Java Language Comparable and Comparator Natural (comparable) vs explicit (comparator) sorting
Example
There are two Collections.sort() methods:
- One that takes a
List<T>as a parameter whereTmust implement Comparable and override thecompareTo()method that determines sort order. - One that takes a List and a Comparator as the arguments, where the Comparator determines the sort order.
First, here is a Person class that implements Comparable:
public class Person implements Comparable<Person> {
private String name;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public int compareTo(Person o) {
return this.getAge() - o.getAge();
}
@Override
public String toString() {
return this.getAge()+"-"+this.getName();
}
}
Here is how you would use the above class to sort a List in the natural ordering of its elements, defined by the compareTo() method override:
//-- usage
List<Person> pList = new ArrayList<Person>();
Person p = new Person();
p.setName("A");
p.setAge(10);
pList.add(p);
p = new Person();
p.setName("Z");
p.setAge(20);
pList.add(p);
p = new Person();
p.setName("D");
p.setAge(30);
pList.add(p);
//-- natural sorting i.e comes with object implementation, by age
Collections.sort(pList);
System.out.println(pList);
Here is how you would use an anonymous inline Comparator to sort a List that does not implement Comparable, or in this case, to sort a List in an order other than the natural ordering:
//-- explicit sorting, define sort on another property here goes with name
Collections.sort(pList, new Comparator<Person>() {
@Override
public int compare(Person o1, Person o2) {
return o1.getName().compareTo(o2.getName());
}
});
System.out.println(pList);
