Comparator Methods and Examples

 In Java, Comparator interface is used to compare objects in order to sort them. Prior to Java 8, Comparator interface supported only two methods - comparator() and equals. However, in Java 8, many new key methods were introduced to the Comparator interface to make sorting easier.

Some of the key methods introduced in Comparator interface are:

• int compare(t1, t2): It compares two objects and returns an integer that indicates their relative order.
• Comparator comparing(KeyExtractor): It accepts a function that extracts a Comparable sort key from a type T, and returns a Comparator that compares by that sort key.
• Comparator thenComparator(KeyExtractor): It returns a lexicographic-order comparator with another comparator.
• Comparator naturalOrder(): It returns a comparator that compares Comparable objects in a natural order.
• Comparator reversed(): It returns a comparator that imposes the reverse ordering of this comparator.
• Comparator reverseOrder(): It returns a comparator that imposes the reverse of the natural ordering.
• Comparator nullFirst(Comparator): It returns a null-friendly comparator that considers null to be less than non-null.
• Comparator nullLast(Comparator): It returns a null-friendly comparator that considers null to be greater than non-null.

Here are some examples that demonstrate how these methods work: Example 1: Sorting an array with nulls first and in natural order.

javascript
String[] arr = {"gfg", null, "ide", null};
Arrays.sort(arr, Comparator.nullsFirst(Comparator.naturalOrder()));
System.out.println(Arrays.toString(arr)); 
Output: [null, null, gfg, ide]

Example 2: Sorting an array in reverse order.

vbnet
String[] arr = {"gfg", "course", "ide"};
Arrays.sort(arr, Comparator.reverseOrder());
System.out.println(Arrays.toString(arr));
Output: [ide, gfg, course]

Example 3: Sorting an array first by name and then by roll number.

csharp
class Student {
    String name;
    int rollNo;
    Student(String n, int r) {
        name = n;
        rollNo = r;
    }
    String getName() {
        return name;
    }
    int getRoll() {
        return rollNo;
    }
    public String toString() {
        return "(" + name + ", " + rollNo + ")";
    }
}
Student[] arr = {new Student("abc", 120), new Student("xyz", 110), new Student("abc", 101)};
Arrays.sort(arr, Comparator.comparing(Student::getName).thenComparing(Student::getRoll));
System.out.println(Arrays.toString(arr));
Output: [(abc, 101), (abc, 120), (xyz, 110)]