java variable scope issue

Question

I'm getting confused at how java works with everything being a class.

I make a class foo.java and in the main method of foo.java I make an instance of foo (a class creating itself makes no sense to me but it seems normal in java ?)

Then I make a instance of another class bar, so I now have instances of foo and bar inside the foo class but how do I access the member variables of foo with the functions in bar?

foo is the processing code and bar is the ui and needs to see all the data in foo so it can display it.

A little sample pf code would be very valuable. Can you post both of your classes so we can see what you have written so far? — Mechkov
– Mechkov, Commented Nov 4, 2011 at 11:57
wouldn't it be more logical to have a foo object in your bar object — Nathan Q
– Nathan Q, Commented Nov 4, 2011 at 11:58

Pablo Santa Cruz · Accepted Answer · 2011-11-04 11:59:24Z

3

There are a lot of possibilities. One is to construct foo with a bar instance:

public class Foo {
    public void someFooMethod() {
    }
    public static void main(String[] args) {
        Foo foo = new Foo();
        Bar bar = new Bar(foo);
        bar.someBarMethod();
    }
}

public class Bar {
    private Foo fooLocalRefence;
    public Bar(Foo foo) {
        this.fooLocalReference = foo;
    }
    public someBarMethod() {
        this.fooLocalReference.someFooMethod();
    }
}

answered Nov 4, 2011 at 11:59

Pablo Santa Cruz

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1 Comment

Anish Dasappan Over a year ago

Yes, composition is always a better choice

Till Helge · Accepted Answer · 2011-11-04 11:59:14Z

1

You're wrong about "a class creating its self makes no sense to me but it seems normal in java", because your terminology is wrong. What you are actually doing is to create an instance of class Foo. And main() is merely the entry point of the application, because the JVM simply needs to know where to start.

And if you want instances of two different classes to share information, you need to provide one of them with a reference to the other. Something like this:

Foo foo = new Foo();
Bar bar = new Bar(foo);

answered Nov 4, 2011 at 11:59

Till Helge

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Comments

Manse · Accepted Answer · 2011-11-04 11:58:12Z

0

I suggest you look at the Java tutorials on the oracle site -> http://download.oracle.com/javase/tutorial/java/index.html

Follow these and this will give you a better understanding of how to write applications in Java - writing code which you do not understand will cause you problems later in the development process .....

answered Nov 4, 2011 at 11:58

Manse

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Comments

John B · Accepted Answer · 2011-11-04 11:59:01Z

0

First, the main method of Foo is probably static, so there is not an instance of Foo creating a Foo.

Second, Foo should expose its data via getter methods and Bar should access the data by calling the getter methods on the provided instance of Foo.

answered Nov 4, 2011 at 11:59

John B

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2 Comments

Skeith Over a year ago

how does bar call the getter methods ? you just moved the problem not solved it

John B Over a year ago

@Skeith I said that "Foo should expose its data via getter methods and Bar should access the data ... on the provided instance of Foo". The assumption is that the getter methods are public so Bar will be able to call them. This is exactly what Pablo posted (except his Foo method returns void).

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