A comparator interface is used to order the objects of user-defined classes. A comparator object is capable of comparing two objects of the same class. Following function compare obj1 with obj2.
Syntax:
public int compare(Object obj1, Object obj2):
Suppose we have an Array/ArrayList of our own class type, containing fields like roll no, name, address, DOB, etc, and we need to sort the array based on Roll no or name?
Method 1: One obvious approach is to write our own sort() function using one of the standard algorithms. This solution requires rewriting the whole sorting code for different criteria like Roll No. and Name.
Method 2: Using comparator interface- Comparator interface is used to order the objects of a user-defined class. This interface is present in java.util package and contains 2 methods compare(Object obj1, Object obj2) and equals(Object element). Using a comparator, we can sort the elements based on data members. For instance, it may be on roll no, name, age, or anything else.
Method of Collections class for sorting List elements is used to sort the elements of List by the given comparator.
public void sort(List list, ComparatorClass c)
To sort a given List, ComparatorClass must implement a Comparator interface.
How do the sort() method of Collections class work?
Internally the Sort method does call Compare method of the classes it is sorting. To compare two elements, it asks “Which is greater?” Compare method returns -1, 0, or 1 to say if it is less than, equal, or greater to the other. It uses this result to then determine if they should be swapped for their sort.
Example
Java
// Java Program to Demonstrate Working of// Comparator Interface// Importing required classesimport java.io.*;import java.lang.*;import java.util.*;// Class 1// A class to represent a Studentclass Student { // Attributes of a student int rollno; String name, address; // Constructor public Student(int rollno, String name, String address) { // This keyword refers to current instance itself this.rollno = rollno; this.name = name; this.address = address; } // Method of Student class // To print student details in main() public String toString() { // Returning attributes of Student return this.rollno + " " + this.name + " " + this.address; }}// Class 2// Helper class implementing Comparator interfaceclass Sortbyroll implements Comparator<Student> { // Method // Sorting in ascending order of roll number public int compare(Student a, Student b) { return a.rollno - b.rollno; }}// Class 3// Helper class implementing Comparator interfaceclass Sortbyname implements Comparator<Student> { // Method // Sorting in ascending order of name public int compare(Student a, Student b) { return a.name.compareTo(b.name); }}// Class 4// Main classclass GFG { // Main driver method public static void main(String[] args) { // Creating an empty ArrayList of Student type ArrayList<Student> ar = new ArrayList<Student>(); // Adding entries in above List // using add() method ar.add(new Student(111, "Mayank", "london")); ar.add(new Student(131, "Anshul", "nyc")); ar.add(new Student(121, "Solanki", "jaipur")); ar.add(new Student(101, "Aggarwal", "Hongkong")); // Display message on console for better readability System.out.println("Unsorted"); // Iterating over entries to print them for (int i = 0; i < ar.size(); i++) System.out.println(ar.get(i)); // Sorting student entries by roll number Collections.sort(ar, new Sortbyroll()); // Display message on console for better readability System.out.println("\nSorted by rollno"); // Again iterating over entries to print them for (int i = 0; i < ar.size(); i++) System.out.println(ar.get(i)); // Sorting student entries by name Collections.sort(ar, new Sortbyname()); // Display message on console for better readability System.out.println("\nSorted by name"); // // Again iterating over entries to print them for (int i = 0; i < ar.size(); i++) System.out.println(ar.get(i)); }} |
Unsorted 111 Mayank london 131 Anshul nyc 121 Solanki jaipur 101 Aggarwal Hongkong Sorted by rollno 101 Aggarwal Hongkong 111 Mayank london 121 Solanki jaipur 131 Anshul nyc Sorted by name 101 Aggarwal Hongkong 131 Anshul nyc 111 Mayank london 121 Solanki jaipur
By changing the return value inside the compare method, you can sort in any order that you wish to, for example: For descending order just change the positions of ‘a’ and ‘b’ in the above compare method.
Sort collection by more than one field
In the previous example, we have discussed how to sort the list of objects on the basis of a single field using the Comparable and Comparator interface But, what if we have a requirement to sort ArrayList objects in accordance with more than one field like firstly, sort according to the student name and secondly, sort according to student age.
Example
Java
// Java Program to Demonstrate Working of// Comparator Interface Via More than One Field// Importing required classesimport java.util.ArrayList;import java.util.Collections;import java.util.Comparator;import java.util.Iterator;import java.util.List;// Class 1// Helper class representing a Studentclass Student { // Attributes of student String Name; int Age; // Parameterized constructor public Student(String Name, Integer Age) { // This keyword refers to current instance itself this.Name = Name; this.Age = Age; } // Getter setter methods 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; } // Method // Overriding toString() method @Override public String toString() { return "Customer{" + "Name=" + Name + ", Age=" + Age + '}'; }}// Class 2// Helper class implementing Comparator interfaceclass CustomerSortingComparator implements Comparator<Student> { // Method 1 // To compare customers @Override public int compare(Student customer1, Student customer2) { // Comparing customers int NameCompare = customer1.getName().compareTo( customer2.getName()); int AgeCompare = customer1.getAge().compareTo( customer2.getAge()); // 2nd level comparison return (NameCompare == 0) ? AgeCompare : NameCompare; }}// Method 2// Main driver methodclass GFG { public static void main(String[] args) { // Create an empty ArrayList // to store Student List<Student> al = new ArrayList<>(); // Create customer objects // using constructor initialization Student obj1 = new Student("Ajay", 27); Student obj2 = new Student("Sneha", 23); Student obj3 = new Student("Simran", 37); Student obj4 = new Student("Ajay", 22); Student obj5 = new Student("Ajay", 29); Student obj6 = new Student("Sneha", 22); // Adding customer objects to ArrayList // using add() method al.add(obj1); al.add(obj2); al.add(obj3); al.add(obj4); al.add(obj5); al.add(obj6); // Iterating using Iterator // before Sorting ArrayList Iterator<Student> custIterator = al.iterator(); // Display message System.out.println("Before Sorting:\n"); // Holds true till there is single element // remaining in List while (custIterator.hasNext()) { // Iterating using next() method System.out.println(custIterator.next()); } // Sorting using sort method of Collections class Collections.sort(al, new CustomerSortingComparator()); // Display message only System.out.println("\n\nAfter Sorting:\n"); // Iterating using enhanced for-loop // after Sorting ArrayList for (Student customer : al) { System.out.println(customer); } }} |
Before Sorting:
Customer{Name=Ajay, Age=27}
Customer{Name=Sneha, Age=23}
Customer{Name=Simran, Age=37}
Customer{Name=Ajay, Age=22}
Customer{Name=Ajay, Age=29}
Customer{Name=Sneha, Age=22}
After Sorting:
Customer{Name=Ajay, Age=22}
Customer{Name=Ajay, Age=27}
Customer{Name=Ajay, Age=29}
Customer{Name=Simran, Age=37}
Customer{Name=Sneha, Age=22}
Customer{Name=Sneha, Age=23}
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