java comparator how to sort by integer

Question

Im trying to learn comparator in java and I have found this great example online  my question is how would this code be changed so that the pet names are ordered by age and in descending order so that the oldest is first and youngest is last    class Dog implements Comparator lt Dog gt   Comparable lt Dog gt   private String name  private int age  Dog       Dog String n  int a     name   n    age   a     public String getDogName      return name     public int getDogAge      return age        Overriding the compareTo method public int compareTo Dog d     return  this name  compareTo d name         Overriding the compare method to sort the age  public int compare Dog d  Dog d1     return d age - d1 age       public class Example  public static void main String args          Takes a list o Dog objects   List lt Dog gt  list   new ArrayList lt Dog gt        list add new Dog  Shaggy  3      list add new Dog  Lacy  2      list add new Dog  Roger  10      list add new Dog  Tommy  4      list add new Dog  Tammy  1      Collections sort list     Sorts the array list    for Dog a  list   printing the sorted list of names      System out print a getDogName                  Sorts the array list using comparator   Collections sort list  new Dog       System out println         for Dog a  list   printing the sorted list of ages      System out print a getDogName                 a getDogAge

User · Answer

If you have access to the Java 8 Comparable API, Comparable.comparingToInt() may be of use. (See Java 8 Comparable Documentation).

For example, a Comparator<Dog> to sort Dog instances descending by age could be created with the following:

Comparable.comparingToInt(Dog::getDogAge).reversed();

The function take a lambda mapping T to Integer, and creates an ascending comparator. The chained function .reversed() turns the ascending comparator into a descending comparator.

Note: while this may not be useful for most versions of Android out there, I came across this question while searching for similar information for a non-Android Java application. I thought it might be useful to others in the same spot to see what I ended up settling on.

User · Answer

One simple way is   Comparator lt Dog gt  ageAscendingComp        Comparator lt Dog gt  ageDescendingComp   Collections reverseOrder ageAscendingComp      then call the sort method   On a side note  Dog should really not implement Comparator  It means you have to do strange things like  Collections sort myList  new Dog  Rex   4        -- why is a new dog being made  What are we even sorting by   Collections sort myList  myList get 0        -- or perhaps more confusingly   Rather you should make Compartors as separate classes   eg   public class DogAgeComparator implments Comparator lt Dog gt        public int compareTo Dog d1  Dog d2            return d1 getAge   - d2 getAge              This has the added benefit that you can use the name of the class to say how the Comparator will sort the list  eg   Collections sort someDogs  new DogNameComparator        now in name ascending order  Collections sort someDogs  Collections reverseOrder new DogAgeComparator         now in age descending order   You should also not not have Dog implement Comparable  The Comparable interface is used to denote that there is some inherent and natural way to order these objects  such as for numbers and strings   Now this is not the case for Dog objects as sometimes you may wish to sort by age and sometimes you may wish to sort by name

User · Answer

From Java 8 you can use    Comparator comparingInt Dog  getDogAge  reversed

User · Answer

Just replace   return d age - d1 age    By   return   Integer d age  compareTo d1 age     Or invert to reverse the list   return   Integer d1 age  compareTo d age     EDIT   Fixed the  memory problem   Indeed  the better solution is change the age field in the Dog class to Integer  because there many benefits  like the null possibility

User · Answer

Simply changing   public int compare Dog d  Dog d1      return d age - d1 age      to  public int compare Dog d  Dog d1      return d1 age - d age      should sort them in the reverse order of age if that is what you are looking for   Update    Arian is right in his comments  one of the accepted ways of declaring a comparator for a dog would be where you declare it as a public static final field in the class itself   class Dog implements Comparable lt Dog gt        private String name      private int age       public static final Comparator lt Dog gt  DESCENDING COMPARATOR   new Comparator lt Dog gt                 Overriding the compare method to sort the age         public int compare Dog d  Dog d1                return d age - d1 age                        Dog String n  int a            name   n          age   a             public String getDogName             return name             public int getDogAge             return age                Overriding the compareTo method     public int compareTo Dog d            return  this name  compareTo d name              You could then use it any where in your code where you would like to compare dogs as follows      Sorts the array list using comparator Collections sort list  Dog DESCENDING COMPARATOR     Another important thing to remember when implementing Comparable is that it is important that compareTo performs consistently with equals  Although it is not required  failing to do so could result in strange behaviour on some collections such as some implementations of Sets  See this post for more information on sound principles of implementing compareTo   Update 2  Chris is right  this code is susceptible to overflows for large negative values of age  The correct way to implement this in Java 7 and up would be Integer compare d age  d1 age  instead of d age - d1 age   Update 3  With Java 8  your Comparator could be written a lot more succinctly as   public static final Comparator lt Dog gt  DESCENDING COMPARATOR        Comparator comparing Dog  getDogAge  reversed      The syntax for Collections sort stays the same  but compare can be written as  public int compare Dog d  Dog d1        return DESCENDING COMPARATOR compare d  d1

User · Answer

public class DogAgeComparator implements Comparator lt Dog gt        public int compare Dog o1  Dog o2            return Integer compare o1 getAge    o2 getId

[java] java comparator, how to sort by integer?

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