Novice wrote:
Lew wrote:
'toString()' should always give a useful way to identify the
specific instance.
That's why I was displaying the name of the table so I'm going to
take that as a "yes" ;-)
It should depend on (and usually only on) the same fields used to
drive 'hashCode()' and 'equals()' and if supported, 'compareTo()'
(which should always be consistent with each other).
I don't touch hashCode(), equals() or compareTo() very often at all.
Or toString() either for that matter. But overriding hashCode() and
equals() solved a big problem for me recently. I was writing a holder
class that contained three fields, a table name, a row number and a
column number, as a key for a hash map and then storing the column
width in the value portion of the map. But when I tried to look up a
given combination of table name, row number and column number I was
never finding values that I knew were there. I googled and found out
that I needed to revise hashCode() and equals() and rougly what those
changes needed to be and then my lookups went fine. But I didn't
touch compareTo(). Hmm, maybe I need to revisit that and make sure it
doesn't need some tweaking too....
The default 'Object#equals()' method gives (almost) the same answer as
the == operator - two instances are equal iff (if and only if) they
are the same instance. That's called "object equality" or "instance
equality".
Often we want value equality. It's not very helpful, for example, for
new Integer(67551) != new Integer(67551)
to be true.
That means equality of different instances, provided their values
match in some defined way.
For 'Integer', value equality depends on only one attribute, the
underlying 'int' field.
For a more complicated type, there might be more than one attribute.
For example, you might regard two widgets as equal iff they have the
same owner and color:
public class Widget
{
private final Owner owner; // assume getter methods for brevity
private final Color color;
public Widget(Owner owner, Color color)
{
if (owner == null || color == null)
{
throw new IllegalArgumentException("null");
}
this.owner = owner;
this.color = color'
assert this.owner != null && this.color != null;
}
@Override public boolean equals(Object other)
{
if (other == this) { return true; }
if (! (other instanceof Widget)) { return false; }
Widget widget = (Widget) other;
return this.getOwner().equals(widget.getOwner())
&& this.getColor().equals(widget.getColor()
}
}
But wait! 'hashCode()', which is a shortcut for 'equals()' (well,
really for "not equals()"), doesn't know anything about those fields
yet! It will think objects are not equal when they really are.
So you have to override 'hashCode()', too, using the same fields, so
that its result are consistent with 'equals()':
@Override public int hashCode()
{
return owner.hashCode() * 31 + color.hashCode();
}