Sorting a collection of objects [duplicate]

Question

If I have a simple list of Strings:

List<String> stringList = new ArrayList<String>(); 

I can sort it with:

Collections.sort(stringList); 

But suppose I have a Person class:

public class Person {    private String name;    private Integer age;    private String country; } 

And a list of it:

List<Person> personList = new ArrayList<Person>(); 

And I want to sort it sometimes by name, sometimes by age, sometimes by country.

What is the easiest way to accomplish that?

I know that I can implement the Comparable interface, but that seems to limit me to sort it by one specific property.

Answer 1

Collections.sort can be called with a custom comparator. And that comparator can be implemented to allow sorting in different sort orders. Here's an example (for your Person model - with age as an Integer):

public class FlexiblePersonComparator implements Comparator<Person> {   public enum Order {Name, Age, Country}    private Order sortingBy = Name;    @Override   public int compare(Person person1, Person person2) {     switch(sortingBy) {       case Name: return person1.name.compareTo(person2.name);       case Age: return person1.age.compareTo(person2.age);       case Country: return person1.country.compareTo(person2.country);     }     throw new RuntimeException("Practically unreachable code, can't be thrown");   }    public void setSortingBy(Order sortBy) {     this.sortingBy = sortingBy;   } } 

And you use it like that (assuming persons is a field):

public void sortPersonsBy(FlexiblePersonComparator.Order sortingBy) {   List<Person> persons = this.persons;  // useless line, just for clarification   FlexiblePersonComparator comparator = new FlexiblePersonComparator();   comparator.setSortingBy(sortingBy);   Collections.sort(persons, comparator); // now we have a sorted list }