non-static method cannot be referenced from a static context

Question

I would like to understand this once and for all.

With this please excuse the mass of code pasted below, but I do not want to leave out any details.

The only

Answer 1

It's a good idea in this case to make the method static. The other way is to use

  new GarageComm().readPositions("holdingsBU.txt")

instead.

So, why this error?

A static method can not call a non-static method in the same class. It sounds bizarre, but there is a reason for it. Consider this:

• Non static methods may depend on the state of the objects, the instance variable to say.

• Static methods can be called from out-side without instanciating

Now, if we allow static methods to play with non-static method (and non static variables), they might fail. So, it is a kind of prevention.

When should I consider making a method static?

Now, come to, why not make the method static?,

Very well, you should make a method static if it's functionality does not depend on the object's state.

If it just uses the passed parameters and some static stuffs (static variable, static methods) and returns (with/without a result, throwing exception anything), consider making it static method.

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update: looked like this post confused a couple of people so updated the answer.

Answer 2

You need to make your function static:

public static HashMap readPositions(String destFile) {
...
}

Creating an instance of GarageComm would work too, but this is bad programming practice in Java, since that object has no state.

Answer 3

This is a typical mindbender for new Java programmers.

A static method does not belong to an object. A non-static method belongs to an object.

You use the main-method convention to have your program started, and it is required that that method must be static.

The trick to get from a static method to a non-static method, is that you must create an object so you can call the method on that. I.e. new GarageComm().readPositions(...). I just don't think you have to here, so it would be simpler to just mark readPositions as static too.

Answer 4

If a method is not static, then it must be called "on" an object. When calling a method from a non-static method, that object is implied -- it's the object the first method was called on. But when calling it from a static method, there is no implied object, so to call the method, you must supply an object:

GarageComm gc = new GarageComm();
hashPos= gc.readPositions("holdingsBU.txt");

Since GarageComm has no state of its own, there's no reason to create a GarageComm object, so it's just as well you mark the method static, so no object is required to call it.

Answer 5

Real solution? Don't put so much stuff in the main() method. That's for noobs.

Java's an object-oriented language. Put the logic inside methods associated with the GarageComm class. main() should do little more than instantiate an instance and call its methods.

Change it like this:

            GarageComm gc = new GarageComm();
            hashPos= gc.readPositions("holdingsBU.txt");//the error is here