java - Non-static variable cannot be referenced from a static context

Question

I've written this test code:

class MyProgram
{
    int count = 0;
    public static void main(String[] args)
    {
        System.out.println(count);
    }
}

But it gives the following error:

Main.java:6: error: non-static variable count cannot be referenced from a static context
        System.out.println(count);
                           ^

How do I get my methods to recognize my class variables?

Answer 1

You must understand the difference between a class and an instance of that class. If you see a car on the street, you know immediately that it's a car even if you can't see which model or type. This is because you compare what you see with the class "car". The class contains which is similar to all cars. Think of it as a template or an idea.

At the same time, the car you see is an instance of the class "car" since it has all the properties which you expect: There is someone driving it, it has an engine, wheels.

So the class says "all cars have a color" and the instance says "this specific car is red".

In the OO world, you define the class and inside the class, you define a field of type Color. When the class is instantiated (when you create a specific instance), memory is reserved for the color and you can give this specific instance a color. Since these attributes are specific, they are non-static.

Static fields and methods are shared with all instances. They are for values which are specific to the class and not a specific instance. For methods, this usually are global helper methods (like Integer.parseInt()). For fields, it's usually constants (like car types, i.e. something where you have a limited set which doesn't change often).

To solve your problem, you need to instantiate an instance (create an object) of your class so the runtime can reserve memory for the instance (otherwise, different instances would overwrite each other which you don't want).

In your case, try this code as a starting block:

public static void main (String[] args)
{
    try
    {
        MyProgram7 obj = new MyProgram7 ();
        obj.run (args);
    }
    catch (Exception e)
    {
        e.printStackTrace ();
    }
}

// instance variables here

public void run (String[] args) throws Exception
{
    // put your code here
}

The new main() method creates an instance of the class it contains (sounds strange but since main() is created with the class instead of with the instance, it can do this) and then calls an instance method (run()).