Question

I have the following two classes:

public class Class1
{
    public Class1 randomvariable; // Variable declared

    public static void main(String[] args)
    {
        randomvariable = new Class1(); // Variable initialized
    }
}

public class Class2
{
    public static void ranMethod()
    {
        randomvariable.getSomething(); // I can't access the member "randomvariable" here even though it's public and it's in the same project?
    }
}

I am very certain that it’s a very fundamental thing I’m missing here, but what am I actually missing? The Class1 member “randomvariable” is public and so is the class and both classes are in the same project.
What do I have to do to fix this problem?

Answer 1

There are two problems:

Firstly, you’re trying to assign a value to randomvariable from main, without there being an instance of Class1. This would be okay in an instance method, as randomvariable would be implicitly this.randomvariable – but this is a static method.

Secondly, you’re trying to read the value from Class2.ranMethod, again without there being an instance of Class1 involved.

It’s important that you understand what an instance variable is. It’s a value associated with a particular instance of a class. So if you had a class called Person, you might have a variable called name. Now in Class2.ranMethod, you’d effectively be writing:

name.getSomething();

That makes no sense – firstly there’s nothing associating this code with Person at all, and secondly it doesn’t say which person is involved.

Likewise within the main method – there’s no instance, so you haven’t got the context.

Here’s an alternative program which does work, so you can see the difference:

public class Person {
    // In real code you should almost *never* have public variables
    // like this. It would normally be private, and you'd expose
    // a public getName() method. It might be final, too, with the value
    // assigned in the constructor.
    public String name;

    public static void main(String[] args) {
        Person x = new Person();
        x.name = "Fred";
        PersonPresenter.displayPerson(x);
    }
}

class PersonPresenter {

    // In a real system this would probably be an instance method
    public static void displayPerson(Person person) {
        System.out.println("I present to you: " + person.name);
    }
}

As you can tell by the comments, this still isn’t ideal code – but I wanted to stay fairly close to your original code.

However, this now works: main is trying to set the value of an instance variable for a particular instance, and likewise presentPerson is given a reference to an instance as a parameter, so it can find out the value of the name variable for that instance.