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

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<Student> { 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 & ages how can i use the method Collections sort(List,Comparator)

Answer 1

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<Student> {      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<Student> NAME = new Comparator<Student>() {             @Override             public int compare(Student o1, Student o2) {                 return o1.name.compareTo(o2.name);             }         };         public static Comparator<Student> AGE = new Comparator<Student>() {             @Override             public int compare(Student o1, Student o2) {                 return o1.age - o2.age;             }         };         public static Comparator<Student> NAMEANDAGE = new Comparator<Student>() {             @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<Student> studentList = new LinkedList<>(); 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<Student> NAME = (Student o1, Student o2) -> o1.name.compareTo(o2.name);     public static final Comparator<Student> AGE = (Student o1, Student o2) -> Integer.compare(o1.age, o2.age);     public static final Comparator<Student> NAMEANDAGE = (Student o1, Student o2) -> NAME.thenComparing(AGE).compare(o1, o2); }