Instantiating object from inside the main of that class in Java

Question

I was looking through my OOP class documentation and I found this example:

class Student {
    private String name;
    public int averageGrade;


    public Student(String n, int avg) {
        name = n;
        averageGrade = avg;
    }

    public static void main(String[] args) {
        Student s = new Student("John", 9);
    }
}

I find it confusing that they are instantiating an object from the main of the same class. Is this considered bad practice? Will the newly created object s have a main method?

Thank you!

Solution 1

There's nothing wrong at all with this. It's entirely normal. (Admittedly it would make more sense for a class with a main method to be something one could obviously execute - a main method in a Student class doesn't make as much sense.)

Objects don't really have methods - classes have methods, either static methods which are called without any particular context, and instance methods which are called on a particular object of that type (or a subclass).

While you could call s.main(...) that would actually just resolve to a call to the static method Student.main; you shouldn't call static methods "via" expressions like this, as it's confusing.

Solution 2

No, it's not bad practice. It's actually fairly frequent. What you missed is that main is a static method. It's not a method of the Student object. It's a method of the Student class. You don't invoke it with someStudent.main(...), but with Student.main(...).

See http://download.oracle.com/javase/tutorial/java/javaOO/classvars.html for more explanations.

Solution 3

This is just fine.

I know it looks a bit recursive, but what happens is that the main() method gets called when you launch this class from the command line, and then the constructor gets called when you actually instantiate an instance of the object. (See Jon's comment as well.)

Solution 4

It is neither bad nor good.

It depends on the usage. In your example the constructor is called from main() method that is static by definition, so you have no other choice.

Yet another example of "good" usage of this pattern is factory method pattern. Enums use this technique too in valueOf() method (that is an example of factory method too).

Solution 5

It's totally fineIt's totally fine... If a member of a class is declared as static,it's an entity lives differently of any particular object of the class.Its something that can be used by itself,without using any objects. OR it's common between different objects.You can actually count the number of objects created from a class,by setting up a static variable in the class like

 class   A
     {
         A()
          {
           count++
           }

     static count=0;
       --- 
       ---
      }

And each time an object of A,created count will add one.

Since static methods does not belong to any objects particularly,Outside the class,its called as classname.method() just like ordinary methods are called as classObject.method()

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Comments

[Kasun Siyambalapitiya]

do we need to create an instance of the Student class in the main method if we want to run some other methods that resides on same class, or is it ok to run other methods of the same class just by saying the method name. For example printResult(); inside the main method of Student class

[Jon Skeet]

@KasunSiyambalapitiya: Well that depends whether you want to invoke the method on a current instance or on a new instance - which state would you want it to work on? Don't just think about what the rules of the language let you do - think about what you want the result to be. You shouldn't be creating objects without having a reason to do so.