使用所有非静态方法但没有非静态字段的类是否有意义？ （或所有静态字段和方法以及构造函数）

Question

I am looking at other peoples' code.

I see a class with no non-static fields but in which most of the methods are non-static, requiring you to make an object to access methods that effectively operate statically.

Is there a possible reason for this, that I am just not understanding?

EDIT

Someone asked for examples. Here is some more info.

For instance there is a file manager class. The only fields are static and are Comparators. There are some methods to do things like sort files in a list, count files, copy files, move files to an archive folder, delete files older than a certain time, or create files (basically take a base name as string, and return a File with given base name and date/time tacked on the end.) 9 non-static methods 5 static methods I don't see a particular rhyme reason for the ones that are static vs non.

One particularly odd thing is that there are two methods for removing files. One that removes a file no matter what, and one that only removes it if it is empty. The former is a static method while the latter is not. They contain the same exact code except the later first checks if the file.length is 0.

Another odd one is a class that does encryption - all fields and methods are static but it has a constructor that does nothing. And an init() method that checks if a static variable contains an object of itself and if not instantiates an object of itself into that field that is then never actually used. (It seems this is done with a lot of classes - init methods that check for an object of itself in a static variable and if not instantiate itself)

 private static File keyfile;
 private static String KEYFILE = "enc.key";
 private static Scrambler sc; 

It has methods to encrypt and decrypt and some methods for dealing with key and file.

Does this make sense to anyone? Am I just not understanding the purpose for this stuff? Or does it seem weird?

Answer 1

Objects don't have to have state. It's a legitimate use case to create an instance of a class with only behaviour.

Why bother to create an instance ? So you can create one and pass it around eg imagine some form of calculator which adheres to a particular interface but each instance performs a calculation differently. Different implements of the interface would perform calculations differently.

I quite often create classes with non-static methods and no members. It allows me to encapsulate behaviour, and I can often add members later as the implementation may demand in the future (including non-functionality related stuff such as instrumentation) I don't normally make these methods static since that restricts my future flexibility.

Answer 2

You can certainly do it that way. You should look carefully at what the instance methods are doing. It's perfectly okay if they're all operating only on parameters passed in and static final static class constants.

If that's the case, it's possible to make all those methods static. That's just a choice. I don't know how the original developers would justify either one. Maybe you should ask them.

Answer 3

Making all methods non-static allows you to override them. This makes it a lot easier to use this class in testing, because instead of the actual implementation you can use a mock that behaves as you want it for the tests. Static methods are, in my book, a code smell and should be avoided unless there's a good reason (eg quite trivial utility methods).

Also, at some point in the future you might want to change the behaviour of the methods in some situation, eg in the form of a strategy.

In the case of your encryption class, you might want to hand your class an instance of the encryption class to handle encrypting/decrypting, but be able to configure the details in some other place. That would allow you to change the algorithm and much more easily test your own code without also having to test the encryption.

Answer 4

Let me rephrase this question a bit,

Even though methods are non-static why would one declare fields as static?

I have taken below quoting from Java Docs ,

Sometimes, you want to have variables that are common to all objects. This is Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.

For example, suppose you want to create a number of Bicycle objects and assign each a serial number, beginning with 1 for the first object. This ID number is unique to each object and is therefore an instance variable. At the same time, you need a field to keep track of how many Bicycle objects have been created so that you know what ID to assign to the next one. Such a field is not related to any individual object, but to the class as a whole.

For Bicycle example, kindly refer the Java Docs .