Java Collections

[Dorothy Taylor]

Can anyone help me with the below given problem:


Assume you are writing an application working with Person objects. The application uses Person objects in Collections, placing them into Collection implementations and querying if a Person is in a Collection using the Collection.contains() method. The application also uses Person objects as keys in Maps to associate other objects with Persons and to efficiently look up those objects based on Person. Given these needs, will the following Person implementation work in our application? If not, please fix it so that it will.

public class Person {	
    private String firstName;
    private String lastName;
    private Date dob;	

    /** Construct a Person given the first name, last name, and birth date. */
    public Person(String firstName, String lastName, Date dob) {
        if (firstName == null || lastName == null || dob == null) {
            throw new IllegalArgumentException();
        }
        this.firstName = firstName;
        this.lastName = lastName;
        this.dob = dob;
    }
}

[Akhilesh Trivedi]

Dorothy Taylor wrote: Given these needs, will the following Person implementation work in our application?

Is it working currently?

[John Jai]

Dorothy Taylor wrote:The application also uses Person objects as keys in Maps to associate other objects with Persons and to efficiently look up those objects based on Person.

If you want to use the Person object as keys then override equals() and the hashCode() methods of the Object class.

[John Jai]

Akhilesh Trivedi wrote:

Dorothy Taylor wrote: Given these needs, will the following Person implementation work in our application?

Is it working currently?

Yes Dorothy - first check and let us know if it's working currently...

[Himanshu Mall]

If not, please fix it so that it will.

[Tomasz Sochanski]

It can't work now. Collection.contains(Object o) implementations use o.equals() method to find element in collection. You don't override equals(), so Object.equals() will be used. Two Person objects created with the same constructor arguments will not be equal. You need to override Object.equals() in your class. If you want to use this class as key in map you will also need to override Object.hashCode() to keep equals() and hashCode() contract. Take a look here if you need more information how to implement missing methods: http://bytes.com/topic/java/insights/723476-overriding-equals-hashcode-methods

[Dorothy Taylor]

Thanks, that was a very useful explanation of the concept. I have one more doubt. So if we need to sort these Person objects based on the natural ordering of each person’s complete name, then is it ok to just extend Comparator interface and implement the compare() method?

[Tomasz Sochanski]

Dorothy Taylor wrote:So if we need to sort these Person objects based on the natural ordering of each person’s complete name, then is it ok to just extend Comparator interface and implement the compare() method?

Dorothy,

You cannot extend interface in class, you can implement it. If you tried your idea (which I strongly suggest) then you already noticed it won't work. Not every Map implementation cares about key ordering, you have to choose the one which does (ie. TreeMap).

TreeMap javadoc stands:

The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

So you can either implement Comparable (not Comparator) interface in Person class, or create implementation of Comparator and provide it during initialization of map. You would also want to read about Collator if you have to care about national characters in names, like 'é', 'ą', 'ö' etc.

[Dorothy Taylor]

Thanks.