Comparator interface is used to order the objects of user-defined classes. A comparator object is capable of comparing two objects of two different classes. Following comparator will sort by ID
Example
package collections;
import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
class Student {
int ID;
String name;
char grade;
public Student(int iD, String name, char grade) {
super();
ID = iD;
this.name = name;
this.grade = grade;
}
@Override
public String toString() {
return "Student [ID=" + ID + ", name=" + name + ", grade=" + grade + "]";
}
}
class IdComparator implements Comparator<Student> {
@Override
public int compare(Student o1, Student o2) {
int result = o1.ID - o2.ID; // Sorting by ID
return result;
}
}
public class StudentSort {
public static void main(String[] args) {
List<Student> stu = new ArrayList<>();
stu.add(new Student(10, "General Motors", 'A'));
stu.add(new Student(100, "Ferrari", 'B'));
stu.add(new Student(5, "Mustang", 'C'));
stu.add(new Student(5, "Ford", 'F'));
System.out.println("Before");
printCollections(stu);
Collections.sort(stu, new IdComparator());
System.out.println("After");
printCollections(stu);
}
private static void printCollections(List<Student> stu) {
for (Student s : stu)
System.out.println(s);
}
}
Output
Before Student [ID=10, name=General Motors, grade=A] Student [ID=100, name=Ferrari, grade=B] Student [ID=5, name=Mustang, grade=C] Student [ID=5, name=Ford, grade=F] After Student [ID=5, name=Mustang, grade=C] Student [ID=5, name=Ford, grade=F] Student [ID=10, name=General Motors, grade=A] Student [ID=100, name=Ferrari, grade=B]
You can define multiple comparators to sort object for different parameters
Example
package collections;
import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
class Student {
int ID;
String name;
char grade;
public Student(int iD, String name, char grade) {
super();
ID = iD;
this.name = name;
this.grade = grade;
}
@Override
public String toString() {
return "Student [ID=" + ID + ", name=" + name + ", grade=" + grade + "]";
}
}
class IdComparator implements Comparator<Student> {
@Override
public int compare(Student o1, Student o2) {
int result = o1.ID - o2.ID; // Sorting by ID
return result;
}
}
class NameComparator implements Comparator<Student> {
@Override
public int compare(Student o1, Student o2) {
int result = o1.name.compareTo(o2.name); // Sorting by name
return result;
}
}
public class StudentSort {
public static void main(String[] args) {
List<Student> stu = new ArrayList<>();
stu.add(new Student(10, "General Motors", 'A'));
stu.add(new Student(100, "Ferrari", 'B'));
stu.add(new Student(5, "Mustang", 'C'));
stu.add(new Student(5, "Ford", 'F'));
System.out.println("Original List");
printCollections(stu);
Collections.sort(stu, new IdComparator()); // comparing by ID
System.out.println("After Comparing by ID");
printCollections(stu);
Collections.sort(stu, new NameComparator()); // comparing by Name
System.out.println("After Comparing by Name");
printCollections(stu);
}
private static void printCollections(List<Student> stu) {
for (Student s : stu)
System.out.println(s);
}
}
Output
Original List Student [ID=10, name=General Motors, grade=A] Student [ID=100, name=Ferrari, grade=B] Student [ID=5, name=Mustang, grade=C] Student [ID=5, name=Ford, grade=F] After Comparing by ID Student [ID=5, name=Mustang, grade=C] Student [ID=5, name=Ford, grade=F] Student [ID=10, name=General Motors, grade=A] Student [ID=100, name=Ferrari, grade=B] After Comparing by Name Student [ID=100, name=Ferrari, grade=B] Student [ID=5, name=Ford, grade=F] Student [ID=10, name=General Motors, grade=A] Student [ID=5, name=Mustang, grade=C]
