Question

i am using Java and i have this hierarchy

class Seq

class For extends Seq

interface SeqIt

Class ForIt implements SeqIt

And ForIt has a constructor: public ForIt( For x ) //takes For objects

Till now everything works fine.
Now I want to make a new class called FU that gets access to any ForIt objects I created.
And this FU class must have a method with this signature public static int sum1(For MyFor)
For example if ForIt objects have inside it an array called myArr. This method sum1 is supposed to return the sum of all elements of the array myArr inside MyFor object

How can I create this class FU?

Answer 1

The 2 ways I can think of are:

• Every time you create a ForIt class, add it to your FU class – when you are done, call the sum method
• Have the ForIt class constructor call a static method of the FU class to let it know a new ForIt was created

I would personally use option 1 as it avoids static variables which can be a pain to work with. Option 2 would work for a simple use case.

//not thread safe
class FU {
    private final List<ForIt> list = new ArrayList<> ();
    private int sum = 0;

    void add(ForIt fi) {
        list.add(fi);
        sum += fi.getValue();
    }

    int sum() {
        return sum;
    }
}

Option 1 is fairly straigthforward (create a FU object, and every time you create a ForIt instance, add it to the FU object).
Option 2:

class ForIt {
    private static final FU fu = new FU();

    ForIt() {
        fu.add(this);
    }

    static int sum() {
        return fu.sum();
    } 
}

public static void main(String... args) {
    new ForIt();
    new ForIt();
    System.out.println(ForIt.sum());
}