Java reference explained

 public static void main(String[] args) {
        T t = new T();
        C c = new C();
        c.a = "a";
        C c1 = new C();
        c1.a = "b";
        t.a.addAll(List.of(c, c1));
        Finder finder = new Finder(t);

        C finded = finder.find("a");

//        finded.a = "C";
        check(finded);
        System.out.println(t.a);
    }

    static void check(C findede) {
        findede.a = "C";
    }

    static class Finder {
        private final T t;
        Finder(T t) {
            this.t = t;
        }
        C find(String c) {
            return t.a.stream().filter(cc -> c.equals(cc.a)).findFirst().get();
        }

    }

    static class C {
        public String a;

        @Override
        public String toString() {
            return "C{" +
                    "a='" + a + '\'' +
                    '}';
        }
    }

    static class T {
       public List<C> a = new ArrayList<>();
    }