How do I add this to an arraylist within a static method?

Question

I'm learning the ropes with Java and I've hit a snag with ArrayLists. The gist of my program is to take some user input parameters, create a class Foo with those parameters, and then add it to an arraylist. The problem is, it complains that I can't reference a non-static type from a static method. The only examples I can find online deal with adding constants ("Cat", "5.0" etc) to arraylists which doesn't really help me.

I put the gist of my code below. I've moved the arraylist off to its own class Bar and added an add method which just does arraylist.add(foo), if only as a crapshoot to make it work (it doesn't). I omitted the loop but it loops a number of times after the definitions, so the arraylist gets populated.

public class MainClass{

public static void main(String[] args){
Scanner scanner = new Scanner(System.in);
int a, b;

a = scanner.nextint();
b = scanner.nextint();

Foo foo = new Foo(a, b);
Bar.add(foo); //Complains here
}
}

Edit: Here is Bar explicitly

import java.util.ArrayList;
public class Bar{
private ArrayList list = new ArrayList();

public void add(Foo foo){
    list.add(foo);
}
}

If it helps, the object foo isn't changed after creation.

How do I get around this? Thanks in advance for any help.

yeah, your gist is missing a declaration of Bar, and/or the error, so it isn't a very good gist — John Gardner
– John Gardner, Commented Dec 4, 2010 at 0:18
edited; the error is pretty much as I said; "non-static method add(java.lang.Object) cannot be referenced from a static context" — BB ON
– BB ON, Commented Dec 4, 2010 at 0:25
if your list only contains one type of object, you should look into using generics, too, and declare that as private List<Foo> list = new ArrayList<Foo>(); — John Gardner
– John Gardner, Commented Dec 4, 2010 at 0:33
This illustrates why you should always quote error messages exactly. You originally said "it complains that I can't reference a non-static type from a static method" but then you said the error was "non-static method add(java.lang.Object) cannot be referenced from a static context". There is a difference between a non-static type and a non-static method. — Laurence Gonsalves
– Laurence Gonsalves, Commented Dec 4, 2010 at 0:42

John Gardner · Accepted Answer · 2010-12-04 00:19:27Z

0

Without information about Bar, or the actual error it complains about, we can't help much.

I'd presume its because the Bar.add method isn't static, or Bar isn't static.

// a class with a public static method that encapsulates a static list
public class Bar
{
    static List<Foo> innerlist = new ArrayList<Foo>();

    public static void add( Foo o )
    {
        innerList.add(o);
    }
}

or do you intend "Bar" to be a static list member of something else?

// a class with a static member
public class OtherClass
{
     public static List<Foo> Bar = new ArrayList<Foo>();
}

then your code would need a OtherClass.Bar.add(o) instead of just Bar.add(o);

edited Dec 4, 2010 at 0:19

answered Dec 4, 2010 at 0:12

John Gardner

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2 Comments

BB ON Over a year ago

Here is Bar; you're right, it's not static. public class Bar{ private Arraylist list = new Arraylist(); public void add(Object O){ this.add(O);}}

John Gardner Over a year ago

you either need to create an instance of Bar, and use that in your main method, or you need to declare the add method to be static on your bar class, and make that list static too.

Aravind Yarram · Accepted Answer · 2010-12-04 00:39:04Z

0

You have 2 options

option 1: create an instance of Bar and add foo to that instance. This is my preferred option

Bar b=new Bar();
b.add(foo);

option 2: make your add(...) method static. this also means your "list" should also be static. both are bad.

answered Dec 4, 2010 at 0:39

Aravind Yarram

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1 Comment

BB ON Over a year ago

I might try this too. Thank you.

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