当遇到要对自己构造的类进行比较时，比如当要构造PriorityQueue实现类、TreeSet实现类、TreeMap实现类等等，或者是使用Collections.sort方法时，会需要类实现Comparable接口或者传入Comparator接口的实现类。

Comparable

当我们构造的类实现 java.lang.Comparable 接口时，需要覆写compareTo()方法。String、Integer等类可直接调用compareTo()方法。
比如编写一个Movie类，用电影年份进行排序，年份小的优先级更高。

// A Java program to demonstrate use of Comparable // A class 'Movie' that implements Comparable 
class Movie implements Comparable<Movie> 
{ private double rating; private String name; private int year; // Used to sort movies by year public int compareTo(Movie m) { return this.year - m.year; } // Constructor public Movie(String nm, double rt, int yr) { this.name = nm; this.rating = rt; this.year = yr; } // Getter methods for accessing private data public double getRating() { return rating; } public String getName()   {  return name; } public int getYear()      {  return year;  } 
} // Driver class 
class Main 
{ public static void main(String[] args) { ArrayList<Movie> list = new ArrayList<Movie>(); list.add(new Movie("Force Awakens", 8.3, 2015)); list.add(new Movie("Star Wars", 8.7, 1977)); list.add(new Movie("Empire Strikes Back", 8.8, 1980)); list.add(new Movie("Return of the Jedi", 8.4, 1983)); Collections.sort(list); System.out.println("Movies after sorting : "); for (Movie movie: list) { System.out.println(movie.getName() + " " + movie.getRating() + " " + movie.getYear()); } } 
} 

Comparator

但如果我还想根据电影评分等信息进行排序，Comparable就做不到了，因为只能覆写一次compareTo方法。于是我们可以编写Comparator的实现类并覆写compare方法来实现多种排序方式。

//A Java program to demonstrate Comparator interface // Class to compare Movies by ratings 
class RatingCompare implements Comparator<Movie> 
{ public int compare(Movie m1, Movie m2) { if (m1.getRating() < m2.getRating()) return -1; if (m1.getRating() > m2.getRating()) return 1; else return 0; } 
} // Class to compare Movies by name 
class NameCompare implements Comparator<Movie> 
{ public int compare(Movie m1, Movie m2) { //String类型实现了Comparable接口，即可调用compareTo方法return m1.getName().compareTo(m2.getName()); } 
} // Driver class 
class Main 
{ public static void main(String[] args) { ArrayList<Movie> list = new ArrayList<Movie>(); list.add(new Movie("Force Awakens", 8.3, 2015)); list.add(new Movie("Star Wars", 8.7, 1977)); list.add(new Movie("Empire Strikes Back", 8.8, 1980)); list.add(new Movie("Return of the Jedi", 8.4, 1983)); // Sort by rating : (1) Create an object of ratingCompare //                  (2) Call Collections.sort //                  (3) Print Sorted list System.out.println("Sorted by rating"); RatingCompare ratingCompare = new RatingCompare(); Collections.sort(list, ratingCompare); for (Movie movie: list) System.out.println(movie.getRating() + " " + movie.getName() + " " + movie.getYear()); System.out.println("\nSorted by name"); NameCompare nameCompare = new NameCompare(); Collections.sort(list, nameCompare); for (Movie movie: list) System.out.println(movie.getName() + " " + movie.getRating() + " " + movie.getYear()); } 
}//也可以直接在构造list时传入Comparator的实现类
//不过这样就直接定好了构造的list的排序方式
//在构造PriorityQueue、TreeSet、TreeMap等实现类时，大多用这样写
ArrayList<Movie> list = new ArrayList<Movie>(RatingCompare); 

所以，如果某个类需要多种排序策略，则可用Comparator实现。