To sort the objects of user-defined classes, a comparator interface is utilized. A comparator object can compare two items of the same type.
This interface may be found in java.util package and includes two methods:
- compare(Object obj1,Object obj2)
- equals (Object element).
It supports different sorting sequences, which means you can sort the items based on any data member, such as rollno, name, age, or anything else.
Syntax:
class MyComparator implements Comparator<Class-name>{}
public int compare(Object obj1, Object obj2)
The following are the rules for utilizing the Comparator interface:
- You must implement the Comparator interface if you want to sort the elements of a collection.
- If you do not mention the type of the object in your Comparator interface, it will presume that you are sorting objects of type Object. As a result, while overriding the compare() method, you must specify the parameter type as Object only.
- If you want to order the user-defined type elements, you must provide the user-defined type generically while implementing the Comparator interface.
- If you do not specify the user-defined type when implementing the interface, it will default to Object and you will be unable to compare the user-defined type components in the collection.
Example:
Student class:
class 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;
}
public String toString()
{
// Returning attributes of Student
return this.rollno + " " + this.name + " "
+ this.address;
}
}
Sortbyroll Class:
Class to sort the entries according to the specified roll numbers.
class Sortbyroll implements Comparator<Student> {
// Method
// Sorting in ascending order of roll number
public int compare(Student a, Student b)
{
return a.rollno - b.rollno;
}
}
Main class:
class Coderz {
// 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, "Rabecca", "London"));
ar.add(new Student(102, "Tom", "Seoul"));
ar.add(new Student(123, "Tiffany", "Jaipur"));
ar.add(new Student(99, "John", "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));
}
}
Output:
Unsorted
111 Rabecca London
102 Tom Seoul
123 Tiffany Jaipur
99 John Hongkong
Sorted by rollno
99 John Hongkong
102 Tom Seoul
111 Rabecca London
123 Tiffany Jaipur
Note: also read about the Comparable Interface in Java
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Staying up to the mark is what defines me. Hi all! I’m Rabecca Fatima a keen learner, great enthusiast, ready to take new challenges as stepping stones towards flying colors.
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