This section provides a tutorial example on how to construct object with the Comparable interface, so that they can be stored in collections using the TreeSet class. Search operations on TreeSet class perform nicely - almost the same as HastSet and LinkedHashSet.
In order to use TreeSet collection, data elements must be comparable. So I
enhanced MySimpleDate class and rename it as MyDate clase:
import java.util.*;
public class MyDate implements Comparable {
private static Random r;
private Date my_date;
public MyDate () {
my_date = new Date();
long l = my_date.getTime();
int i = (int) (l/1000/60/60/24);
if (r==null) r = new Random();
i = r.nextInt(i);
l = ((long) i)*24*60*60*1000;
my_date.setTime(l);
}
public long getTime() {
return my_date.getTime();
}
public int compareTo(Object o) {
if (getTime()<((MyDate) o).getTime()) return -1;
else if (getTime()==((MyDate) o).getTime()) return 0;
else return 1;
}
public boolean equals(Object o) {
return compareTo(o)==0;
}
public int hashCode() {
return (int)(getTime()^(getTime()>>>32));
}
}
Note that:
Comparable interface is implemented in this class. It requires to implement the
compareTo() method.
If the compareTo() method is implemented, the equals() method must be overriden,
in order to maintain the following logic: if x.compareTo(y)==0, then x.equals(y)==true.
The default equals() method compares two objects based on their reference numbers
(addresses in memory),
not looking at any data inside the objects. So two separate objects, even with identical
data, will always fail the default equals() method.
If the equals() method is overriden, the hashCode() method must also be overriden,
in order to maintain the following logic: if x.equals(y)==true, then x.hashCode()==y.hashCode().
The default hashCode() method generates hash value also based on the object's reference
number (address in memory), not looking at any data inside the object. So two separate
objects with identical data may not have the same hash value, if the default hashCode()
method is used.
I copied the hash code generation expression from the Long class.
Using the MyDate class with my own compareTo(), equals(), and hashCode() methods,
I repeated the search operation performance tests. Here is the results:
HashSet, LinkedHashSet, and TreeSet did very well. The well designed hash
value helped to reduce the number of objects to be fetched for comparison.
Vector, ArrayList, and LinkedList took 3 times longer than the MySimpleDate
version. My guess is that the overriden equals() method is 3 times slower than
the default equals() method, which just checks the memory addresses of the objects.
TreeMap and HashMap took a little bit longer than the MySimpleDate version.
IdentityHashMap and Hashtable took about the same time as the MySimpleDate
version.
