Java sorting - comparable v comparator
In Java, you can sort objects that implement the Collection interface, using either the comparable interface or a comparator class. What's the difference?
interfaced class must contain a method called compareTo
to compare two objects (one being the object on which it is called and the other being passed as a paramater) which returns an integer, negative for a < b
, positive for a > b
and 0 otherwise. It must also contain an equals method, returning a boolean, on the same parameters.
is a class in its own right, which implements the Comparator interface; that means it must contain a method called compare
(two objects as parameters) which returns a negative, zero or positive integer depensing on whether the first object is less than, equal to, or greater than the second.
If you want to sort a collection using its comparable interface, you simply call the static Collections.sort method on it ... so if we had an ArrayList called Actable which implemented Comparable, we could write:
To sort a collection using a Comparator class, you need to pass an extra parameter into the Collections.sortmethod - that parameter being an instance of a Comparator object. Thus:
Collections.sort(Actable, new Bynum());
So in effect, using the comparable
interface gives you a default sort order of your own making, whereas the comparator
is more flexible as it allows you define a whole series of different types of sorting that can be applied to a whole lot of different types of objects. (written 2007-02-02, updated 2007-02-05)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesJ714 - Java - Fundamental classes 
String handling - from first steps to practical examples - (2010-11-13) 
Sorting - naturally, or into a different order - (2010-08-14) 
for and foreach in Java - (2010-04-22) 
Length, size or capacity in Java? - (2010-02-24) 
Sorting Collections of Objects in Java - (2009-09-25) 
Viv.java uses unchecked or unsafe operations - explanation and cure - (2009-09-24) 
Java Collection Objects in the java.util package - (2009-08-05) 
Java - Generics - (2008-11-27) 
Java, sorting, ArrayList example, generics - (2008-01-11) 
Do languages change? - (2004-09-08)
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