Unable to override compare() method of Comparator

Question

I’m not sure why I get this below error though I already implemented the method.

“The method compare(Student, Student) of type NameComparator must override or implement a supertype method” in NameComparator.java while implementing the compare method

Student.java

public class Student {
    private String name;
    private int age;
    private String lesson;
    private int grade;

    public Student() {
    }

    public Student(String name, int age, String lesson, int grade) {
        super();
        this.name = name;
        this.age = age;
        this.lesson = lesson;
        this.grade = grade;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getLesson() {
        return lesson;
    }

    public void setLesson(String lesson) {
        this.lesson = lesson;
    }

    public int getGrade() {
        return grade;
    }

    public void setGrade(int grade) {
        this.grade = grade;
    }

    @Override
    public String toString() {
        return "[name=" + this.name + ", age=" + this.age + ", lesson="
                + this.lesson + ", grade=" + this.grade + "]";
    }

}

NameComparator.java

import java.util.Comparator;

@SuppressWarnings("rawtypes")
public class NameComparator implements Comparator {

// I’m getting this error for below method "The method compare(Student, Student) of type NameComparator must override or implement a super type method"

    @Override
    public int compare(Student s1, Student s2) {
        String name1 = o1.getName();
        String name2 = o2.getName();

        // ascending order (descending order would be: name2.compareTo(name1))
        return name1.compareTo(name2);
    }

}

Answer 1

Change

public class NameComparator implements Comparator {

to

public class NameComparator implements Comparator<Student> {

When you implement the raw Comparator interface (which is not advised), your compare method expects Object arguments.

Answer 2

The Comparator<T> interface is a generic interface. As is, it took the default implementation of Comparator<Object>, which would mean that it is expecting public int compare(Object s1, Object s2) { to be implemented.

To fix this, simply replace Comparator with Comparator<Student> in you class decleration.

Answer 3

The method declaration must looks like:

@Override
public int compare(Object s1, Object s2) {

Because you ot add the generic type at the implements clause.

Change the class declaration to:

public class NameComparator implements Comparator<Student> {

And the error will gone.