What is the point of final class in Java

Question

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

[java] What is the point of "final class" in Java?

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