Comparator Interface in Java

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.

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 criterion like Roll No. and Name.

Method 2:

Using comparator interface- Comparator interface is used to order the objects of 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 comparator, we can sort the elements based on data members. For instance it may be on rollno, 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.

// To sort a given list. ComparatorClass must implement 
// Comparator interface.
public void sort(List list, ComparatorClass c)

How does Collections.Sort() 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 its sort.

class Sortbyroll implements Comparator<Student> 
{ 
    // Used for sorting in ascending order of 
    // roll number 
    public int compare(Student a, Student b) 
    { 
        return a.rollno - b.rollno; 
    } 
} 

class Sortbyname implements Comparator<Student> 
{ 
    // Used for sorting in ascending order of 
    // roll name 
    public int compare(Student a, Student b) 
    { 
        return a.name.compareTo(b.name); 
    } 
} 

Collections.sort(ar, new Sortbyroll()); 
Collections.sort(ar, new Sortbyname()); 

Sort collection by more than one field:

    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; 
            } 
        } 
    }