Defining Comparisons

A list of strings can be sorted using the sort() method, but a list of Student objects cannot.  However, the programmer can include the __eq__ and __lt__ methods in the Studentclass to solve this problem.  Each method uses the students’ names as the comparable attributes.

Note that the concepts of less than and greater than are more restrictive than equality.  That is, the types of objects being compared are now assumed to be the same, so any errors will be raised at run time.

Example:

class Student:

    NUM_GRADES = 5

    def __init__(self, name):
        self.name = name
        self.grades = []
        for i in range(Student.NUM_GRADES):
            self.grades.append(0)

    def __eq__(self, other):
        if self is other:
            return True
        elif type(self) != type(other):
            return False
        else:
            return self.name == other.name

    def __lt__(self, other):
        return self.name < other.name

Usage:

s1 = Student(‘Mary’)
s2 = Student(‘Bill’)
print(s1 < s2)            # displays False
print(s1 > s2)            # displays True

public interface Comparable<E>{

    public int compareTo(E element);
}

This interface is generic; the element type E is filled in by the implementing class (usually, the name of that class itself).  The method compares the relevant attributes of the receiver object and the parameter object.  compareToreturns 0 if the two objects are equal (using the equals method), an integer less than 0 if the receiver is less than the parameter, and an integer greater than 0 otherwise.  The following example uses the students’ names as the comparable attributes:

Example:

public class Student implements Comparable<Student>{

    private String name;

    public Student(String name){
        this.name = name;
    }

    public int compareTo(Student other){
        return this.name.compareTo(other.name);
    }
}

Usage:

Student s1 = new Student(“Mary”);
Student s2 = new Student(“Bill”);
System.out.println(s1.comapreTo(s2) < 0);   // displays false
System.out.println(s1.compareTo(s2) > 0);   // displays true