非抽象类不能重写Comparable中的抽象方法compareTo吗？

Question

I have a class Vertex<T> which implements IVertex<T> which implements Comparable . Whenever, I compile my code, I get the error:

Vertex is not abstract and does not override abstract method compareTo(IVertex) in Comparable

The issue with this is that, I CANNOT change any of the code within the interface IVertex as that is what my teacher has instructed. How can I solve this issue? I have included my code below:

Vertex:

 package student_solution;


import graph_entities.*;

import java.util.*;

public class Vertex<T> implements IVertex<T>{

  // Add an edge to this vertex.

  public void addEdge(IEdge<T> edge){

  } 

  // We get all the edges emanating from this vertex:  

    public Collection< IEdge<T> > getSuccessors(){

    }

    // See class Label for an an explanation:

    public Label<T> getLabel(){

    }

    public void setLabel(Label<T> label){

    }

  }

IVertex:

package graph_entities;

import java.util.Collection;

public interface IVertex<T> extends Comparable<IVertex<T>>
{

  // Add an edge to this vertex.

  public void addEdge(IEdge<T> edge);

  // We get all the edges emanating from this vertex:  

public Collection< IEdge<T> > getSuccessors();

  // See class Label for an an explanation:

public Label<T> getLabel();

public void setLabel(Label<T> label);

}

Thank you in advance!

Answer 1

As the error suggests, your class implements an interface which extends Comparable . Now, in order to make your class concrete, you have to override all the methods of interfaces your class is implementing.

So, in your case, all you need to do is to override compareTo method in Vertex class , eg:

@Override
public int compareTo(IVertex<T> o) {
    // implementation
    return 0;
}

Here 's Oracle's documentation on interfaces and inheritance.