feature because they greatly simplify the code and increase readability and as a consequence the maintainability of the code.
I'm no Java specialist but I'm using C# 3.5 and closures are one of my favorite features of the language, for example take the following statement as an example:
// Example #1 with closures
public IList<Customer> GetFilteredCustomerList(string filter) {
//Here a closure is created around the filter parameter
return Customers.Where( c => c.Name.Contains(filter)).ToList();
}
now take an equivalent example that doesn't use closures
//Example #2 without closures, using just basic OO techniques
public IList<Customer> GetFilteredCustomerList(string filter) {
return new Customers.Where( new CustomerNameFiltrator(filter));
}
...
public class CustomerNameFiltrator : IFilter<Customer> {
private string _filter;
public CustomerNameFiltrator(string filter) {
_filter = filter;
}
public bool Filter(Customer customer) {
return customer.Name.Contains( _filter);
}
}
I know this is C# and not Java but the idea is the same, closures are useful for conciseness, and make code shorter and more readable. Behind the scenes, the closures of C# 3.5 do something that's looks very similar to example #2 meaning the compiler creates a private class behind the scenes and passes the 'filter' parameter to it.
Java doesn't need closures to work, as a developer you don't need them either, but, they are useful and provide benefits so that means that they are desirable in a language that is a production language and one of it's goals is productivity.
I've been reading a lot lately about the next release of Java possibly supporting closures. I feel like I have a pretty firm grasp on what closures are, but I can't think of a solid example of how they would make an Object-Oriented language "better."
Well, most people who use the term "closure" actually mean "function object", and in this sense, function objects make it possible to write simpler code in certain circumstances such as when you need custom comparators in a sort function.
For example, in Python:
def reversecmp(x, y):
return y - x
a = [4, 2, 5, 9, 11]
a.sort(cmp=reversecmp)
This sorts the list a in reverse order by passing the custom comparison functoin reversecmp. The addition of the lambda operator makes things even more compact:
a = [4, 2, 5, 9, 11]
a.sort(cmp=lambda x, y : y - x)
Java does not have function objects, so it uses "functor classes" to simulate them. In Java you do the equivalent operation by implementing a custom version of the Comparator class, and passing that to the sort function:
class ReverseComparator implements Comparator {
public compare(Object x, Object y) {
return (Integer) y - (Integer) x;
}
...
List<Integer> a = Arrays.asList(4, 2, 5, 9, 11);
Collections.sort(a, new ReverseComparator());
As you can see, it gives the same effect as closures, but is clumsier and more verbose. However, the addition of anonymous inner classes obviates most of the pain:
List<Integer> a = Arrays.asList(4, 2, 5, 9, 11);
Comparator reverse = new Comparator() {
public Compare(Object x, Object y) {
return (Integer) y - (Integer) x;
}
}
Collections.sort(a, reverse);
So I would say that the combination of functor classes + anonymous inner classes in Java is sufficient to compensate for the lack of true function objects, making their addition unnecessary.
Java has had closures since 1.1, just in a very cumbersome and limited way.
They are often useful wherever you have a callback of some description. A common case is to abstract away control flow, leaving the interesting code to call an algoritm with a closure that has no external control flow.
A trivial example is for-each (although Java 1.5 already has that). Whilst you can implement a forEach method in Java as it stands, it's far too verbose to be useful.
An example which already makes sense with existing Java is implementing the "execute around" idiom, whereby resource acquisition and release is abstracted. For instance, file open and close can be done within try/finally, without the client code having to get the details right.
When closures finally arrive in Java, I will gleefully get rid of all my custom comparator classes.
myArray.sort( (a, b) => a.myProperty().compareTo(b.myProperty() );
...looks a helluva lot better than...
myArray.sort(new Comparator<MyClass>() {
public int compare(MyClass a, MyClass b) {
return a.myProperty().compareTo(b.myProperty();
}
});
A few people have said, or implied, that closures are just syntactic sugar - doing what you could already do with anonymous inner classes and making it more convenient to pass parameters in.
They are syntactic sugar in the same sense that Java is syntactic sugar for assembler (that "assembler" could be bytecode, for sake of argument). In other words they raise they level of abstraction, and this is an important concept.
Closures promote the concept of the function-as-object to a first class entity - one that increases the expressiveness of code, rather than cluttering it with even more boilerplate.
An example that's close to my heart has already been mentioned by Tom Hawtin - implementing the Execute Around idiom, which is just about the only way to get RAII into Java. I wrote a blog entry on exactly that subject a couple of years ago when I first heard closures might be coming.
Ironically, I think the very reason that closures would be good for Java (more expressiveness with less code) may be what rattles many Java advocates. Java has a mindset of "spell everything out the long way". That and the fact that closures are a nod towards a more functional way of doing things - which I also see as a Good Thing, but may water down the pure OO message that many in the Java community hold dear.
I have been thinking a lot about the topic of this very interesting question in
the last few days. First of all, if I have understood correctly, Java already has
some basic notion of closures (defined through anonymous classes) but the new feature
that is going to be introduced is the support for closures based on anonymous functions.
This extension will definitely make the language more expressive but I am not sure
if it really fits with the rest of the language.
Java has been designed as an object-oriented language with no support for functional programming: Will the new semantics be easy to understand? Java 6 does not even have functions, will Java 7 have anonymous functions but no "normal" functions?
My impression is that as new programming styles or paradigms like functional
programming become more popular, everyone wants to use them in their
favourite OOP language. This is understandable: one wants to continue to use
a language they're familiar with while adopting new features. But in this way
a language can become really complex and lose coherence.
So my attitude at the moment is to stick to Java 6 for OOP (I hope Java 6 will still
be supported for a while) and, in case I really get interested in doing OOP + FP,
to take a look at some other language like Scala (Scala was defined to be multi-
paradigm from the beginning and can be well integrated with Java) rather than switching
to Java 7.
I think Java owes its success to the fact that it combines a simple language with very
powerful libraries and tools, and I do not think that new features like closures will
make it a better programming language.
Now that JDK8 is about to be released there is more information available that can enrich the answers to this question.
Bria Goetz, language architect at Oracle, has published a series of papers (yet drafts) on the current state of lambda expressions in Java. It does also cover closures as they are planning to release them in the upcoming JDK, which should be code complete around January 2013 and should be released around midyear 2013.
The State of Lambda: in the first page or two this article attempts to answer the question presented here. Although I still found it short in arguments, but is is full of examples.
The State of Lambda - Libraries Edition: this is also very interesting because it covers advantages like lazy evaluation and parallelism.
The Translation of Lambda Expressions: which basically explains the desugaring process done by the Java compiler.
As a java developer who is trying to teach themselves lisp in an attempt to become a better programmer, I would say that I would like to see the Josh Block proposal for closures implemented. I find myself using anonymous inner classes to express things like what to do with each element of a list when aggregating some data. To would be nice to represent that as a closure, instead of having to create an abstract class.
Closures in an imperative language (examples: JavaScript, C#, the forthcoming C++ refresh) are not the same as anonymous inner classes. They need to be able to capture modifiable references to local variables. Java's inner classes can only capture local final variables.
Almost any language feature can be criticised as non-essential:
for, while, do are all just syntactic sugar over goto/if.
Inner classes are syntactic sugar over classes with a field pointing to the outer class.
Generics are syntactic sugar over casts.
Exactly the same "non-essential" argument should have blocked the inclusion of all the above features.
Java Closure Examples
Not only that benjismith, but I love how you can just do...
myArray.sort{ it.myProperty }
You only need the more detailed comparator you've shown when the natural language comparison of the property doesn't suit your needs.
I absolutely love this feature.
What about readability and maintainability...one-liner closures are harder to understand and debug, imo
Software has looong life and you can get people with rudimentary knowledge of the language to maintain it...So spread out logic better than one-liners for easy maintenance...You generally don't have a software star looking after software after its release...
You might want to look at Groovy, a language that's mostly compatible with Java, and runs on the JRE, but supports Closures.
The lack of binding in anonymous function [i.e. if the variables (and method arguments if there is an enclosing method) of the outer context are declared final then they are available but not otherwise], I don't quite understand what that restriction actually buys.
I use "final" profusely anyways. So, if my intent was to use the same objects inside the closure, I would indeed declare those objects final in the enclosing scope. But what would be wrong in letting the "closure [java a.i.c.]" just get a copy of the reference as if passed via a constructor (well that in fact is how it is done).
If the closure wants to overwrite the reference, so be it; it will do so without changing the copy that the enclosing scope sees.
If we argue that that would lead to unreadable code (e.g. maybe it's not straight-forward to see what the object reference is at the time of the constructor call for the a.i.c.), then how about at least making the syntax less verbose? Scala? Groovy?