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javahashmapcomparablecompareto

When to include what?

I created a class Person (as the book says) to hold the name and last name of a person entered from the keyboard and then there is another class PhoneNumber which encapsulates the country code, area code and the number of a person as a String.
Person is intended to be used as the key in a Hashmap.
Class BookEntry encapsulates both Person and PhoneNumber. A lot of BookEntry objects make up a HashMap that represents a phonebook.

Person implements Comparable<Person> so it contains CompareTo(Person) method. Later the book adds equals(Object anotherPerson)method.
My question is, isn't the CompareTo method enough for comparing two keys? or is it that the internal mechanics of the HashMap<> requires me to include equals() method to compare two keys?
compareTo() 

public int compareTo(Person person) {
    int result = lastName.compareTo(person.lastName);
    return result==0? firstName.compareTo(person.firstName):result;
}

equals() 

public boolean equals(Object anotherPerson){
    return compareTo((Person)person)==0;
}
Solution

  • compareTo() method is used in sorting,

    This method's implementation will determine who is greater(lesser, same) between two person, also at what degree

    while equals() & hashcode() will be used in Hash based data structure (HashMap) in your case

    user-defined class as a key of HashMap

    yes you need to implement hashcode() and equals() properly

    Also See