Komposition created by romangraef - https://repl.it/@romangraef/Komposition

Futter.java

class Futter {
  String name;
  float mass;

  public Futter(String name)  {
    this.name = name;
    this.mass = 100f;
  }

  public String toString(){
    return "Futter: "+name + ", "+mass+"g.";
  }

}

Futternapf.java

import java.util.Optional;

class Futternapf {

  Optional<Futter> futter;

  public Futternapf() {
    futter = Optional.empty();
  }


  public void refillFutter(String name) {
    futter = Optional.of(new Futter(name));
  }

  public String toString() {
    return "Futternapf. Futter: "+futter;
  }
}

Main.java

class Main {
  public static void main(String[] args) {
    Futternapf f = new Futternapf();
    System.out.println(f);
    f.refillFutter("Premium Futter");
    System.out.println(f);
  }
}