what is the work-around to "non-static variable cannot be referenced from a static context"?

Question

There's a particular idiom to putting a method, or perhaps anonymous inner class, somehow, into the main method of a driver class:

package net.bounceme.dur.misc;

import net.bounceme.dur.misc.Foo;

public class StaticRef {

    Foo f = Foo.INSTANCE;

    public static void main(String[] args) throws Exception {

        f.connect();  //move this to inside "run"
        /*
        public void run(){
           StaticRef sf = new StaticRef();
           //do stuff here
        }
         */
    }
}

to prevent the below error:

init:
Deleting: /home/thufir/NetBeansProjects/nntp/build/built-jar.properties
deps-jar:
Updating property file: /home/thufir/NetBeansProjects/nntp/build/built-jar.properties
Compiling 1 source file to /home/thufir/NetBeansProjects/nntp/build/classes
/home/thufir/NetBeansProjects/nntp/src/net/bounceme/dur/misc/StaticRef.java:11: non-static variable f cannot be referenced from a static context
        f.connect();  //move this to inside "run"
1 error
/home/thufir/NetBeansProjects/nntp/nbproject/build-impl.xml:626: The following error occurred while executing this line:
/home/thufir/NetBeansProjects/nntp/nbproject/build-impl.xml:245: Compile failed; see the compiler error output for details.
BUILD FAILED (total time: 0 seconds)

However, I can't find anything so far. I see something similar in threading examples, but can't quite get the syntax.

The following is largely what I want, but I don't think it's quite correct:

package net.bounceme.dur.misc;

public class StaticRef {

    Foo f = Foo.INSTANCE;

    public static void main(String[] args) throws Exception {

        StaticRef sf = new StaticRef();
        sf.f.connect();
    }
}

What I would like is to put the instantiation of sf into...I'm not quite sure. Maybe the above is correct and "ok"?

In your example, Foo is not static. f.connect() will not work. Is your question "How do I call f.connect() inside public static main?". If so, you can't without creating an instance of StaticRef. — kufudo, Feb 18 '13 at 07:13
Pardon, yes, Foo is not static, nor do I wish it to be so. What's the work-around to instantiating Foo without declaring it to be static or final? I'll revise the question, I might've gotten mixed up. — Thufir, Feb 18 '13 at 07:19
@Thufir. After your updated question, I really don't understand now what you want. Because your updated code will work fine. — Rohit Jain, Feb 18 '13 at 07:19
static missing in front of `Foo f = ` to allow writing `sf.f.connect()`... — Aubin, Feb 18 '13 at 07:23
Only it's a bit awkward to reference the Foo instance, which I'd rather not be either static nor final, only private. I believe there's an idiom which "runnable" uses, and I've seen it in Swing generated code as well. — Thufir, Feb 18 '13 at 07:23
possible duplicate of [non-static variable cannot be referenced from a static context (java)](http://stackoverflow.com/questions/2559527/non-static-variable-cannot-be-referenced-from-a-static-context-java) — user207421, Feb 18 '13 at 09:15
@EJP I don't think it's not a real question because it has a real answer. I was asking for the work around, and found it -- see my answer. Answers like "declare variable static" were clearly not what I was asking. Please do **comment** on my answer rather than just downvoting. — Thufir, Feb 18 '13 at 10:33
@Thufir: Don't jump to conclusions: If you look at EJP's profile you'll see no downvotes have been made today. Marking a question as a dup is fair comment. — Jon Egerton, Feb 18 '13 at 10:51
@JonEgerton I did jump to conclusions, I didn't know you could look at downvotes like that. It's just annoying in that there 20million questions on that error message, and, here and there, you can find the answer I was looking for -- but it's a real needle in a haystack. In fact, I referenced my sources in my answer for that reason. I didn't find the answer in a question, but saw a few things which triggered recalling this "idiom", if it is such. Is it an idiom? As best I could I crafted a careful question on a topic with, admittedly, bazillions of duplicate questions. Sorry EJP. — Thufir, Feb 18 '13 at 11:00

score 1 · Answer 1 · answered Feb 18 '13 at 07:23

1

You have some options:

Move Foo into the scope of main
Declare Foo as a static variable
```
static Foo f = Foo.INSTANCE;
```
Create an instance of StaticRef and use the object from that
```
new StaticRef().f.connect();
```

answered Feb 18 '13 at 07:23

Makoto

104,088
27
192
230

only for the second option, referencing the Foo object f is a bit awkward. – Thufir Feb 18 '13 at 07:25
If it's public, protected or package visibility, it's possible - but I do agree a bit awkward. Proper OO style would indicate that a getter be used for it (`getFoo()`), but if that's how it's done, then... – Makoto Feb 18 '13 at 07:26

score 1 · Answer 2 · edited May 23 '17 at 12:26

Your instance variable cannot be referenced from a static context. You need an object of the class to get (a reference to) it's contents.

You can write a Singleton pattern:

public class SingletonDemo {
    private static SingletonDemo instance = null;

    private SingletonDemo() {       }

    public static SingletonDemo getInstance() {
            if (instance == null) {
                instance = new SingletonDemo ();
            }
            return instance;
    }

}

If thread safety is an issue, you can use an enum:

public enum Singleton{
    INSTANCE;
    private Singleton(){ ... }
}

or

public class Singleton{
    private final static Singleton instance = new Singleton();
    private Singleton(){ ... }
    public static Singleton getInstance(){ return instance; }
}

See here

[Double-checked locking](http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java) done this way isn't thread safe. — Makoto, Feb 18 '13 at 07:28

score 1 · Answer 3 · answered Feb 18 '13 at 07:25

1

Sorry, can't comment yet so I have to post it as an answer.

The updated code is fine, but since Foo.INSTANCE seems to be a use of the Singleton pattern, it makes sense to define it as static and use it as in your first example. At most there will be a single value of Foo.INSTANCE, so it doesn't make sense to make each instance of StaticRef to have its own reference to Foo.INSTANCE.

answered Feb 18 '13 at 07:25

Javier

12,100
5
46
57

Why can't you comment everywhere? That only requires 50 reputation. – Makoto Feb 18 '13 at 07:52
1

I didn't have so "much" reputation 37 minutes ago (or I didn't notice it had been awarded). – Javier Feb 18 '13 at 08:04

score 0 · Accepted Answer · edited May 23 '17 at 10:29

If this code is "faulty" or violates any OOP principle, please do comment:

package net.bounceme.dur.misc;

public class StaticRef {

    private Foo f = Foo.INSTANCE;

    public StaticRef() throws Exception {
        f.connect();
    }

    public static void main(String[] args) throws Exception {
        new StaticRef();
    }
}

The justification I've heard for this approach is that, for re-use, StaticRef can be easily modified to act as a Java Bean by doing little more than removing the main method. Please do comment. See also this answer, for a similar solution, or this other answer.

what is the work-around to "non-static variable cannot be referenced from a static context"?

4 Answers4

Linked