Use Comparable if you want to define a default (natural) ordering behaviour of the object in question, a common practice is to use a technical or natural (database?) identifier of the object for this. Use Comparator if you want to define an external controllable ordering behaviour, this can override the default ordering behaviour.
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 … Read more
The text below comes from Comparator vs Comparable Comparable A comparable object is capable of comparing itself with another object. The class itself must implements the java.lang.Comparable interface in order to be able to compare its instances. Comparator A comparator object is capable of comparing two different objects. The class is not comparing its instances, … Read more
This is a weakness in the compiler’s type inferencing mechanism. In order to infer the type of u in the lambda, the target type for the lambda needs to be established. This is accomplished as follows. userList.sort() is expecting an argument of type Comparator<User>. In the first line, Comparator.comparing() needs to return Comparator<User>. This implies … Read more
Is the array sort callback function called many times during the course of the sort? Yes If so, I’d like to know which two numbers are passed into the function each time You could find out your self with: array.sort((a,b) => { console.log(`comparing ${a},${b}`); return a > b ? 1 : a === b ? … Read more
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: if (card1.getRarity() < card2.getRarity()) { return 1; you do … 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 … Read more
Your comparator is not transitive. Let A be the parent of B, and B be the parent of C. Since A > B and B > C, then it must be the case that A > C. However, if your comparator is invoked on A and C, it would return zero, meaning A == C. … Read more
Since Date implements Comparable, it has a compareTo method just like String does. So your custom Comparator could look like this: public class CustomComparator implements Comparator<MyObject> { @Override public int compare(MyObject o1, MyObject o2) { return o1.getStartDate().compareTo(o2.getStartDate()); } } The compare() method must return an int, so you couldn’t directly return a boolean like you … Read more
Both Comparable and Comparator are interfaces. All they do is define a way of asking whether one object is greater than, equal to, or less than another. The interfaces don’t enforce what that means — that’s up to the class that implements the interface. So one Comparator<Employee> might say that Adam is “greater than” Bill … Read more