I am reading a book about Java and it says that you can declare the whole class as final. I cannot think of anything where I'd use this.
I am just new to programming and I am wondering if programmers actually use this on their programs. If they do, when do they use it so I can understand it better and know when to use it.
If Java is object oriented, and you declare a class final, doesn't it stop the idea of class having the characteristics of objects?
First of all, I recommend this article: Java: When to create a final class
If they do, when do they use it so I can understand it better and know when to use it.
A final class is simply a class that can't be extended.
(It does not mean that all references to objects of the class would act as if they were declared as final.)
When it's useful to declare a class as final is covered in the answers of this question:
Good reasons to prohibit inheritance in Java?
If Java is object oriented, and you declare a class final, doesn't it stop the idea of class having the characteristics of objects?
In some sense yes.
By marking a class as final you disable a powerful and flexible feature of the language for that part of the code. Some classes however, should not (and in certain cases can not) be designed to take subclassing into account in a good way. In these cases it makes sense to mark the class as final, even though it limits OOP. (Remember however that a final class can still extend another non-final class.)
In Java, items with the final modifier cannot be changed!
This includes final classes, final variables, and final methods:
A final class cannot be extended by any other class
A final variable cannot be reassigned another value
A final method cannot be overridden
One scenario where final is important, when you want to prevent inheritance of a class, for security reasons. This allows you to make sure that code you are running cannot be overridden by someone.
Another scenario is for optimization: I seem to remember that the Java compiler inlines some function calls from final classes. So, if you call a.x() and a is declared final, we know at compile-time what the code will be and can inline into the calling function. I have no idea whether this is actually done, but with final it is a possibility.
The best example is
public final class String
which is an immutable class and cannot be extended.
Of course, there is more than just making the class final to be immutable.
If you imagine the class hierarchy as a tree (as it is in Java), abstract classes can only be branches and final classes are those that can only be leafs. Classes that fall into neither of those categories can be both branches and leafs.
There's no violation of OO principles here, final is simply providing a nice symmetry.
In practice you want to use final if you want your objects to be immutable or if you're writing an API, to signal to the users of the API that the class is just not intended for extension.
Relevant reading: The Open-Closed Principle by Bob Martin.
Key quote:
Software Entities (Classes, Modules,
Functions, etc.) should be open for
Extension, but closed for
Modification.
The final keyword is the means to enforce this in Java, whether it's used on methods or on classes.
The keyword final itself means something is final and is not supposed to be modified in any way. If a class if marked final then it can not be extended or sub-classed. But the question is why do we mark a class final? IMO there are various reasons:
Standardization: Some classes perform standard functions and they are not meant to be modified e.g. classes performing various functions related to string manipulations or mathematical functions etc.
Security reasons: Sometimes we write classes which perform various authentication and password related functions and we do not want them to be altered by anyone else.
I have heard that marking class final improves efficiency but frankly I could not find this argument to carry much weight.
If Java is object oriented, and you declare a class final, doesn't it
stop the idea of class having the characteristics of objects?
Perhaps yes, but sometimes that is the intended purpose. Sometimes we do that to achieve bigger benefits of security etc. by sacrificing the ability of this class to be extended. But a final class can still extend one class if it needs to.
On a side note we should prefer composition over inheritance and final keyword actually helps in enforcing this principle.
final class can avoid breaking the public API when you add new methods
Suppose that on version 1 of your Base class you do:
public class Base {}
and a client does:
class Derived extends Base {
public int method() { return 1; }
}
Then if in version 2 you want to add a method method to Base:
class Base {
public String method() { return null; }
}
it would break the client code.
If we had used final class Base instead, the client wouldn't have been able to inherit, and the method addition wouldn't break the API.
A final class is a class that can't be extended. Also methods could be declared as final to indicate that cannot be overridden by subclasses.
Preventing the class from being subclassed could be particularly useful if you write APIs or libraries and want to avoid being extended to alter base behaviour.
In java final keyword uses for below occasions.
Final Variables
Final Methods
Final Classes
In java final variables can't reassign, final classes can't extends and final methods can't override.
Be careful when you make a class "final". Because if you want to write an unit test for a final class, you cannot subclass this final class in order to use the dependency-breaking technique "Subclass and Override Method" described in Michael C. Feathers' book "Working Effectively with Legacy Code". In this book, Feathers said, "Seriously, it is easy to believe that sealed and final are a wrong-headed mistake, that they should never have been added to programming languages. But the real fault lies with us. When we depend directly on libraries that are out of our control, we are just asking for trouble."
If the class is marked final, it means that the class' structure can't be modified by anything external. Where this is the most visible is when you're doing traditional polymorphic inheritance, basically class B extends A just won't work. It's basically a way to protect some parts of your code (to extent).
To clarify, marking class final doesn't mark its fields as final and as such doesn't protect the object properties but the actual class structure instead.
TO ADDRESS THE FINAL CLASS PROBLEM:
There are two ways to make a class final. The first is to use the keyword final in the class declaration:
public final class SomeClass {
// . . . Class contents
}
The second way to make a class final is to declare all of its constructors as private:
public class SomeClass {
public final static SOME_INSTANCE = new SomeClass(5);
private SomeClass(final int value) {
}
Marking it final saves you the trouble if finding out that it is actual a final, to demonstrate look at this Test class. looks public at first glance.
public class Test{
private Test(Class beanClass, Class stopClass, int flags)
throws Exception{
// . . . snip . . .
}
}
Unfortunately, since the only constructor of the class is private, it is impossible to extend this class. In the case of the Test class, there is no reason that the class should be final. The Test class is a good example of how implicit final classes can cause problems.
So you should mark it final when you implicitly make a class final by making it's constructor private.
One advantage of keeping a class as final :-
String class is kept final so that no one can override its methods and change the functionality. e.g no one can change functionality of length() method. It will always return length of a string.
Developer of this class wanted no one to change functionality of this class, so he kept it as final.
The other answers have focused on what final class tells the compiler: do not allow another class to declare it extends this class, and why that is desirable.
But the compiler is not the only reader of the phrase final class. Every programmer who reads the source code also reads that. It can aid rapid program comprehension.
In general, if a programmer sees Thing thing = that.someMethod(...); and the programmer wants to understand the subsequent behaviour of the object accessed through the thing object-reference, the programmer must consider the Thing class hierarchy: potentially many types, scattered over many packages. But if the programmer knows, or reads, final class Thing, they instantly know that they do not need to search for and study so many Java files, because there are no derived classes: they need study only Thing.java and, perhaps, it's base classes.
Yes, sometimes you may want this though, either for security or speed reasons. It's done also in C++. It may not be that applicable for programs, but moreso for frameworks.
http://www.glenmccl.com/perfj_025.htm
think of FINAL as the "End of the line" - that guy cannot produce offspring anymore. So when you see it this way, there are ton of real world scenarios that you will come across that requires you to flag an 'end of line' marker to the class. It is Domain Driven Design - if your domain demands that a given ENTITY (class) cannot create sub-classes, then mark it as FINAL.
I should note that there is nothing stopping you from inheriting a "should be tagged as final" class. But that is generally classified as "abuse of inheritance", and done because most often you would like to inherit some function from the base class in your class.
The best approach is to look at the domain and let it dictate your design decisions.
As above told, if you want no one can change the functionality of the method then you can declare it as final.
Example: Application server file path for download/upload, splitting string based on offset, such methods you can declare it Final so that these method functions will not be altered. And if you want such final methods in a separate class, then define that class as Final class. So Final class will have all final methods, where as Final method can be declared and defined in non-final class.
Let's say you have an Employee class that has a method greet. When the greet method is called it simply prints Hello everyone!. So that is the expected behavior of greet method
public class Employee {
void greet() {
System.out.println("Hello everyone!");
}
}
Now, let GrumpyEmployee subclass Employee and override greet method as shown below.
public class GrumpyEmployee extends Employee {
#Override
void greet() {
System.out.println("Get lost!");
}
}
Now in the below code have a look at the sayHello method. It takes Employee instance as a parameter and calls the greet method hoping that it would say Hello everyone! But what we get is Get lost!. This change in behavior is because of Employee grumpyEmployee = new GrumpyEmployee();
public class TestFinal {
static Employee grumpyEmployee = new GrumpyEmployee();
public static void main(String[] args) {
TestFinal testFinal = new TestFinal();
testFinal.sayHello(grumpyEmployee);
}
private void sayHello(Employee employee) {
employee.greet(); //Here you would expect a warm greeting, but what you get is "Get lost!"
}
}
This situation can be avoided if the Employee class was made final. Just imagine the amount of chaos a cheeky programmer could cause if String Class was not declared as final.
Final class cannot be extended further. If we do not need to make a class inheritable in java,we can use this approach.
If we just need to make particular methods in a class not to be overridden, we just can put final keyword in front of them. There the class is still inheritable.
Final classes cannot be extended. So if you want a class to behave a certain way and don't someone to override the methods (with possibly less efficient and more malicious code), you can declare the whole class as final or specific methods which you don't want to be changed.
Since declaring a class does not prevent a class from being instantiated, it does not mean it will stop the class from having the characteristics of an object. It's just that you will have to stick to the methods just the way they are declared in the class.
Android Looper class is a good practical example of this.
http://developer.android.com/reference/android/os/Looper.html
The Looper class provides certain functionality which is NOT intended to be overridden by any other class. Hence, no sub-class here.
I know only one actual use case: generated classes
Among the use cases of generated classes, I know one: dependency inject e.g. https://github.com/google/dagger
Object Orientation is not about inheritance, it is about encapsulation. And inheritance breaks encapsulation.
Declaring a class final makes perfect sense in a lot of cases. Any object representing a “value” like a color or an amount of money could be final. They stand on their own.
If you are writing libraries, make your classes final unless you explicitly indent them to be derived. Otherwise, people may derive your classes and override methods, breaking your assumptions / invariants. This may have security implications as well.
Joshua Bloch in “Effective Java” recommends designing explicitly for inheritance or prohibiting it and he notes that designing for inheritance is not that easy.
Related
I was recently going through one of the Netflix open source project
There I found use of both final class along with private constructor. I fully aware that
final is to avoid inheritance
private is to disallow instantiation
But m just curious to know why they are both used together. Although methods are static, so we can use them without instantiation but still eager to know design principle behind it.
With this code you will have this features
Not allow anyone subclass (extends) your class
Not allow instantiating your class
Making a variables or classes final increase the performance (not much, but it does and used as common practice in big projects will make a difference)
In this case I can't see a singleton pattern to get an instance, so, IMHO, you're looking to a helper/util class in the Netflix API, where the developer team used some standard practices to ensure users use their classes in the correct way:
StaticFinalClassExample.methodYouWantToCall();
Also, looking at the class you linked:
/**
* This class consists exclusively of static methods that help verify the compliance of OP1A-conformant....
*/
And:
//to prevent instantiation
private IMFConstraints()
{}
ADD ON:
If you want further info, take a look at Item 4 from Joshua Bloch's Effective Java (2nd Edition):
Item 4: Enforce noninstantiability with a private constructor
Occasionally you’ll want to write a class that is just a grouping of static methods and static fields. Such classes have acquired a bad reputation because some people abuse them to avoid thinking in terms of objects, but they do have valid uses.
They can be used to group related methods on primitive values or arrays, in the manner of java.lang.Math or java.util.Arrays.
They can also be used to group static methods, including factory methods (Item 1), for objects that implement a particular interface, in the manner of java.util.Collections.
Lastly, they can be used to group methods on a final class, instead of extending the class.
Such utility classes were not designed to be instantiated: an instance would be nonsensical. In the absence of explicit constructors, however, the compiler provides a public, parameterless default constructor. To a user, this constructor is indistinguishable from any other. It is not uncommon to see unintentionally instantiable classes in published APIs.
Attempting to enforce noninstantiability by making a class abstract does
not work. The class can be subclassed and the subclass instantiated. Furthermore, it misleads the user into thinking the class was designed for inheritance (Item 17).
There is, however, a simple idiom to ensure noninstantiability. A default constructor is generated only if a class contains no explicit constructors, so a class can be made noninstantiable by including a private constructor.
That class consists of static so called "utility" methods, and therefore you don't need an instance of it, and further, it's WRONG to try to get an instance of it. The class is final so that a client developer doesn't have the option of coming along and extending the class, because that would be against the intention of the original class.
There are basically 2 uses for private constructors: to tightly control instantiation in the case of a class that you want to restrict creation of (for example, if it requires a ton of resources). In this first case, you have to provide static factory methods that create an object for the client.
ie:
public static IMFConstraints getInstance()
The other case is if it's never valid to make an instance. In that case, you provide static methods, which are called on the class itself. ie:
public static void checkIMFCompliance(List<PartitionPack> partitionPacks)
You would call the above method like so:
// your cool client code here...
IMFConstraints.checkIMFCompliance(myPartitionPacks);
// more of your awesome code...
The class you linked is the latter case.
So, I'm beginning to learn Java and I think it's an awesome programming language, however I've come across the static keyword which, to my understanding, makes sure a given method or member variable is accessible through the class (e.g. MyClass.main()) rather than solely through the object (MyObject.main()). My question is, is it possible to make certain methods only accessible through the class and not through the object, so that MyClass.main() would work, however MyObject.main() would not? Whilst I'm not trying to achieve anything with this, I'd just like to know out of curiosity.
In my research I couldn't find this question being asked anywhere else, but if it is elsewhere I'd love to be pointed to it!
Forgive me if it's simple, however I've been thinking on this for a while and getting nowhere.
Thanks!
Any static method or member belongs to the class, whereas non-static members belong to the object.
Calling a static method (or using a static member) by doing myObject.method() is actually exactly the same as MyClass.method() and any proper IDE will give a suggestion to change it to the second one, since that one is actually what you are doing regardless of which of the two you use.
Now to answer the actual question:
is it possible to make certain methods only accessible through the class and not through the object
No, not as far as i know, but like I said, any proper IDE will give a warning, since it makes little sense and it gives other readers of the code an instant hint that you're dealing with static members.
Yes, short answer is no.
But you can put your static members in a dedicated class, so that no instances share any one of them.
MyObject is instance of MyClass, and you aggregate all you static parts in MyStaticThing.
Using static member on an instance can be misleading, so it is a bad practice
http://grepcode.com/file/repo1.maven.org/maven2/org.sonarsource.java/java-checks/3.4/org/sonar/l10n/java/rules/squid/S2209.html
While it is possible to access static members
from a class instance, it's bad form, and considered by most to be
misleading because it implies to the readers of your code thatthere's
an instance of the member per class instance.
Another thing, do not use static things, because you cannot do abstraction and replace implementations to extend your code.
Being able to switch between implementations is useful for maintenance and tests.
In Java, you can crete an object with these keywords.(new keyword, newInstance() method, clone() method, factory method and deserialization) And when you create an object,it can also use classes abilities which is like static methods.
Short answer:No.
Is it possible to make certain methods only accessible through the class and not through the object?
Yes, it is. You achieve this by preventing any instances of the class to ever be created, by making the class non-instantiable: declare its constructor private.
public final class NonInstantiable {
private NonInstantiable() {
throw new RuntimeException(
"This class shouldn't be instantiated -- not even through reflection!");
}
/* static methods here... */
}
Now, it only makes sense to declare any methods of the class static -- and they can only be called through the class name. Such a class is often called a utility class.
When the Gang of four introduced the singleton pattern, they also had to explain, why not to use static class fields and method instead. The reason was: the possibility to inherit. For Java it had sense - we cannot normally inherit the class fields and methods.
Later the "Effective Java" book appeared. And we know now that the existence of reflection destroys the singularity of the singleton class with private constructor. And the only way to make a real SINGLEton is to make it as a single item of an enumeration. Nice. I had done some myself this way.
But a question remains: While we cannot inherit from enumeration, what is the use of this singleton? Why we don't use these old good static/class fields and methods?
Edit. Thanks to the #bayou.io I see that in https://softwareengineering.stackexchange.com/a/204181/44104 there is a code that can trick the enum, too, and create again two exemplars of the enum singleton. The other problems are mentioned there, too. So, there is no need to use enum instead of the usual singleton class pattern, too? BTW, all enum pluses that are mentioned here till now, work for singleton classes, too.
what is the use of this singleton? Why we don't use these old good static/class fields and methods?
Because enum is an object so it can not only be passed around but also implement interfaces.
Also since we are making a class, we can use the different public/private options available to all kinds of classes.
So in practice, we can make a singleton that implements an interface and then pass it around in our code and the calling code is non the wiser. We can also make the enum class package private but still pass it around to other classes in other packages that expect the interface.
If we used the static methods version, then the calling class would have to know that this object is a singleton, and our singleton class would have to be public so the other classes can see it and use it's methods.
There's nothing particularly wrong with the "good old fashioned singleton", enum "singletons" are just convenient - it saves you the need to muck around with boiler-plated code that looks the same in every singelton.
To me, a singleton makes sense wherever you want to represent something which is unique in its kind.
As an example, if we wanted to model the Sun, it could not be a normal class, because there is only one Sun. However it makes sense to make it inherit from a Star class. In this case I would opt for a static instance, with a static getter.
To clarify, here is what I'm talking about :
public class Star {
private final String name;
private final double density, massInKg;
public Star(String name, double density, double massInKg) {
// ...
}
public void explode() {
// ...
}
}
public final class Sun extends Star {
public static final Sun INSTANCE = new Sun();
private Sun() { super("The shiniest of all", /**...**/, /**...**/); }
}
Sun can use all the methods of Star and define new ones. This would not be possible with an enum (extending a class, I mean).
If there is no need to model this kind of inheritance relationships, as you said, the enum becomes better suited, or at least easier and clearer. For example, if an application has a single ApplicationContext per JVM, it makes sense to have it as a singleton and it usually doesn't require to inherit from anything or to be extendable. I would then use an enum.
Note that in some languages such as Scala, there is a special keyword for singletons (object) which not only enables to easily define singletons but also completely replaces the notion of static method or field.
ENUM singletons are easy to write. It will occupy very less code, which is clean & elegant if you compare with implementation of lazy singleton with double synchronized blocks
public enum EasySingleton{
INSTANCE;
}
Creation of ENUM instance is thread safe.
ENUM singletons handled serialization by themselves.
conventional Singletons implementing Serializable interface are no longer remain Singleton because readObject() method always return a new instance just like constructor in Java. you can avoid that by using readResolve() method and discarding newly created instance by replacing with Singeton
private Object readResolve(){
return INSTANCE;
}
Have a look at this article on singleton
In his book Effective Java, Joshua Bloch recommends against using Interfaces to hold constants,
The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class’s exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the con-stants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.
His reasoning makes sense to me and it seems to be the prevailing logic whenever the question is brought up but it overlooks storing constants in interfaces and then NOT implementing them.
For instance,
public interface SomeInterface {
public static final String FOO = "example";
}
public class SomeOtherClass {
//notice that this class does not implement anything
public void foo() {
thisIsJustAnExample("Designed to be short", SomeInteface.FOO);
}
}
I work with someone who uses this method all the time. I tend to use class with private constructors to hold my constants, but I've started using interfaces in this manner to keep our code a consistent style. Are there any reasons to not use interfaces in the way I've outlined above?
Essentially it's a short hand that prevents you from having to make a class private, since an interface can not be initialized.
I guess it does the job, but as a friend once said: "You can try mopping a floor with an octopus; it might get the job done, but it's not the right tool".
Interfaces exist to specify contracts, which are then implemented by classes. When I see an interface, I assume that there are some classes out there that implement it. So I'd lean towards saying that this is an example of abusing interfaces rather than using them, simply because I don't think that's the way interfaces were meant to be used.
I guess I don't understand why these values are public in the first place if they're simply going to be used privately in a class. Why not just move them into the class? Now if these values are going to be used by a bunch of classes, then why not create an enum? Another pattern that I've seen is a class that just holds public constants. This is similar to the pattern you've described. However, the class can be made final so that it cannot be extended; there is nothing that stops a developer from implementing your interface. In these situations, I just tend to use enum.
UPDATE
This was going to be a response to a comment, but then it got long. Creating an interface to hold just one value is even more wasteful! :) You should use a private constant for that. While putting unrelated values into a single enum is bad, you could group them into separate enums, or simply use private constants for the class.
Also, if it appears that all these classes are sharing these unrelated constants (but which make sense in the context of the class), why not create an abstract class where you define these constants as protected? All you have to do then is extend this class and your derived classes will have access to the constants.
I don't think a class with a private constructor is any better than using an interface.
What the quote says is that using implements ConstantInterface is not best pratice because this interface becomes part of the API.
However, you can use static import or qualified names like SomeInteface.FOO of the values from the interface instead to avoid this issue.
Constants are a bad thing anyway. Stuffing a bunch of strings in a single location is a sign that your application has design problems from the get go. Its not object oriented and (especially for String Constants) can lead to the development of fragile API's
If a class needs some static values then they should be local to that class. If more classes need access to those values they should be promoted to an enumeration and modeled as such. If you really insist on having a class full of constants then you create a final class with a private no args constructor. With this approach you can at least ensure that the buck stops there. There are no instantiations allowed and you can only access state in a static manner.
This particular anti-pattern has one serious problem. There is no mechanism to stop someone from using your class that implements this rouge constants interface.Its really about addressing a limitation of java that allows you to do non-sensical things.
The net out is that it reduces the meaningfulness of the application's design because the grasp on the principles of the language aren't there. When I inherit code with constants interfaces, I immediately second guess everything because who knows what other interesting hacks I'll find.
Creating a separate class for constants seems silly. It's more work than making an enum, and the only reason would be to do it would be to keep unrelated constants all in one place just because presumably they all happen to be referenced by the same chunks of code. Hopefully your Bad Smell alarm goes of when you think about slapping a bunch of unrelated stuff together and calling it a class.
As for interfaces, as long as you're not implementing the interface it's not the end of the world (and the JDK has a number of classes implementing SwingConstants for example), but there may be better ways depending on what exactly you're doing.
You can use enums to group related constants together, and even add methods to them
you can use Resource Bundles for UI text
use a Map<String,String> passed through Collections.unmodifiableMap for more general needs
you could also read constants from a file using java.util.Properties and wrap or subclass it to prevent changes
Also, with static imports there's no reason for lazy people to implement an interface to get its constants when you can be lazy by doing import static SomeInterface.*; instead.
This question already has answers here:
Should private helper methods be static if they can be static
(20 answers)
Closed 9 years ago.
What do you think about using private static methods?
Personally, I prefer using a static private method to non-static as long as it does not require access to any instance fields.
But I heard that this practice violates OOP principles.
Edit: I am wondering from style prospective of view, not performance.
A private static method by itself does not violate OOP per se, but when you have a lot of these methods on a class that don't need (and cannot*) access instance fields, you are not programming in an OO way, because "object" implies state + operations on that state defined together. Why are you putting these methods on that class, if they don't need any state?
(*) = In principle, due to the class level visibility in Java, a static method on a class has access to instance fields of an object of that class, for example:
class Test
{
int field = 123;
private static void accessInstance(Test test)
{
System.out.println(test.field);
}
}
You need to pass in the reference to an instance (this pointer) yourself of course, but then you are essentially mimicking instance methods. Just mentioning this for completeness.
As mentioned above, private static methods are often useful for organizing re-used logic and reducing/eliminating repeated code. I'm surprised that I haven't noticed any mention of performance in this discussion. From Renaud Waldura's 'The Final Word on Final':
(Note, private static methods are implicitly final)
"Since a final method is only implemented in the declaring class, there is no need to dynamically dispatch a call to a final method, and static invocation can be used instead. The compiler can emit a direct call to the method, bypassing entirely the usual virtual method invocation procedure. Because of this, final methods are also candidates for inlining by a Just-In-Time compiler or a similar optimization tool. (Remember, private/static methods are already final, therefore always considered for this optimization.)"
Check out the whole paper: http://renaud.waldura.com/doc/java/final-keyword.shtml
private or public doesn't make a difference - static methods are OK, but if you find you're using them all the time (and of course instance methods that don't access any instance fields are basically static methods for this purpose), then you probably need to rethink the design. It's not always possible, but most of the time methods should reside with the data they operate on - that's the basic idea of OOP.
I don't necessarily see any real problem with what you are doing, but my first question would be if the method doesn't require access to any instance fields, then what is it doing in that class in the first place?
It's a matter of taste, but I make methods that don't react to a state within the object static. This way I don't have to rewrite code if a static function needs similar functionality. A sorting function would be a good example of such a case.
I tend not to use private static methods. I do use public static methods and group them into Util classes to promote reuse.
Private static methods can for example operate on private static members of their class. This can be utilized to encapsulate and unify certain class specific operations.
The major drawback of using static methods is in my opinion the fact that one throws away the possibility to override. Since classes in Java are not like, let's say, classes in Smalltalk, you can not override static methods.
Since your question relates to private static methods, overriding is out of option anyway.
I tend to use static methods only in case of utility classes (like java.lang.Math) or patterns like the Singleton pattern. All of these require a higher visibility than private, because they represent services provided by their class to others.
Final thought: If you have one or more private static methods, think about extracting them to a dedicated utility class and making them public. Even better, make them instance methods and use the Singleton pattern.