void*


class MyCl extends Object
class MyCl extends XYZ /* extends ... extends ... extends Object */
{
    int i;
    String s;

    @Override /* annotation */
//    public String toString(String s)
    public String toString()
    {
        return String.format("MyCl {%d, %s}", i, s);
//        return String.format("MyCl {%d, %s}%n", i, s);
    }
}

new MyCl(5, "abc").toString();    ->   MyCl {5, "abc"}


equivalence relation
- reflexive         a.equals(a)
- symmetrical       a.equals(b) ====> b.equals(a)
- transitive        a.equals(b) && b.equals(c) ====> a.equals(c)


MUST: a.equals(b)  ==(necessary)==> a.hashCode() == b.hashCode()

GOOD: !a.equals(b) ==(good idea)==> a.hashCode() != b.hashCode()
                   (not necessary)

    // possible but BAD
    public int hashCode() { return 0; }