Assuming you have something like so:

public class MyFirstClass {
   ...
   public ArrayList<Integer> myNumbers()    {
    ArrayList<Integer> numbers = new ArrayList<Integer>();
    numbers.add(5);
    numbers.add(11);
    numbers.add(3);
    return(numbers);
   }
   ...
}

You can call that method like so:

public class MySecondClass {
    ...
    MyFirstClass m1 = new MyFirstClass();
    List<Integer> myList = m1.myNumbers();
    ...
}

Since the method you are trying to call is not static, you will have to create an instance of the class which provides this method. Once you create the instance, you will then have access to the method.

Note, that in the code example above, I used this line: List<Integer> myList = m1.myNumbers();. This can be changed by the following: ArrayList<Integer> myList = m1.myNumbers();. However, it is usually recommended to program to an interface, and not to a concrete implementation, so my suggestion for the method you are using would be to do something like so:

public List<Integer> myNumbers()    {
    List<Integer> numbers = new ArrayList<Integer>();
    numbers.add(5);
    numbers.add(11);
    numbers.add(3);
    return(numbers);
   }

This will allow you to assign the contents of that list to whatever implements the List interface.

You need to instantiate the class it is contained within, or make the method static.

So if it is contained within class Foo:

Foo x = new Foo();
List<Integer> stuff = x.myNumbers();

or alternatively shorthand:

List<Integer> stuff = new Foo().myNumbers();

or if you make it static like so:

public static List<Integer> myNumbers()    {
    List<Integer> numbers = new ArrayList<Integer>();
    numbers.add(5);
    numbers.add(11);
    numbers.add(3);
    return(numbers);
}

you can call it like so:

List<Integer> stuff = Foo.myNumbers();

List<Integer> numbers = new TheClassName().myNumbers();

If Foo is the class enclose this method

class Foo{
  public ArrayList<Integer> myNumbers()    {
     //code code code
  }
}

then

new Foo().myNumbers();

MyClass obj = new MyClass();
ArrayList<Integer> numbers = obj.myNumbers();

You can use on another class

public ArrayList<Integer> myNumbers = new Foo().myNumbers();

or

Foo myClass = new Foo();
 public ArrayList<Integer> myNumbers = myclass.myNumbers();

Your method can be called and the arraylist can be stored like this

YourClassName class = new YourClassName();
Arraylist<Integer> numbers = class.numbers(); 

This also allows the arraylist to be manipulated further in this class

you can also create method like prime_f and then call ArrayList like:

   public static ArrayList<Integer> prime_f(int number) {
   ArrayList<Integer> counter = new ArrayList<Integer>();
   for (int i = 2;i < number;i++) {
       while(number%i == 0) {
           counter.add(i);
           number = number/i;
       }
   }
   if (number > 2) {
       counter.add(number);
   }
   return counter;
}