gistfile1.txt

import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;


public class Main {

    public static class Thingy {
        private DifferentThingy nestedThingy;

        public Thingy(DifferentThingy nestedThingy) {
            this.nestedThingy = nestedThingy;
        }

        private DifferentThingy getNestedThingy() {
            return this.nestedThingy;
        }
    }

    public static class DifferentThingy {
        private String attr;

        public DifferentThingy(String attr) {
            this.attr = attr;
        }

        private String getAttr() {
            return this.attr;
        }
    }

    public static void main(String... args) {
        getUniqueBasedOnDifferentThingyAttr(Arrays.asList(new Thingy(new DifferentThingy("1")),
                new Thingy(new DifferentThingy("1")),
                new Thingy(new DifferentThingy("1")),
                new Thingy(new DifferentThingy("2")),
                new Thingy(new DifferentThingy("3")),
                new Thingy(new DifferentThingy("1"))));
    }

    private static Collection<Thingy> getUniqueBasedOnDifferentThingyAttr(List<Thingy> originalList) {
        return originalList.stream()
                .collect(Collectors.toMap(thing -> thing.getNestedThingy().getAttr(), p -> p, (p, q) -> p))
                .values();
    }
}