One ArrayList for all classes

Question

I want to have one list for accessing from any class. But when I try to reach a list from another class it throws NullPointerException. I tried to solve it with constructors, but in this case every class which have an object of the main class (where the list stores) create a new list by calling constuctor. But I need unique list which can be accessed, modified and etc. from other classes. How could I manage this?

You'd probably start to manage this by providing some more information, e.g. a [mcve]. Currently I have no idea what you're actually after or which problems you try to solve - some sort of code is very likely to clarify it. — Thomas, Apr 18 '17 at 12:24
Look for keywords like: static, singleton Make a try and when you then still have problem, post some code and we can help you — Rene M., Apr 18 '17 at 12:28
Just be careful when designing around singletons. They are [often considered an antipattern](http://stackoverflow.com/a/1448407/6902543) when testing your code. — Michael Peyper, Apr 18 '17 at 12:36
So what's the difference between singleton pattern and static? — Hasan Shans, Apr 18 '17 at 15:52

score 0 · Accepted Answer · answered Apr 21 '17 at 06:41

0

I've got it. I just needed to declare my ArrayList static. Thank you so much!

answered Apr 21 '17 at 06:41

Hasan Shans

57
11

score -1 · Answer 2 · answered Apr 18 '17 at 12:30

Use the concept of Singleton class.

class List
{
    private static ArrayList<int> intlist;
    private static List list;

    //private constructor [singleton class]
    private List(){
       intlist==new ArrayList<int>();
    };

    private static List getList()
    { 
       if(this.list==null)
          list=new List();

       return list;
    }

    public getIntList(){return this.intlist;}
}

class A
{
    public void Test()
    {
        List l=List.getList();
        l.getIntList().add(1);
    }
}

class B
{
    public void Test()
    {
        List l=List.getList();
        l.getIntList().add(2);
    }
}

Now class A and class B are getting same list to put value into it.

One ArrayList for all classes

2 Answers2