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Thread: How do I sort a TreeSet<Person> by Person's age?

  1. #1
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    Default How do I sort a TreeSet<Person> by Person's age?

     
    public class Person implements Comparable<Person>
    {
        // the age of the person
        private int age;
        //the name of the person
        private String name;
        //the Integer object to wrap the age value;
        private Integer ageWrap;
     
       //Constructor and accessors methods omitted. 
                             .
                             .
                             .
     /**
         * Compare the ages of the persons
         */
        public int compareTo(Person person)
        {
            return getAgeWrap().compareTo(person.getAgeWrap());
        }
    }
     
    import java.util.*;
    public class PersonTreeSet
    {
        // instance variables 
        private TreeSet<Person> setOfPersons;
     
        private String[] persons = {"Vasilis", "Jenny", "Varvara", "Thanasis", "Dimitris", "Dionisia", "Kostas",
                "Eugenia", "Hara", "Antonia"};
        private int maxAge;
     
          //Constructor and main method omitted.
                             .
                             .
                             .
     /**
         * Create people and add them in the TreeSet
         */
        public void createPersons()
        {
            Random rand = new Random();
            for(int index = 0; index < persons.length; index++) {
                Person person = new Person(rand.nextInt(maxAge), persons[index]);
                setOfPersons.add(person);
            }
            System.out.println(setOfPersons.size());
        }
     
        /**
         * Print all the elements of the treeSet
         */
        public void printPersons()
        {
            for(Person person : setOfPersons) {
                System.out.println(person.getName() + " is " + person.getAge() + " years old");
            }
        }
       /**
         * sort persons by age.
         */
        public void sortByAge()
        {
            ArrayList<Person> sortPersons = new ArrayList<Person>();
            sortPersons.addAll(setOfPersons);
            Collections.sort(sortPersons);
            for(Person person : sortPersons) {
                System.out.println(person.getName() + " is " + person.getAge() + " years old");
            }
        }
    }

    The collection library has a class named TreeSet, which is an example of
    a sorted set. Elements in this set are kept in order. Carefully read the description of this class,
    and then write a class Person that can be inserted into a TreeSet, which will then sort the
    Person objects by age.
    This is the exercise I am trying to solve. And this is as far as I have gotten to.
    Is it possible to sort my setOfPersons TreeSet directly? I tried to use
    Collections.sort(setOfPersons)
    method but it wont compile, and I realized that it is not applicable to TreeSet. So I made the
    sortByAge()
    method to do it indirectly...
    I am puzzled though because in the exercise it states
    write a class Person that can be inserted into a TreeSet, which will then sort the Person objects by age.
    meaning that the TreeSet will sort the Person Objects and not my class..
    Any suggestions?


  2. #2
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    Default Re: How do I sort a TreeSet<Person> by Person's age?

    A TreeSet is always sorted. It will sort itself.
    Please read the API documentation before using a class, it clearly states that:
    All elements inserted into the [Tree]set must implement the Comparable interface. Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the set. If the user attempts to add an element to the set that violates this constraint (for example, the user attempts to add a string element to a set whose elements are integers), the add call will throw a ClassCastException.
    What you need to do is implement the Comparable interface in your Person class.
    The Comparable interface defines a method that will be used for comparison. In this method you can use your age attribute.
    For example:
    public int compare(Person a, Person b) {
            return Integer.compare(a.age, b.age);
    }

  3. #3
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    Default Re: How do I sort a TreeSet<Person> by Person's age?

    I am confused now...I do implement the comparable interface dont I? I implement the compareTo method in Person Class ... The API documentation for the Comparable interface mentions only compareTo(T o) method and this is the one I implemented. So if I got it right I will replace that with the one you suggest and the Persons objects will be sorted by age automatically?

    --- Update ---

    I am SO STUPID.... I didnt notice that I have already solve the exercise.... Just Bu providint the implementation of the compareTo method the the elements of the TreeSet are already sorted by age... Cornix the suggestion you provided me doesnt implement the compareTo method and it doesnt compile since Implement the Comparable<Person> interface in my class signature... Thanks for your time anyway

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