Comparator In Java :
- Use: Order the objects of user-defined classes.
- Package: java.util package
- Comparing two objects of two different classes.
- Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort) to allow precise control over the sort order.
Syntax
public int compare(Object obj1, Object obj2):
import java.util.*;
class Student {
int rollno;
String name, address;
// Constructor
public Student(int rollno, String name, String address) {
this.rollno = rollno;
this.name = name;
this.address = address;
}
@Override
public String toString() {
return this.rollno + " " + this.name
+ " " + this.address;
}
}
class Sortbyroll implements Comparator<Student> {
@Override
public int compare(Student a, Student b) {
return a.rollno - b.rollno;
}
}
class Sortbyname implements Comparator<Student> {
@Override
public int compare(Student a, Student b) {
return a.name.compareTo(b.name);
}
}
class Main {
public static void main(String[] args) {
ArrayList<Student> ar = new ArrayList<Student>();
ar.add(new Student(111, "kamlesh", "ahmedabad"));
ar.add(new Student(131, "jay", "surat"));
ar.add(new Student(121, "snehal", "mumbai"));
System.out.println("Unsorted");
for (int i = 0; i < ar.size(); i++) {
System.out.println(ar.get(i));
}
Collections.sort(ar, new Sortbyroll());
System.out.println("\nSorted by rollno");
for (int i = 0; i < ar.size(); i++) {
System.out.println(ar.get(i));
}
Collections.sort(ar, new Sortbyname());
System.out.println("\nSorted by name");
for (int i = 0; i < ar.size(); i++) {
System.out.println(ar.get(i));
}
}
}
output
Unsorted
111 kamlesh ahmedabad
131 jay surat
121 snehal mumbai
Sorted by rollno
111 kamlesh ahmedabad
121 snehal mumbai
131 jay surat
Sorted by name
131 jay surat
111 kamlesh ahmedabad
121 snehal mumbai
Sort collection by more than one field
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Comparator;
class Student {
// instance member variables
String Name;
int Age;
// parameterized constructor
public Student(String Name, Integer Age) {
this.Name = Name;
this.Age = Age;
}
public String getName() {
return Name;
}
public void setName(String Name) {
this.Name = Name;
}
public Integer getAge() {
return Age;
}
public void setAge(Integer Age) {
this.Age = Age;
}
// overriding toString() method
@Override
public String toString() {
return "Customer{" + "Name=" + Name + ", Age=" + Age + '}';
}
static class CustomerSortingComparator implements Comparator<Student> {
@Override
public int compare(Student customer1, Student customer2) {
// for comparison
int NameCompare = customer1.getName().compareTo(customer2.getName());
int AgeCompare = customer1.getAge().compareTo(customer2.getAge());
// 2-level comparison using if-else block
if (NameCompare == 0) {
return ((AgeCompare == 0) ? NameCompare : AgeCompare);
} else {
return NameCompare;
}
}
}
public static void main(String[] args) {
List<Student> al = new ArrayList<>();
Student obj1 = new Student("Kamlesh", 27);
Student obj2 = new Student("Vijay", 23);
Student obj3 = new Student("Samir", 37);
Student obj4 = new Student("Vijay", 22);
Student obj5 = new Student("Jay", 29);
Student obj6 = new Student("Mohit", 22);
// add customer objects to ArrayList
al.add(obj1);
al.add(obj2);
al.add(obj3);
al.add(obj4);
al.add(obj5);
al.add(obj6);
// before Sorting arraylist: iterate using Iterator
Iterator<Student> custIterator = al.iterator();
System.out.println("Before Sorting:\n");
while (custIterator.hasNext()) {
System.out.println(custIterator.next());
}
// sorting using Collections.sort(al, comparator);
Collections.sort(al, new CustomerSortingComparator());
// after Sorting arraylist: iterate using enhanced for-loop
System.out.println("\n\nAfter Sorting:\n");
for (Student customer : al) {
System.out.println(customer);
}
}
}
output
Before Sorting:
Customer{Name=Kamlesh, Age=27}
Customer{Name=Vijay, Age=23}
Customer{Name=Samir, Age=37}
Customer{Name=Vijay, Age=22}
Customer{Name=Jay, Age=29}
Customer{Name=Mohit, Age=22}
After Sorting:
Customer{Name=Jay, Age=29}
Customer{Name=Kamlesh, Age=27}
Customer{Name=Mohit, Age=22}
Customer{Name=Samir, Age=37}
Customer{Name=Vijay, Age=22}
Customer{Name=Vijay, Age=23}
Java 8 Comparator
Comparator<Student> cm1=Comparator.comparing(Student::getName);
Collections.sort(al,cm1);
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
class Student {
int rollno;
String name;
int age;
Student(int rollno, String name, int age) {
this.rollno = rollno;
this.name = name;
this.age = age;
}
public int getRollno() {
return rollno;
}
public void setRollno(int rollno) {
this.rollno = rollno;
}
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;
}
}
public class Main {
public static void main(String args[]) {
ArrayList<Student> al = new ArrayList<Student>();
al.add(new Student(101, "Kamlesh", 23));
al.add(new Student(106, "Hiren", 27));
al.add(new Student(105, "Vijay", 21));
Comparator<Student> cm1 = Comparator.comparing(Student::getName);
//Comparator<Student> cm1 = Comparator.comparing(Student::getName,Comparator.nullsFirst(String::compareTo)); //null first
//Comparator<Student> cm1 = Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)); //null last
Collections.sort(al, cm1);
System.out.println("Sorting by Name");
for (Student st : al) {
System.out.println(st.rollno + " " + st.name + " " + st.age);
}
//Sorting age
Comparator<Student> cm2 = Comparator.comparing(Student::getAge);
Collections.sort(al, cm2);
System.out.println("Sorting by Age");
for (Student st : al) {
System.out.println(st.rollno + " " + st.name + " " + st.age);
}
}
}
output
Sorting by Name
106 Hiren 27
101 Kamlesh 23
105 Vijay 21
Sorting by Age
105 Vijay 21
101 Kamlesh 23
106 Hiren 27
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