import java.util.*;
class GenericTest02 {

class Element { // chemical element
  final String name;
  final int z; // sequential number in Mendeleev table;
  Element(String n, int z) { name = n; this.z = z; }
  public String toString() { return "name="+name+"_seqNo="+z; }
  public boolean equals(Object o) { return (o instanceof Element) && (this.name.equals( ((Element)o).name)); }
  public int hashCode() { return name.hashCode() + 31*z; }
}
class Metal extends Element {
  final double mohsHardness;
  Metal(String n, int z, double h) { super(n, z); mohsHardness = h; }
  public String toString() { return super.toString()+"_hardness="+mohsHardness; }
}

class ElementComparator implements Comparator<Element> {
  public int compare(Element e1, Element e2) {
    return e1.name.compareTo(e2.name);
  }
}

  public static void main(String[] aa) {
    new GenericTest02().go();
  }
  void go() {
    Set<Element> e = new TreeSet<Element>( new ElementComparator() );

    e.add( new Metal( "ferrum",  26, 4)  );
    e.add( new Metal( "chrom",  24,  8.5 ) );
    e.add( new Metal( "titan",   22, 6   ) );

    showAll( "initially my old collection:", e );

    addSome( e ); // pass collection declared as "Set<Element>" - super class for Metal

    showAll( "after all, my new collection is:", e );

  }

  void showAll( String title, Set<? extends Element> e) { 
    System.out.println( "\n"+title );
    for (Element x : e) System.out.println( x );
  };


  void addSome( Set<? super Metal> m ) {

    m.add( new Metal( "cuprum", 29, 3 ) );
    
    System.out.println( "\nafter adding one element:" );
    for(Iterator i = m.iterator(); i.hasNext(); ) { System.out.println( i.next() ); }

    // incompatible types  found: java.lang.Object required: GenericTest02.Metal
    // -!- for ( Metal x : m) System.out.println( x ); // Why ?

    // incompatible types  found: java.lang.Object required: GenericTest02.Element
    // -!- for ( Element x : m) System.out.println( x ); // Why ?

  }

}

initially my old collection:
name=chrom_seqNo=24_hardness=8.5
name=ferrum_seqNo=26_hardness=4.0
name=titan_seqNo=22_hardness=6.0

after adding one element:
name=chrom_seqNo=24_hardness=8.5
name=cuprum_seqNo=29_hardness=3.0
name=ferrum_seqNo=26_hardness=4.0
name=titan_seqNo=22_hardness=6.0

after all, my new collection is:
name=chrom_seqNo=24_hardness=8.5
name=cuprum_seqNo=29_hardness=3.0
name=ferrum_seqNo=26_hardness=4.0
name=titan_seqNo=22_hardness=6.0

  void addSome( Set<? super Metal> m ) {

    m.add( new Metal( "cuprum", 29, 3 ) );
    System.out.println( "\nafter adding one element:" );
    for ( Object x : m) System.out.println( x ); // OK
    //for(Iterator i = m.iterator(); i.hasNext(); ) { System.out.println( i.next() ); }
  }