Do I need a equals and Hashcode method if my class implements comparable in Java?

Question

I found this comment on can StringBuffer objects be keys in TreeSet in Java?

"There are 2 identifying strategies used with Maps in Java (more-or-less).

Hashing: An input "Foo" is converted into a best-as-possible attempt to generate a number that uniquely accesses an index into an array. (Purists, please don't abuse me, I am intentionally simplifying). This index is where your value is stored. There is the likely possibility that "Foo" and "Bar" actually generate the same index value meaning they would both be mapped to the same array position. Obviously this can't work and so that's where the "equals()" method comes in; it is used to disambiguate

Comparison: By using a comparative method you don't need this extra disambiguation step because comparison NEVER produces this collision in the first place. The only key that "Foo" is equal to is "Foo". A really good idea though is if you can is to define "equals()" as compareTo() == 0; for consistency sake. Not a requirement."

my question is as follows: if my class implements comparable, then does it mean I dont have to override equals and hashcode method for using my objects as keys in Hash collections. eg

class Person implements Comparable<Person> {
     int id;
     String name;

     public Person(int id, String name) {
        this.id=id;
        this.name=name;
     }

    public int compareTo(Person other) {
      return this.id-other.id;
    }
}

Now, can I use my Person objects in Hashable collections?

Answer 1

The article you brough is talking on TreeSet. a tree set is a tree with each node has a place defined by it's value in compare to the other values already in the tree.

a hashTable stores key/value pairs in a hash table. When using a Hashtable, you specify an object that is used as a key, and the value that you want linked to that key. The key is then hashed, and the resulting hash code is used as the index at which the value is stored within the table.

the difference between Hashable and TreeSet is that treeset don't need hashCode, it just need to know if you need the take the item left or right in the tree. for that you can use Compare and nothing more.

in hashTable a compare will suffice, because it's build differently, each object get to his cell by hashing it, not by comparing it to the items already in the collection.

so the answer is no, you can' use Person in hashtable just with compareTo. u must override hashCode() and equals() for that

i also suggest you read this article on hashtables