FormingClosure

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;
import java.util.function.Supplier;

public class FormingClosure {

    Supplier<List<String>> supplier;

    public static void main(String[] args) {
        FormingClosure fc = new FormingClosure();
        fc.go();
    }

    public void go() {
        final List<String> someList = new ArrayList<>();
        
        // THIS is a side effect!!!
        someList.add("one");
        someList.add("two");

        // JUST formed a closure right here on someList. Booooooo
        supplier = () -> someList;

        // THIS is a side effect!!!
        someList.add("three");
        someList.add("four");

        System.out.println(" someList:");
        someList.forEach(System.out::println);
        System.out.println(" --------------------- ");
        System.out.println(" closure:");
        supplier.get().forEach(System.out::println);

        // The RIGHT way to do it:
        // Everyone understand this right? It's a Function that takes a List of Strings and
        // returns Supplier that supplies a List of Strings....
        Function<List<String>, Supplier<List<String>>> consumer = list -> () -> list;
    }
}