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 instanc

Answer 1

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