Java - Using Instance Class from Static Method?

Question

I've been programming for years using .NET and I'm taking the plunge into Java with some simple starter programs.

I'm having a bit of trouble though...

When I create my start-up class with public void main, the compiler won't let me instantiate any of the classes I've written?

The error I'm getting is "non-static variable _processor cannot be referenced from a static context" where _processor is the object I'm trying to instantiate from the Processor class that I wrote.

The program will compile and run just fine when I change Processor to a static class, but I don't want to have to make all of my classes static.

Any way around this?

Thanks in advance!

Here's everything I've written. It won't compile in the current state:

class Lab
{
    public static void main(String[] args)
    {
        Processor proc = new Processor();

        proc.Go();
    }

    private class Processor
    {
        private Random _rand = new Random();

        public void Processor() {}

        public void Go()
        {

        }
    }
}

That's not everything you've written - the error message refers to _processor, but that doesn't occur in the code you've shown. — Jon Skeet
– Jon Skeet, Commented Apr 17, 2012 at 17:06

Péter Török · Accepted Answer · 2012-04-17 17:19:32Z

2

Is Processor an inner class of Lab by any chance? (yes, now that you published your code, my suspicion is confirmed).

In Java, nonstatic inner classes contain an implicit reference to the containing object of the outer class, so they can't be instantiated from static context (from your main method).

So

either create an instance of Lab (e.g. myLab), and then call myLab.new Processor(), or
declare Processor static (as you did), or
turn Processor into a top level class.

edited Apr 17, 2012 at 17:19

answered Apr 17, 2012 at 17:03

Péter Török

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Comments

Piotr Kochański · Accepted Answer · 2012-04-17 17:06:59Z

1

Is this your problem, by any chance?

public class DemoClass{

  String field;

  public static void main(String[] args){
    field = "Hey"; //forbidden - can't access instance field from static context
    DemoClass dc = new DemoClass();
    dc.field = "Hey"; //this is ok
  }

}

answered Apr 17, 2012 at 17:06

Piotr Kochański

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Comments

Youssef G. · Accepted Answer · 2012-04-17 17:04:02Z

0

You should call a constructor from main:

public static void main(String[] args){
new MyClass();
}

And put your instantiations in the constructor.

answered Apr 17, 2012 at 17:04

Youssef G.

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toto2 · Accepted Answer · 2012-04-17 17:10:07Z

0

but I don't want to have to make all of my classes static.

Most inner classes in Java are static, in my experience. If you can write an inner class in a separate file (it does not use directly the members of the containing class), it should then be defined as static. It really is only for convenience that Java lets you write inner static classes inside other classes.

answered Apr 17, 2012 at 17:10

toto2

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Comments

Speckpgh · Accepted Answer · 2012-04-17 17:10:54Z

0

Your process class is an inner class of your lab class, you can't create an instance of your process class until you instantiate an instance of you lab class, unless that inner class is static or something.

Main will run in your lab class, because it is static, but when you attempt to create an instance of the private inner class, you can't because this class is a "private inner" class of Lab, and you don't have a lab instance, so you can't reference it directly.

You can try making your processor class static, or at the very least public, or better yet you can instantiate an instance of your lab class first, and reference creation of the processor class through your instantiate lab class.

answered Apr 17, 2012 at 17:10

Speckpgh

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