Any possibility to divide a class into multiple physical files using Java?
No, the whole of a class has to be in a single file in Java.
If you're thinking of C#'s "partial types" feature, there's no equivalent in Java. (If you weren't thinking of C#, ignore this :)
Yes You Can!
For the sake of completion:
Since Java 8, you have the concept of default methods.
you can split up your class into multiple files/subclasses by gently abusing interfaces
observe:
MyClassPartA.java
interface MyClassPartA{
public default int myMethodA(){return 1;}
}
MyClassPartB.java
interface MyClassPartB{
public default String myMethodB(){return "B";}
}
and combine them:
MyClass.java
public class MyClass implements MyClassPartA, MyClassPartB{}
and use them:
MyClass myClass = new MyClass();
System.out.println(myClass.myMethodA());
System.out.println(myClass.myMethodB());
You can even pass variables between classes/files with abstract getters and setters that you will need to realize/override in the main class, or a superclass of that however.
This might be a good idea if the class is really so large such that the implemented concepts are not easy to grasp. I see two different ways to do this:
Use inheritance: Move general concepts of the class to a base class and derive a specialized class from it.
Use aggregation: Move parts of your class to a separate class and establish a relationship to the second class using a reference.
As previously mentioned, there is no concept like partial classes in Java, so you really have to use these OOP mechanisms.
Using just javac, this is not possible. You could of course combine multiple files into a single .java file as part of your build process, and invoke javac afterwards, but that would be cumbersome on so many levels that it is unlikely to be useful.
Maybe you could explain your problem, then we can help better.
If you feel your .java files are too large, you should probably consider refactoring.
Of course it is possible, but I don't think it's useful at all.
To start off, divide isn't really the question I guess, you just compile the file and split it up whichever way you want.
Now to put them back together all you need to do is to write a custom class loader which loads all the pieces, combines them into a single byte array, then calls defineClass().
Like I said, it does look pretty pointless and is probably not what you want and definitely not what you need, but it is technically possible.
(I did something similar once as a joking way of obfuscating code: bytes of the class file were scattered in constants of all the other classes in the application. It was fun, I have to admit.)
No, in Java this can not be done.
No you can't. If your class is too big than you should split it into two or more.
Related
In an OOP program, where would I put functions for basic operations?
For example, if I had a class that, in one of the functions needed code that could invert an array, I could just make a method called invertArray() within the class.
On the other hand, I could create a whole new Functions class, where I could dump all these basic functions like inverting an array into. However, with this approach, I would have to instantiate this class in pretty much every other class I use. In addition, it isn't really an "object," but more of a conglomeration of functions that don't belong anywhere else, which kind of defeats the purpose of "object-oriented" programming.
Which implementation is better? Is there a better implementation I should use?
Thanks in advance.
Edit: Should this kind of post even belong in Stack Overflow? If not, could you please guide me to a more appropriate Stack Exchange website? Thanks.
Depending on your language it can depend where you put things.
However, given your an example, an invertArray lives on an Array class. In some languages you might make an ArrayHelper or ArrayExtension class. But the principle is "invert" is something you want to tell an array.
You will generally find all your functions will generally live somewhere on some class and there will be a logical place for them.
It's generally not a good idea to make a class that holds a mishmash of functions. You can end up with things like "Math" which is a bunch of "static" functions ( they don't work on an object ) they simply do some calculation with parameters and return a result. But they are still grouped by the idea they are common mathmatical functions
As per your question is regarding Java:
if I had a class that, in one of the functions needed code that could invert an array, I could just make a method called invertArray() within the class.
Then yes you can do this, but if you are willing to implement OOPS concept in Java the you can also do :
I could create a whole new Functions class, where I could dump all these basic functions like inverting an array into.
For this part :
I would have to instantiate this class in pretty much every other class I use.
You can also create an interface as Java provides you this functionality where in you can just declare you functions and provide its implementation in their respective classes. This is also helpful if in case you want different functionality with same function then you can choose this way and you don't have to rewrite your function definitions again and again.
And, OOPS concept comes handy when your are dealing with big projects with n number of classes. It depends whether you are learning or implementing on projects.
I have a very large and bloated class and I want to split it into separate files, but it should be completely transparent to the user and compatible with existing projects that use the class.
In particular, I have my own ImageMatrix class and it defines a ton of unary functions, a ton of binary functions with a scalar, a ton of binary functions with another image, etc. To keep the class clean and maintainable, I wish to put each class of operators in a separate file.
Is there a way to just cut/paste these methods into a file and include them in the source?
So I wish that I can still do this, but the methods actually reside in different files:
ImageMatrix img = new ImageMatrix(800, 600, 3);
img.clear(0.5f, 0.0f, 0,0f);
img.addSelf(anotherImg);
img.normalize();
img.abs();
img.addSelf(0.5);
Each method is around 15-30 lines of code because the implementations are extremely optimized for performance, the like of which is impossible to achieve with Bufferedimage or even a plain int[]. There is a lot of code duplication, which is the main reason to group similar methods together. If I add a function or if I change global behavior (like error checking), I can easily keep things coherent.
Unfortunately it is not possible to split a Java class definition into multiple files.
Try to restructure your code so that a huge class definition isn't necessary. Often this means exposing some state through getter and setter methods, and writing supporting classes which use these methods to add functionality to your main class.
In your case, you might consider adding supporting classes like ImageFilters, or even something more narrow like ImageNormalizer if you like very small classes.
You can also use delegate methods. This means the definitions of your methods are still in ImageMatrix, but they are implemented in another class. Depending on the complexity of your methods this could reduce the amount of code in ImageMatrix.
class ImageMatrix {
MatrixHelperOne helperOne = new MatrixHelperOne();
...
public void complexMethod1(Arg arg) {
helperOne.complexMethod1(arg);
}
...
}
Well, you could create a more generic class and put more generic methods there and then have ImageMatrix extends that one. Also, if you have a lot of matrix manipulation functions you will probably end up with a lot of code duplication, i.e. repeating the same code in different methods instead of moving the common code into an aux method and calling it from different places, etc.
You can't split a Java class across files. If each of your methods is self-contained, you could create classes with static methods. So to a certain extent, you would only need to cut and paste code.
Possible ways to refactor the code which had Java interface solely used to define lots of constants.. You can now imagine how this class is used to access these consts.
This is known as a constant interface anti-pattern. Although the previous link provides a way to fix this (using a class and static imports), I think there is a better way of refactoring this. Follow the suggestion here to fix this. Overall it is better to move the constants to the appropriate classes/abstractions rather than using one utility constant class. For e.g Calendar class defines only the constants that are relevant to its operations. Also as CoolBeans suggested try converting those String constants to enums where applicable.
So you have a big bag of constants (you don't say how big, but I've seen things like this with thousands of entries). Importing all of these values is a mess, as you get all the unrelated values imported into everything.
Rather than automating the changes, what I'd do is separate the constants into logically coherent groups, and then moving each group into the class hierarchy where they make sense. e.g. if you've got constants for COLOR_RED, COLOR_GREEN, DATE_FIELD, WEEK_FIELD, you'd probably want to appropriately split them into the color and data hierarchies. For the first pass ignore edge cases where you can't decide immediately - anything you can do to trim the constants down to coherent groups will help.
Seems like a good usecase for enums. So take out the interface and replace it with an enum. Since it is a collection of constants enum fits the bill nicely. Moreover, enums are one of the most efficient ways to implement a Singleton in Java.
Instead of duplicating answers, take a look at these good relevant questions.
Java Enum Singleton
Efficient way to implement singleton pattern in Java
Alternatively as Pangea mentioned you can do static imports. I think both approaches are fine but enums in my opinion will be a better placeholder for organizing your unrelated constants in relevant meaningful groups.
You could attack the source files with a shell script that does the following:
for all .java files:
if (content matches " class ? imports Singleton {"):
replace "imports Singleton" with ""
append "import static Singleton.*;\n" after package declaration
This is far from perfect (it just ignores cases where a class imports Singleton and other interfaces...) but it could be a practical strategy - and maybe it's OK to solve 80% with a quick script and correct the remaining 20% manually (IDE will report errors).
Is it a bad practice to have a package with only one class in it? Would it make more sense just to move the single class to a util package that would contain other random useful classes?
Is it a bad practice to have a package with only one class in it?
Not necessarily. It could be a sign of somebody getting obsessed with classifying things. On the other hand, it could just be a logical consequence of a sensible general classification scheme applied in an unusual case.
An example of the latter might be where you have a general API, and multiple implementations of that API, where each of the implementations consists of multiple classes. But one of those implementations (lets call it the Null implementation) consists of just one class.
The real test is whether the package structure is serving its purpose(s):
Is it making it easier to find library classes?
Do the packages organize the application classes along the lines of the application's logical module structure?
Does the structure allow you to effectively make use of "package private" visibility?
Would it make more sense just to move the single class to a util package that would contain other random useful classes?
Not necessarily. If the class is just another "randomly useful" leaf class, then there is a good case for moving it. On the other hand, if it has a specific function and is not intended to be used generally, then it may be better to leave it where it is.
It is best not to get too obsessed with creating elegant package hierarchies, or with rejigging them when they turn out to be not as elegant (or useful) as you first thought. There are usually more important things to do, like implementing functionality, writing tests, writing documentation and so on.
No
Package is used to put similar classes together,
In your system if there is no similar class then obviously you can put it .
Is it a bad practice to have a package with only one class in it?
Not necessarily. Packages are using to group together logically related entities. It doesn't prevent you from having just one such entity in a package.
Would it make more sense just to move the single class to a util package that would contain other random useful classes?
Not to me, for two reasons:
Util has a specific meaning. Moving an arbitrary entity to util for reasons of loneliness would be a borderline case of util-abuse.
This is premature organization. With Java the IDE support is rich enough to reorganize easily and effectively using a few clicks. Wait a while to see how your project evolves and then take a call.
There are different stategies for static util classes. I use this one :
if your util class is generic (String utils, DB utils, etc.), I put it in a "util" package, that is used in all the application.
if the util class is specific to a domain, I call it "DomainHelper" by convention, and put it in the domain package, at the same level as domain classes.
Yes, it's a definite code smell.
This doesn't mean it's necessarily wrong, but there should be a really good reason for a lone class in a package.
Most instances of a package with a single class that I've seen have been erroneous.
Packages should implement features. It's rare that a feature is implemented using only a single class.
Its not 'bad' to have a single class in a package, Create a new package to group more than one related classes and in case if you expect more related classes to your present single logically unrelated class in future to avoid refactoring. Moving all the random utility type classes to a single package is a common practice seen in many places.Its a matter of choice really.
I guess it depends. It is quite rare in to have a package with one class in it because in addition to the answers listed above, packages also serve the purpose of creating a layered system. A package with only one class in it indicates that the decomposition of the system has not surfaced some objects in the system. So, yes, I would take a closer look at this package and question what the purpose is.
It is better not to stick random stuff in an Util package precisely because of the reason mentioned above. You should ask yourself whether you would think to look in Util for your class in the future before putting it there. When Util grows large it starts to get difficult finding the Utility one is looking for.
This is language agnostic, but I'm working with Java currently.
I have a class Odp that does stuff. It has two private helper methods, one of which determines the max value in an int[][], and the other returns the occurrences of a character in a String.
These aren't directly related to the task at hand, and seem like they could be reused in future projects. Where is the best place to put this code?
Make it public -- bad, because Odp's functionality is not directly related, and these private methods are an implementation detail that don't need to be in the public interface.
Move them to a different class -- but what would this class be called? MiscFunctionsWithNoOtherHome? There's no unifying theme to them.
Leave it private and copy/paste into other classes if necessary -- BAD
What else could I do?
Here's one solution:
Move the method that determines te max value in a two-dimensional int array to a public class called IntUtils and put the class to a util package.
Put the method that returns the occurrences of a character in a String to a puclic class called StringUtils and put the class to a util package.
There's nothing particularly bad about writing static helper classes in Java. But make sure that you don't reinvent the wheel; the methods that you just described might already be in some OS library, like Jakarta Commons.
Wait until you need it!
Your classes wil be better for it, as you have no idea for now how your exact future needs will be.
When you are ready, in Eclipse "Extract Method".
EDIT: I have found that test driven development give code that is easier to reuse because you think of the API up front.
A lot of people create a Utility class with a lot of such methods declared as static. Some people don't like this approach but I think it strikes a balance between design, code reuse, and practicality.
If it were me, I'd either:
create one or more Helper classes that contained the methods as static publics, naming them as precisely as possible, or
if these methods are all going to be used by classes of basically the same type, I'd create an abstract base class that includes these as protected methods.
Most of the time I end up going with 1, although the helper methods I write are usually a little more specific than the ones you've mentioned, so it's easier to come up with a class name.
I not know what the other languages do but I have the voice of experience in Java on this: Just move to the end-brace of your class and write what you need ( or nested class if you prefer as that is accepted canonical convention in Java )
Move the file scope class ( default access class right there in the file ) to it's own compilation unit ( public class in it's own file ) when the compiler moans about it.
See other's comments about nested classes of same name if differing classes have the same functionality in nested class of same name. What will happen on larger code bases is the two will diverge over time and create maintainability issues that yield to Java's Name of class as type of class typing convention that forces you to resolve the issue somehow.
What else could I do?
Be careful not to yield to beginner impulses on this. Your 1-2 punch nails it, resist temptation.
In my experience, most large projects will have some files for "general" functions, which are usually all sorts of helper functions like this one which don't have any builtin language library.
In your case, I'd create a new folder (new package for Java) called "General", then create a file to group together functions (for Java, this will just be a class with lots of static members).
For example, in your case, I'd have something like: General/ArrayUtils.java, and in that I'd throw your function and any other function you need.
Don't worry that for now this is making a new class (and package) for only one function. Like you said in the question, this will be something you'll use for the next project, and the next. Over time, this "General" package will start to grow all sorts of really great helper classes, like MathUtils, StringUtils, etc. which you can easily copy to every project you work on.
You should avoid helper classes if you can, since it creates redundant dependencies. Instead, if the classes using the helper methods are of the same type (as kbrasee wrote), create an abstract superclass containing the methods.
If you do choose to make a separate class do consider making it package local, or at least the methods, since it may not make sense for smaller projects. If your helper methods are something you will use between projects, then a library-like approach is the nicest to code in, as mentioned by Edan Maor.
You could make a separate project called utils or something, where you add the classes needed, and attach them as a library to the project you are working on. Then you can easily make inter-project library updates/fixes by one modification. You could make a package for these tools, even though they may not be that unified (java.util anyone?).
Option 2 is probably your best bet in Java, despite being unsatisfying. Java is unsatisfying, so no surprise there.
Another option might be to use the C Preprocessor as a part of your build process. You could put some private static functions into file with no class, and then include that file somewhere inside a class you want to use it in. This may have an effect on the size of your class files if you go overboard with it, of course.