public class Employee {
String name;
int age;
double salary;
long mobile;
// constructors, getters & setters
}
employees = new Employee[] { ... };
[Employee(name=John, age=25, salary=3000.0, mobile=9922001),
Employee(name=Ace, age=22, salary=2000.0, mobile=5924001),
Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)]
두 매개변수 객체의 value가 아닌 key
로 비교하여 정렬하는 로직이기 때문에
keyExtractor
: 정렬 key를 뽑아내는 함수적 인터페이스keyComparator
: 정렬 key를 비교하는 함수적 인터페이스@Test
public void whenComparing_thenSortedByName() {
Comparator<Employee> employeeNameComparator
= Comparator.comparing(Employee::getName);
Arrays.sort(employees, employeeNameComparator);
assertTrue(Arrays.equals(employees, sortedEmployeesByName));
}
keyExtractor
를 Employee::getName
으로 받아 직원의 이름 순으로 정렬되었다.
[Employee(name=Ace, age=22, salary=2000.0, mobile=5924001),
Employee(name=John, age=25, salary=3000.0, mobile=9922001),
Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)]
@Test
public void whenComparingWithComparator_thenSortedByNameDesc() {
Comparator<Employee> employeeNameComparator
= Comparator.comparing(
Employee::getName, (s1, s2) -> {
return s2.compareTo(s1);
});
Arrays.sort(employees, employeeNameComparator);
assertTrue(Arrays.equals(employees, sortedEmployeesByNameDesc));
}
keyExtractor
로는 Employee::getName
을 받고, keyComparator
로는 (s1, s2) -> {return s2.compareTo(s1);}
람다식을 받아 직원의 이름 기준 내림차순 정렬되었다.
[Employee(name=Keith, age=35, salary=4000.0, mobile=3924401),
Employee(name=John, age=25, salary=3000.0, mobile=9922001),
Employee(name=Ace, age=22, salary=2000.0, mobile=5924001)]
원본 정렬을 기준으로 반대로 정렬
@Test
public void whenReversed_thenSortedByNameDesc() {
Comparator<Employee> employeeNameComparator
= Comparator.comparing(Employee::getName);
Comparator<Employee> employeeNameComparatorReversed
= employeeNameComparator.reversed();
Arrays.sort(employees, employeeNameComparatorReversed);
assertTrue(Arrays.equals(employees, sortedEmployeesByNameDesc));
}
이름 순으로 정렬한 뒤 내림차순으로 정렬되도록 역순 정렬
[Employee(name=Keith, age=35, salary=4000.0, mobile=3924401),
Employee(name=John, age=25, salary=3000.0, mobile=9922001),
Employee(name=Ace, age=22, salary=2000.0, mobile=5924001)]
Employee 배열에 임의로 null 데이터 2개를 중간에 삽입
@Test
public void whenNullsFirst_thenSortedByNameWithNullsFirst() {
Comparator<Employee> employeeNameComparator
= Comparator.comparing(Employee::getName);
Comparator<Employee> employeeNameComparator_nullFirst
= Comparator.nullsFirst(employeeNameComparator);
Arrays.sort(employeesArrayWithNulls,
employeeNameComparator_nullFirst);
assertTrue(Arrays.equals(
employeesArrayWithNulls,
sortedEmployeesArray_WithNullsFirst));
}
[null,
null,
Employee(name=Ace, age=22, salary=2000.0, mobile=5924001),
Employee(name=John, age=25, salary=3000.0, mobile=9922001),
Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)]
null 데이터 2개가 우선적으로 정렬
@Test
public void whenNullsLast_thenSortedByNameWithNullsLast() {
Comparator<Employee> employeeNameComparator
= Comparator.comparing(Employee::getName);
Comparator<Employee> employeeNameComparator_nullLast
= Comparator.nullsLast(employeeNameComparator);
Arrays.sort(employeesArrayWithNulls, employeeNameComparator_nullLast);
assertTrue(Arrays.equals(
employeesArrayWithNulls, sortedEmployeesArray_WithNullsLast));
}
[Employee(name=Ace, age=22, salary=2000.0, mobile=5924001),
Employee(name=John, age=25, salary=3000.0, mobile=9922001),
Employee(name=Keith, age=35, salary=4000.0, mobile=3924401),
null,
null]
null 데이터 2개가 후순위로 정렬
Reference (1) - Comparator.comparing (Tistory)
Reference (2) - Comparator.comparing (Baeldung)
References (3) - Comparable vs Comparator (Tistory)