Collections sort List T Comparator super T method example

Question

Possible Duplicate    Sorting Java objects using multiple keys       I can t find any example of using this method  all examples give the second parameter  null   I heard that this method used for sorting classes according to more than one criterion but no example where found   public class Student implements Comparable lt Student gt    String name  int age   public Student String name  int age        this name   name      this age   age      Override public String toString         return name         age      Override public int compareTo Student o        Integer myAge   age      Integer oAge   o age      return myAge compareTo oAge          for this class if i want to sort a list of Student according to their names  amp  ages how can i use the method Collections sort List Comparator

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You probably want something like this   Collections sort students  new Comparator lt Student gt                           public int compare Student s1  Student s2                               if s1 getName      null  amp  amp  s2 getName      null  amp  amp  s1 getName   comareTo s1 getName       0                                   return s1 getName   compareTo s2 getName                                  else                                return s1 getAge   compareTo s2 getAge                                                              This sorts the students first by name  If a name is missing  or two students have the same name  they are sorted by their age

User · Answer

To use Collections sort List Comparator    you need to create a class that implements Comparator Interface  and code for the compare   in it  through Comparator Interface  You can do something like this   class StudentComparator implements Comparator       public int compare  Student s1 Student s2                   code to compare 2 students           To sort do this    Collections sort List new StudentComparator

User · Answer

Building upon your existing Student class  this is how I usually do it  especially if I need more than one comparator    public class Student implements Comparable lt Student gt         String name      int age       public Student String name  int age           this name   name         this age   age              Override     public String toString             return name         age              Override     public int compareTo Student o            return Comparators NAME compare this  o               public static class Comparators            public static Comparator lt Student gt  NAME   new Comparator lt Student gt                   Override             public int compare Student o1  Student o2                    return o1 name compareTo o2 name                                    public static Comparator lt Student gt  AGE   new Comparator lt Student gt                   Override             public int compare Student o1  Student o2                    return o1 age - o2 age                                   public static Comparator lt Student gt  NAMEANDAGE   new Comparator lt Student gt                   Override             public int compare Student o1  Student o2                    int i   o1 name compareTo o2 name                   if  i    0                        i   o1 age - o2 age                                    return i                                     Usage   List lt Student gt  studentList   new LinkedList lt  gt     Collections sort studentList  Student Comparators AGE     EDIT  Since the release of Java 8 the inner class Comparators may be greatly simplified using lambdas  Java 8 also introduces a new method for the Comparator object thenComparing  which removes the need for doing manual checking of each comparator when nesting them  Below is the Java 8 implementation of the Student Comparators class with these changes taken into account    public static class Comparators       public static final Comparator lt Student gt  NAME    Student o1  Student o2  - gt  o1 name compareTo o2 name       public static final Comparator lt Student gt  AGE    Student o1  Student o2  - gt  Integer compare o1 age  o2 age       public static final Comparator lt Student gt  NAMEANDAGE    Student o1  Student o2  - gt  NAME thenComparing AGE  compare o1  o2

User · Answer

This might be simplest way -  Collections sort listOfStudent new Comparator lt Student gt                          public int compare Student s1 Student s2                                 Write your logic here                              Using Java 8 lambda expression  -  listOfStudent sort  s1  s2  - gt  s1 age - s2 age

[java] Collections sort(List<T>,Comparator<? super T>) method example

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