Java, how to inherit methods from abstract class

Question

I have an abstract class Person and and interface comparable, which is also used for some other part of the program. Currently I have a method compareTo() in Person. When I try to compile, I get :

The type Student must implement the inherited abstract method 
 Comparable<Person>.compareTo(Person, Person)

What exactly do I have to do? I don't wont to implement this method in any of the subclasses, because I need this method for all of them, Student, Tutor, Professor, etc... Is there a better way of doing this?

Interface:

interface Comparable<Element> {
    public int compareTo(Element nodeA, Element nodeB);
}

Abstract class Person:

abstract class Person implements Comparable<Person> {

    protected String name;

    public Person(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String newName) {
        name = newName;
    }
    public String toString() {
        return name;
    }

    public int compareTo(Person personB) {
        int comp = this.name.compareTo(personB.getName());
        return comp;
    }
}

And class Student

class Student extends Person implements Comparable<Person> {

    private int id;

    public Student(String name, int id) {
        super(name);
        this.id = id;
    }

    public int getID() {
        return id;
    }

    public void setID(int newID) {
        id = newID;
    }

    public String toString() {
        return id + ", " + name;
    }
}

Answer 1

Your Comparable<Element> class declares a method public int compareTo(Element nodeA, Element nodeB); , but in your Person class, you implement public int compareTo(Person personB) , which is not the same method signature.

You need to either implement public int compareTo(Person personA, Person personB) , or alter your Comparable<Element> class's method definition to be public int compareTo(Element other); to override the core Comparable class's compareTo method .

Also, as @murat mentions below in the comment, using the @Override annotation would help you out (assuming you're on Java version 1.5 or higher). If you add @Override to a method that you're not actually overriding from a superclass (such as your two-argument compareTo method), then it will be a compiler error.

Answer 2

Your Comparable interface has a method compareTo(Element nodeA, Element nodeB) . This method is not defined in Student, and it's not defined in Person either. Person has the following method:

public int compareTo(Person personB)

, which doesn't override compareTo(Person nodeA, Person nodeB)

Why are you redefining the standard java.util.Comparable interface?

Answer 3

You should implement the method as it appears in the interface, ie with two arguments

public int compareTo(Person nodeA, Person nodeB)

To avoid such problems in the future use the @Override annotation:

@Override
public int compareTo(Person nodeA, Person nodeB)

This will cause a compilation error if you try to override a method, but make a mistake in its signature.

Also, consider using Java's standard Comparable .

Answer 4

Change your interface from:

interface Comparable<Element> 
{     
   public int compareTo(Element nodeA, Element nodeB); 
}

to:

interface Comparable<Element> 
{     
   public int compareTo(Element nodeA); 
}

And make your Person class be defined as:

abstract class Person implements Comparable<? extends Person> { /* ... */ }

And make your Student (and other Person-subclasses be):

class Student extends Person { /* ... */ }

That is all.

Answer 5

You need to implement

public int compareTo(Person nodeA, Person nodeB)

In your Person class. Currently you only have:

public int compareTo(Person personB)

Answer 6

您需要在Student类中创建compareTo方法。

Answer 7

interface Comparable<Element> {
    public int compareTo(Element nodeA, Element nodeB);
}

It makes sense if it was either:

interface Comparator<Element> { //comparator compares two instances of same type
    public int compare(Element nodeA, Element nodeB);
}

or (has to be something like this for your case):

interface Comparable<Element> { //comparable compares itself with another instance of same type
    public int compareTo(Element that);
}

But for both cases, you should use the standard Java interfaces: Even though you are only using Comparable also see Comparator .