Treeset to order elements in descending order
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11-09-2019 - |
Question
Here is the piece of code that I have used for Java 5.0
TreeSet<Integer> treeSetObj = new TreeSet<Integer>( Collections.reverseOrder() ) ;
Collections.reverseOrder() is used to obtain a comparator in order to reverse the way the elements are stored and iterated.
Is there a more optimized way of doing it?
Solution
Why do you think this approach won't be optimized? The reverse order Comparator
is simply going to be flipping the sign of the output from the actual Comparator
(or output from compareTo
on the Comparable
objects being inserted) and I would therefore imagine it is very fast.
An alternative suggestion: Rather than change the order you store the elements in you could iterate over them in descending order using the descendingIterator()
method.
OTHER TIPS
TreeSet::descendingSet
In Java 6 and later, there is a method on TreeSet
called descendingSet()
producing a NavigableSet
interface object.
public NavigableSet descendingSet()
The descending set is backed by this set, so changes to the set are reflected in the descending set, and vice-versa. If either set is modified while an iteration over either set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined.
The returned set has an ordering equivalent to
Collections.reverseOrder(comparator()). The expression s.descendingSet().descendingSet() returns a view of s essentially equivalent to s.
Specified by: descendingSet in interface NavigableSet<E> Returns: a reverse order view of this set Since: 1.6
TreeSet<Integer> treeSetObj = new TreeSet<Integer>(new Comparator<Integer>()
{
public int compare(Integer i1,Integer i2)
{
return i2.compareTo(i1);
}
});
there is need to flip the result. But I guess this is just a micro-optimization... Do you really need this ?
Reverse compare
You can reverse the order of the two arguments in the compare
method of your Comparator
.
TreeSet t = new TreeSet(new MyComparator());
{
class MyComparator implements Comparator
{
public int compare(Integer i1,Integer i2)
{
Integer I1=(Integer)i1;
Integer I2=(Integer)i2;
return I2.compareTo(I1); // return -I1compareTo(I2);
}
}
}