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); }