CODEWITHSUNDEEP
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Motombo
Motombo

Reputation: 1787

Why Must this method be static(Java)?

For some background, I'm currently on chapter 8 in my book, we finished talking about arraylists, arrays, if statements, loops etc. Now this part of the book talks about call by reference,value and some other pretty neat things that seem odd to me at first.I've read What situation to use static and some other SO questions, and learned quite a bit as well.

Consider the following example my book gave (among many examples)

There is another reason why static methods are sometimes necessary. If a method manipulates a class that you do not own, you cannot add it to that class. Consider a method that computes the area of a rectangle. The Rectangle class in the standard library has no such feature, and we cannot modify that class. A static method solves this problem:

public class Geometry
{
  public static double area(Rectangle rect)
  {
    return rect.getWidth() * rect.getHeight();
  }
// More geometry methods can be added here.
}

Now we can tell you why the main method is static. When the program starts, there aren’t any objects. Therefore, the first method in the program must be a static method.

Ok, thats pretty cool, up until now I've just been really blindly putting public in front of all my methods, so this is great to know. But the review small problem on the next page caught my attention

The following method computes the average of an array list of numbers:

public static double average(ArrayList<Double> values)

Why must it be a static method?

Here I was like wait a sec. I'm pretty sure I did this without using static before. So I tried doing this again and pretty easily came up with the following

import java.util.ArrayList;
class ArrList
{
    private double sum;

    public ArrList()
    {
        sum = 0;
    }

    public double average(ArrayList <Double> values)
    {
        for(int i = 0; i < values.size() ; i++)
        {
            sum+=values.get(i);
        }

        return sum / values.size();

    }
}

public class Average
{
    public static void main(String [] args)
    {
        ArrList arrListObj = new ArrList();
        ArrayList<Double> testArrList = new ArrayList<Double>();
        testArrList.add(10.0);
        testArrList.add(50.0);
        testArrList.add(20.0);
        testArrList.add(20.0);
        System.out.println(arrListObj.average(testArrList));

    }
}

TLDR

Why does my book say that public static double average(ArrayList<Double> values) needs to be static?

ATTEMPT AT USING STATIC

public class Average
{
    public static void main(String [] args)
    {
        ArrayList<Double> testArrList = new ArrayList<Double>();
        ArrayList<Double> testArrListTwo = new ArrayList<Double>();
        testArrList.add(10.0);
        testArrList.add(50.0);
        testArrList.add(20.0);
        testArrList.add(20.0);
        testArrListTwo.add(20.0);
        testArrListTwo.add(20.0);
        testArrListTwo.add(20.0);
        System.out.println(ArrList.average(testArrList));
        System.out.println(ArrList.average(testArrListTwo)); // we don't get 20, we get 53.3333!


    }
}

Upvotes: 0

Views: 272

Answers (3)

SomeDude
SomeDude

Reputation: 14238

Static methods like the area, average are usually utility functions. You don't need any object to use an utility function. For example consider Math.pow you don't need to instantiate any object to use the power function, just use Math.pow(10.0, 2.0) to get (10.0)^2

In short : Static method means class method, that is no instance of that object is needed to invoke. whereas your average method is an instance method, you need an object to invoke that method.

Upvotes: 1

David
David

Reputation: 218847

It doesn't.

The only method which needs to be static is the initial main() method. Anything and everything else is up to you as the programmer to decide what makes sense in your design.

static has nothing to do with public accessors (as you allude to), and it has nothing to do with the technical operation being performed. It has everything to do with the semantics of the operation and the class which holds it.

An instance (non-static) method exists on a particular instance of a class. Semantically it should perform operations related to that specific instance. A static method exists on a class in general and is more conceptual. It doesn't do anything to a particular instance (unless it's provided an instance of something as a method argument of course).

So you really just need to ask yourself about the semantics of the operation. Should you need new instance of an object to perform an operation? Or should the operation be available without an instance? That depends on the operation, on what the objects represent, etc.

Upvotes: 1

djechlin
djechlin

Reputation: 60768

  1. If it is not static, then any other class that wants to use this method must first create an instance of this object.

From some other class:

Average.average(new ArrayList<Double>()); // legal only if static
new Average().average(new ArrayList<Double>()); // necessary if not static
// and only makes sense if Average can be instantiated in the first place

  1. It's legal to make it an instance (i.e. not static) variable, but the method is actually harder to understand. If it is static then whoever reads the code knows it does not use any member variables of the class.

    // In the class body
int x = 0; // member variable
public static double average() {
    x = x + 1; // illegal
}

The less something can do, the easier to understand what it does do.

Upvotes: 1

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