When to use static methods

Question

I am wondering when to use static methods  Say if I have a class with a few getters and setters  a method or two  and I want those methods only to be invokable on an instance object of the class  Does this mean I should use a static method  Example  Obj x   new Obj    x someMethod        or  Obj someMethod       Is this the static way   I m rather confused

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No  static methods aren t associated with an instance  they belong to the class   Static methods are your second example  instance methods are the first

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One rule-of-thumb  ask yourself  quot Does it make sense to call this method  even if no object has been constructed yet  quot   If so  it should definitely be static  So in a class Car you might have a method  double convertMpgToKpl double mpg      which would be static  because one might want to know what 35mpg converts to  even if nobody has ever built a Car   But this method  which sets the efficiency of one particular Car   void setMileage double mpg      can t be static since it s inconceivable to call the method before any Car has been constructed   By the way  the converse isn t always true  you might sometimes have a method which involves two Car objects  and still want it to be static   E g   Car theMoreEfficientOf Car c1  Car c2   Although this could be converted to a non-static version  some would argue that since there isn t a  quot privileged quot  choice of which Car is more important  you shouldn t force a caller to choose one Car as the object you ll invoke the method on   This situation accounts for a fairly small fraction of all static methods  though

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Static methods can be used if   One does not want to perform an action on an instance  utility methods      As mentioned in few of above answers in this post  converting miles to kilometers  or calculating temperature from Fahrenheit to Celsius and vice-versa  With these examples using static method  it does not need to instantiate whole new object in heap memory  Consider below   1  new ABCClass double farenheit  convertFarenheitToCelcium    2  ABCClass convertFarenheitToCelcium double farenheit       the former creates a new class footprint for every method invoke  Performance  Practical  Examples are Math and Apache-Commons library StringUtils class below    Math random   Math sqrt double  Math min int  int  StringUtils isEmpty String  StringUtils isBlank String   One wants to use as a simple function  Inputs are explictly passed  and getting the result data as return value  Inheritence  object instanciation does not come into picture  Concise  Readable    NOTE   Few folks argue against testability of static methods  but static methods can be tested too  With jMockit  one can mock static methods  Testability  Example below   new MockUp lt ClassName gt           Mock     public int doSomething Input input1  Input input2           return returnValue

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Define static methods in the following scenarios only    If you are writing utility classes and they are not supposed to be changed  If the method is not using any instance variable  If any operation is not dependent on instance creation   If there is some code that can easily be shared by all the instance methods  extract that code into a static method  If you are sure that the definition of the method will never be changed or overridden  As static methods can not be overridden

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A static method is one type of method which doesn t need any object to be initialized for it to be called  Have you noticed static is used in the main function in Java  Program execution begins from there without an object being created   Consider the following example    class Languages          public static void main String   args                   display                 static void display                    System out println  Java is my favorite programming language

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Static methods are not associated with an instance  so they can not access any non-static fields in the class    You would use a static method if the method does not use any fields  or only static fields  of a class    If any non-static fields of a class are used you must use a non-static method

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Static methods and variables are controlled version of  Global  functions and variables in Java  In which methods can be accessed as classname methodName   or classInstanceName methodName    i e  static methods and variables can be accessed using class name as well as instances of the class   Class can t be declared as static because it makes no sense  if a class is declared public  it can be accessed from anywhere   inner classes can be declared static

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Static methods are the methods in Java that can be called without creating an object of class   It is belong to the class   We use static method when we no need to be invoked method using instance

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There are some valid reasons to use static methods    Performance  if you want some code to be run  and don t want to instantiate an extra object to do so  shove it into a static method  The JVM also can optimize static methods a lot  I think I ve once read James Gosling declaring that you don t need custom instructions in the JVM  since static methods will be just as fast  but couldn t find the source - thus it could be completely false   Yes  it is micro-optimization  and probably unneeded  And we programmers never do unneeded things just because they are cool  right  Practicality  instead of calling new Util   method arg   call Util method arg   or method arg  with static imports  Easier  shorter  Adding methods  you really wanted the class String to have a removeSpecialChars   instance method  but it s not there  and it shouldn t  since your project s special characters may be different from the other project s   and you can t add it  since Java is somewhat sane   so you create an utility class  and call removeSpecialChars s  instead of s removeSpecialChars    Sweet  Purity  taking some precautions  your static method will be a pure function  that is  the only thing it depends on is its parameters  Data in  data out  This is easier to read and debug  since you don t have inheritance quirks to worry about  You can do it with instance methods too  but the compiler will help you a little more with static methods  by not allowing references to instance attributes  overriding methods  etc      You ll also have to create a static method if you want to make a singleton  but    don t  I mean  think twice   Now  more importantly  why you wouldn t want to create a static method  Basically  polymorphism goes out of the window  You ll not be able to override the method  nor declare it in an interface  pre-Java 8   It takes a lot of flexibility out from your design  Also  if you need state  you ll end up with lots of concurrency bugs and or bottlenecks if you are not careful

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I am wondering when to use static methods      A common use for static methods is to access static fields   But you can have static methods  without referencing static variables  Helper methods without referring static variable can be found in some java classes like java lang Math  public static int min int a  int b        return  a  lt   b    a   b     The other use case  I can think of these methods combined with synchronized method is implementation of class level locking in multi threaded environment        Say if I have a class with a few getters and setters  a method or two  and I want those methods only to be invokable on an instance object of the class  Does this mean I should use a static method    If you need to access method on an instance object of the class  your method should should be non static   Oracle documentation page provides more details    Not all combinations of instance and class variables and methods are allowed    Instance methods can access instance variables and instance methods directly  Instance methods can access class variables and class methods directly  Class methods can access class variables and class methods directly  Class methods cannot access instance variables or instance methods directly   they must use an object reference  Also  class methods cannot use the this keyword as there is no instance for this to refer to

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Static methods in java belong to the class  not an instance of it   They use no instance variables and will usually take input from the parameters  perform actions on it  then return some result  Instances methods are associated with objects and  as the name implies  can use instance variables

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Use a static method when you want to be able to access the method without an instance of the class

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Static  Obj someMethod  Use static when you want to provide class level access to a method  i e  where the method should be callable without an instance of the class

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A static method has two main purposes    For utility or helper methods that don t require any object state  Since there is no need to access instance variables  having static methods eliminates the need for the caller to instantiate the object just to call the method   For the state that is shared by all instances of the class  like a counter  All instance must share the same state  Methods that merely use that state should be static as well

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Static methods don t need to be invoked on the object and that is when you use it  Example  your Main    is a static and you don t create an object to call it

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Whenever you do not want to create an object to call a method in your code just declare that method as static  Since the static method does not need an instance to be called with but the catch here is not all static methods are called by JVM automatically  This privilege is enjoyed only by the main    public static void main String    args   method in java because at Runtime this is the method Signature public  static  void main   sought by JVM as an entry point to start execution of the code   Example   public class Demo      public static void main String    args              Demo d   new Demo           System out println  This static method is executed by JVM            Now to call the static method Displ   you can use the below methods             Displ      By method name itself           Demo Displ      By using class name  Recommended          d Displ      By using instance   Not recommended          public static void Displ              System out println  This static method needs to be called explicitly              Output - This static method is executed by JVM This static method needs to be called explicitly This static method needs to be called explicitly This static method needs to be called explicitly

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After reading Misko s articles I believe that static methods are bad from a testing point of view  You should have factories instead maybe using a dependency injection tool like Guice    how do I ensure that I only have one of something     only have one of something   The problem of    how do I ensure that I   only have one of something    is nicely   sidestepped  You instantiate only a   single ApplicationFactory in your   main  and as a result  you only   instantiate a single instance of all   of your singletons    The basic issue with static methods is they are procedural code     The basic issue with static methods is   they are procedural code  I have no   idea how to unit-test procedural code    Unit-testing assumes that I can   instantiate a piece of my application   in isolation  During the instantiation   I wire the dependencies with   mocks friendlies which replace the   real dependencies  With procedural   programing there is nothing to  wire    since there are no objects  the code   and data are separate

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In eclipse you can enable a warning which helps you detect potential static methods   Above the highlighted line is another one I forgot to highlight

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I found a nice description  when to use static methods  There is no hard and fast  well written rules  to decide when to make a method static or not  But there are few observations based upon experience  which not only help to make a method static but also teaches when to use static method in Java  You should consider making a method static in Java    If a method doesn t modify state of object  or not using any instance variables   You want to call method without creating instance of that class   A method is good candidate of being static  if it only work on arguments provided to it e g  public int factorial int number     this method only operate on number provided as argument   Utility methods are also good candidate of being static e g  StringUtils isEmpty String text   this a utility method to check if a String is empty or not   If function of method will remain static across class hierarchy e g  equals   method is not a good candidate of making static because every Class can redefine equality    Source is here

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If you apply static keyword with any method  it is known as static method    A static method belongs to the class rather than object of a class  A static method invoked without the need for creating an instance of a class  static method can access static data member and can change the value of it  A static method can be accessed just using the name of a class dot static name        example   Student9 change    If you want to use non-static fields of a class  you must use a non-static method      Program of changing the common property of all objects static field      class Student9     int rollno     String name     static String college    ITS       static void change       college    BBDIT            Student9 int r  String n      rollno   r     name   n           void display    System out println rollno     name     college       public static void main String args       Student9 change       Student9 s1   new Student9  111  Indian      Student9 s2   new Student9  222  American      Student9 s3   new Student9  333  China       s1 display      s2 display      s3 display             O P    111 Indian BBDIT        222 American BBDIT        333 China BBDIT

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Static methods should be called on the Class  Instance methods should be called on the Instances of the Class   But what does that mean in reality   Here is a useful example   A car class might have an instance method called Accelerate     You can only Accelerate a car  if the car actually exists  has been constructed  and therefore this would be an instance method   A car class might also have a count method called GetCarCount     This would return the total number of cars created  or constructed    If no cars have been constructed  this method would return 0  but it should still be able to be called  and therefore it would have to be a static method

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Actually  we use static properties and methods in a class  when we want to use some part of our program should exists there until our program is running  And we know that  to manipulate static properties  we need static methods as they are not a part of instance variable  And without static methods  to manipulate static properties is time consuming

[java] When to use static methods

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