The Arrays class has versions of sort() and binarySearch() which don’t require a Comparator. For example, you can use the version of Arrays.sort() which just takes an array of objects. These methods call the compareTo() method of the objects in the array.
Since Date implements Comparable, it has a compareTo method just like String does. So your custom Comparator could look like this: The compare() method must return an int, so you couldn’t directly return a boolean like you were planning to anyway. Your sorting code would be just about like you wrote: A slightly shorter way to write all this, if you don’t need to reuse your comparator, is to … Read more
You can not sort TreeMap on values. A Red-Black tree based NavigableMap implementation. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used You will need to provide comparator for Comparator<? super K> so your comparator should compare on keys. To provide sort on … Read more
There are a couple of awkward things with your example class: it’s called People while it has a price and info (more something for objects, not people); when naming a class as a plural of something, it suggests it is an abstraction of more than one thing. Anyway, here’s a demo of how to use a Comparator<T>: EDIT And an … Read more
[…] How should Java Comparator class be declared to sort the arrays by their first elements in decreasing order […] Here’s a complete example using Java 8: Output: For Java 7 you can do: If you unfortunate enough to work on Java 6 or older, you’d do:
The exception message is actually pretty descriptive. The contract it mentions is transitivity: if A > B and B > C then for any A, B and C: A > C. I checked it with paper and pencil and your code seems to have few holes: you do not return -1 if card1.getRarity() > card2.getRarity(). … Read more
You’re passing the comparator as the key function. You should be passing it as the cmp, wrapped in some kind of function that turns it into a proper comparator. (Although actually, you should be using key functions: Also note that sortedDict will not actually be a dict, so the name is rather confusing.)
When your class implements Comparable, the compareTo method of the class is defining the “natural” ordering of that object. That method is contractually obligated (though not demanded) to be in line with other methods on that object, such as a 0 should always be returned for objects when the .equals() comparisons return true. A Comparator is its own definition of how to … Read more