all about comapring objects

[Arun Prasath]

Can anyone tell me in detail about the following and what is the difference between

equals, == and compareTo methods?

[Stan James]

With objects, the == operator compares the two object references and tells whether they refer to the same object. If you've done C it's a lot like comparing two pointers.

equals() is implemented in code to compare the value or content of two objects. It often compares a combination of fields. For example, a Person class might say

public boolean equals( ... )
{
   return this.firstName.equals(other.firstName) 
       && this.lastName.equals(other.lastName);
}

compareTo() compares the values again but tells whether one object is "greater than" or "less than" the other. That Person class might order by last name:

public int compareTo( ... )
{
   rc = this.lastName.compareTo(other.lastName);
   if(rc == 0)
      rc = this.firstName.compareTo(other.firstName);
   return rc;
}

Both of these methods need a bit more housekeeping. They should check for nulls, only compare to other instances of exactly the same class and so on.

An important rule is to make equals(), compareTo() and hashcode() compatible. If two objects are equal according to one, they should be equal in all three.

Did that help?

[Ernest Friedman-Hill]

Moving to Java in General (Beginner.)

[Campbell Ritchie]

[NITPICK]

equals(), compareTo() and hashcode() compatible. If two objects are equal according to one, they should be equal in all three.

Not quite. You can, at least in theory, have two different objects return identical hash codes.[/NITPICK]

[Adam Nace]

Originally posted by Campbell Ritchie:
[NITPICK] Not quite. You can, at least in theory, have two different objects return identical hash codes.[/NITPICK]

Indeed. Furthermore, the API for compareTo() specifies that compareTo() does not have to return equal simply because equals() does (and vice-versa).

It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is "Note: this class has a natural ordering that is inconsistent with equals."

- Adam
[ July 31, 2006: Message edited by: Adam Nace ]

[Stan James]

Good catch, guys. I once had written if equals() says true the others should agree. That much I think is actually true. Donno why I changed it. And I did say "should" rather than "MUST" on the other.