with Java: when to use static methods




when to use static methods c# (16)

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.

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?

e.g

Obj x = new Obj();
x.someMethod

or

Obj.someMethod

(is this the static way?)

I'm rather confused!


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


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.


A static method has two main purposes:

  1. 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.
  2. 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.

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.


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.


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.


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.


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)


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.


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<ClassName>() {
    @Mock
    public int doSomething(Input input1, Input input2){
        return returnValue;
    }
};

One rule-of-thumb: ask yourself "does it make sense to call this method, even if no Obj has been constructed yet?" 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 void setMileage(double mpg) (which sets the efficiency of one particular Car) can't be static since it's inconceivable to call the method before any Car has been constructed.

(Btw, 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 "privileged" 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.)


Define static methods in the following scenarios only:

  1. If you are writing utility classes and they are not supposed to be changed.
  2. If the method is not using any instance variable.
  3. If any operation is not dependent on instance creation.
  4. If there is some code that can easily be shared by all the instance methods, extract that code into a static method.
  5. If you are sure that the definition of the method will never be changed or overridden. As static methods can not be overridden.

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.


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.");
     }
  }

If you apply static keyword with any method, it is known as static method.

  1. A static method belongs to the class rather than object of a class.
  2. A static method invoked without the need for creating an instance of a class.
  3. static method can access static data member and can change the value of it.

//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





static-methods