Question

I am writing a program that stores an ArrayList of Person objects (input is from a text file).

This is the code for the Person class, which I will create Person objects from:

import java.io.Serializable;

public class Person implements Comparable<Person>, Serializable
{
private String firstName;
private String lastName;
private int age;

public Person(String firstName, String lastName, int age)
{
    this.firstName = firstName;
    this.lastName = lastName.toUpperCase();
    this.age = age;
}

public int getAge() 
{
    return age;
}

public String getName()
{
    return firstName;
}

/**
 * @return a String of the details of a person in the format:
 *      Name: <firstName> <lastName> Age: <age>
 */
public String toString()
{       
    return

            "Name: " + firstName + "" + lastName + "\t\t" + "Age: " + age;
}

/**
 * Compare the age of the current instance of Person to another age of the specified Person
 * @return negative number this < p
 * @return 0 if this == p
 * @return positive number if this > p
 */
public int compareTo(Person p) {
          return ((Integer)this.getAge()).compareTo(p.getAge());
}

And I created a Comparable interface:

public interface Comparable<T> {
public int compareTo(T o);
}

And here is the code for the class called Collection which will create an ArrayList to store Person objects, I ommitted parts of the code that were not relevant as it is long:

import java.io.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.StringTokenizer;
import java.util.Iterator;

public class Collection
{
private ArrayList<Person> people;

public Collection()
{
    people = new ArrayList<Person>();
}   

public void readFromFile(String filename)
{
    // code that will get input to assign values to fields to a Person

    Person newPerson = new Person(firstNameToken, lastNameToken, ageToken);
}

/**
 * Prints the details of each person held in the people ArrayList
 */
public void printDetails()
{       
    Iterator<Person> it = people.iterator();

        while(it.hasNext())
        {
        Person p = it.next();
        System.out.println(p.toString());
        }
}

public static void main(String [] args) throws FileNotFoundException
{
    Collection c = new Collection(); 

    // check
    //for(Person person : c.people) 
    //{
    //  System.out.println(person);
    //}

    Collections.sort(c.people);
}
}

However I get this error, the sort does not work:

Exception in thread “main” java.lang.Error: Unresolved compilation problem: Bound mismatch: The generic method sort(List) of type Collections is not applicable for the arguments (ArrayList). The inferred type Person is not a valid substitute for the bounded parameter >

Does anyone know why? I am looking furiously around on google for solutions, I can’t see what I am missing. I’ve implemented comparable..

Answer 1

And I created a Comparable interface: public interface Comparable { public int compareTo(T o); }

You are not supposed to create your own interface. Use java.lang.Comparable<T>, the one that the Collections.sort() method expects your object to implement