Need for static methods and variables

Can anyone please explain me the need for static methods and variables?

Thanks,
Krithika

Welcome to JavaRanch, Krithika Sundaram

Krithika Sundaram wrote:Can anyone please explain me the need for static methods and variables?

Static Variables

When we want a variable to be shared across all instances of a class, we need a global variable. Static variables are of rescue here. Otherwise, all the variables will be specific to a particular instance (object) of the class.

One good example : If you want to count the # of instances being created for a particular class. Where will you have such variable/count? It should be outside the object scope and that is nothing but the class level scope -- static variable.

Static Methods

There are cases when we have methods doing some activity NOT specific to an instance of a class but in general. Say for example, if you want to print a message based on the person's age, you can have it as an instance method for example, isEligible() which says whether the person is eligible or not based on his age. If you look at this, 'age' is a specific property of each object. So this method has to be an instance method.

whereas, if you look at the method named getSystemDate() that does NOT need to print anything based on the object's state/data. Hence it is common to all objects and does NOT need any instance to invoke it. -- Class level or static method it is called as.

class Person
{
   
    /** instance level variable -- each object shares its own copy */
    private int age;

/** static variable -- one for all objects (common to a class) */
   static int instanceCounter = 0;

/**
       *  Overloaded one argument Constructor
       *  of the class
       */
    public Person(int age)
    {
        this.age = age;
        instanceCounter++;
    }

/**
      *  This method returns true/false based on the 
      *  eligibility criteria (which is calculated on the
      *  person's age. 
      *
      *  The result might vary based on the each object's 
      *  state (that has the member data 'age')
      */
    public boolean isEligible()
    {
          if (this.age >= 18)
              return true;

return false;
    }

/**
       * This is the 'static' method -- which does NOT
       * need any instance specific data to do its intended
       * operation. 
       * 
       * It acts in the same fashion for all the callers including
       * the objects of its own. (i.e., The result would always
       * be the same for any type of caller).
       */
    public static java.util.Date getSystemDate()
    {
        return new java.util.Date();
    }

public static void main(String... args)
    {

Person p1 = new Person(21);
        Person p2 = new Person(17);

System.out.println("p1 eligible ? : " + p1.isEligible());
        System.out.println("p2 eligible ? : " + p2.isEligible());

System.out.println("Though static methods are generally called with the class name, I can even call with the object references");

System.out.println(p1.getSystemDate());
        System.out.println(p2.getSystemDate());

System.out.println("I am calling the static methods with the class name for the same/independent results");
        System.out.println(Person.getSystemDate());

System.out.println("The total # of instances being created so far is : "+Person.instanceCounter);

}
}

Guess this helps

You can read sun's online tutorial for more!

You can read sun's online tutorial for more!

And here is the link:
http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html

Regards,
Rok

Raghavan Muthu wrote:Welcome to JavaRanch, Krithika Sundaram

Thanks Raghavan!

It was very helpful. I appreciate your elaborate response. Thanks to Raghavan and Rok.

You are welcome Krithika and it is our pleasure