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Java: when to use static methods

Posted by: admin November 2, 2017 Leave a comment

Questions:

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!

Answers:

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.)

Questions:
Answers:

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.
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Answers:

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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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.

Questions:
Answers:

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.");
     }
  }

Questions:
Answers:

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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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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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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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.

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Answers:

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

Questions:
Answers:

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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Answers:

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 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 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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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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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;
    }
};

Questions:
Answers:

I am wondering when to use static methods?

  1. A common use for static methods is to access static fields.
  2. 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 <= b) ? a : b;
    }
    
  3. 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:

  1. Instance methods can access instance variables and instance methods directly.
  2. Instance methods can access class variables and class methods directly.
  3. Class methods can access class variables and class methods directly.
  4. 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.
Questions:
Answers:

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)

eclipse setting

Questions:
Answers:

Static methods are shared between all instances of the class, developers normally use static keyword with utility methods which do some common functionality that is regularly used anywhere in the application.

If the following conditions apply, then make your method static:

  1. Your method does a common functionality which is regularly used in the whole application.
  2. Your method doesn’t use instance variables.
  3. Your method accepts input as arguments, do some common functionality then return some result.

P.S: It is worth to mention that it is very difficult or even (not possible) to mock a static method, mocking is a mechanism used in unit testing in order to fake the business of some methods.

For more details, check: Static keyword in java