When to make a method static in Java

Making a method static in Java is an important decision . Though, static
keyword is one of the fundamental concepts, many times programmers gets
confused to make a particular method static or not. In Java programming, main
motivation for making a method static is convenience.
You can call a static method without creating any object, just by using it's
class name. So if you need a method, which you want to call directly by class
name, make that method static. Utility classes e.g. java.lang.Math or StringUtils, are good
examples of classes, which uses static methods. Before making a method static,
you should look into limitation of static methods as well, as you
can not override static method in Java. By keeping these properties in
mind, we can make few rules, which will help to decide when to make a method static in Java and when to use them. In this
Java article, we will learn more about benefits and limitation of making a method
static, and also see couple of examples of static methods from JDK to learn
when and how to use static method in Java.

What does static method do in Java

When you see a static method in Java code, What do you assume? What
reasoning and assumptions a reader make, when he sees a static method? This is
important to learn to  ensure we are
using static method correctly.

1) Static method doesn't modify state of object. Since state of object is
maintained as instance variables, and Java doesn't allow non static variables
on static context. Modern days IDE like Netbeans also shows static
method in italics to differentiate it from other methods.

2) Static method mostly operates on arguments, almost all static method
accepts arguments, perform some calculation and return value.

Rules
to make a method static in Java

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 :

1) If a method doesn't modify state of object, or not using any instance variables.

2) You want to call method without creating instance of that class.

3) 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.

4) 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.

5) 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.

When to use static method in Java

Now, we know the benefits and limitation of making a method static in
Java, we can see couple of scenarios where we can use static methods. Factory
design pattern provides a good use of static method. You can use static method
to create instance of a class. Even Effective Java advises about
using static factory method, couple of example of these in Java library is
creating thread pool from Executors class. Executors provides
lots of static methods to create different types of thread pool e.g. public
static ExecutorService newCachedThreadPool(), public static
ExecutorService newFixedThreadPool(int nThreads) etc. Another interesting
use of static methods from JDK is collection classes e.g. Collections and Arrays which
provides lot of static utility methods to operate on different kinds of
collection. Static method can also be combined with variable arguments to
create a collection of explicitly elements e.g. EnumSet.of(E first,
E... rest). Apart from these, if you loot at Apache commons lang library, you will
find a pattern of utils class e.g. StringUtils, ArrayUtils, which
provides utility methods to operate on String and
arrays. One more interesting use of static method I have seen is valueOf() method
inside different value classes e.g. java.lang.String, though
this is also an example of factory method, but it's also
a nice way to convert one type to another. For example valueOf() can also
be used to convert String to Integer in Java. In short, it make sense to use
static methods :

1) Along with creational design pattern e.g. Factory and Singleton.

2) As utility method, which operate on arguments.

3) A conversion tool e.g. valueOf().

That's all about when to make a method static in Java. We have seen
benefits and limitation of making a method static, and few examples of static
methods from JDK. JDK examples will also help you to decide when to use static
method in Java.

Mentioned books in this article

Effective Java 2nd Edition from Joshua Bloch

Source:http://javarevisited.blogspot.com/2013/07/when-to-make-method-static-in-java.html