What is the reason behind "non-static method cannot be referenced from a static context"?

Asked 2008-11-14T17:59:30.660

389

The very common beginner mistake is when you try to use a class property "statically" without making an instance of that class. It leaves you with the mentioned error message:

You can either make the non static method static or make an instance of that class to use its properties.

What the reason behind this? Am not concern with the solution, rather the reason.

private java.util.List<String> someMethod(){
    /* Some Code */
    return someList;            
}

public static void main(String[] strArgs){          
     // The following statement causes the error. 
    java.util.List<String> someList = someMethod();         
}

java static

Edit Report

3 Answers

103

The method you are trying to call is an instance-level method; you do not have an instance.

static methods belong to the class, non-static methods belong to instances of the class.

answered 2008-11-14T18:04:17.810

I think it is worth pointing out that by the rules of the Java language the Java compiler inserts the equivalent of "this." when it notices that you're accessing instance methods or instance fields without an explicit instance. Of course, the compiler knows that it can only do this from within an instance method, which has a "this" variable, as static methods don't.

Which means that when you're in an instance method the following are equivalent:

instanceMethod();
this.instanceMethod();

and these are also equivalent:

... = instanceField;
... = this.instanceField;

The compiler is effectively inserting the "this." when you don't supply a specific instance.

This (pun intended) bit of "magic help" by the compiler can confuse novices: it means that instance calls and static calls sometimes appear to have the same syntax while in reality are calls of different types and underlying mechanisms.

The instance method call is sometimes referred to as a method invocation or dispatch because of the behaviors of virtual methods supporting polymorphism; dispatching behavior happens regardless of whether you wrote an explicit object instance to use or the compiler inserted a "this.".

The static method call mechanism is simpler, like a function call in a non-OOP language.

Personally, I think the error message is misleading, it could read "non-static method cannot be referenced from a static context without specifying an explicit object instance".

What the compiler is complaining about is that it cannot simply insert the standard "this." as it does within instance methods, because this code is within a static method; however, maybe the author merely forgot to supply the instance of interest for this invocation — say, an instance possibly supplied to the static method as parameter, or created within this static method.

In short, you m

answered 2012-07-15T19:20:31.090

The answers so far describe why, but here is a something else you might want to consider:

You can can call a method from an instantiable class by appending a method call to its constructor,

Object instance = new Constuctor().methodCall();

primitive name = new Constuctor().methodCall();

This is useful it you only wish to use a method of an instantiable class once within a single scope. If you are calling multiple methods from an instantiable class within a single scope, definitely create a referable instance.

answered 2008-11-15T07:18:41.420

What is the reason behind "non-static method cannot be referenced from a static context"?

3 Answers

Your Answer