Object comparison using equality & comparable interface

Question

Could anyone please explain me whether there's any difference in the codes below when comparing objects and more specifically their values.

Code 1

x.equals(y)

Code 2

x.compareTo(y) == 0

Are the codes above interchangeable. What's the difference if any?

`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."` — nachokk, Jan 17 '14 at 14:25
_"The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C. Note that null is not an instance of any class, and **e.compareTo(null)** should throw a **NullPointerException** even though **e.equals(null)** returns **false**."_ — Alexis C., Jan 17 '14 at 14:25

nachokk · Answer 1 · 2014-01-17T14:51:41.163

From Comparable

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."

As @ZouZou mention

The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C. Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

That means they are not interchangable.

An example where this happen in java api is in BigDecimal

import java.math.BigDecimal;

public class Test{
 public static void main(String args[])  {

       BigDecimal big = BigDecimal.ZERO;
       BigDecimal zero  = new BigDecimal("0.00");

       System.out.println("Compare "+ (big.compareTo(zero) == 0) ); //prints true
       System.out.println("Equals "+big.equals(zero)); // prints false      
 } 
}

One notable example of a class where `equals()` is inconsistent with `compareTo()` is [`BigDecimal`](http://docs.oracle.com/javase/7/docs/api/java/math/BigDecimal.html) — axtavt, Jan 17 '14 at 14:46
And a reminder that if you are overriding equals(Object) make sure you also override hashCode() to be consistent with equals. ["If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result."](http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html#hashCode()) — Victor, Jan 17 '14 at 15:29

score 1 · Answer 2 · edited May 23 '17 at 10:31

1

compareTo doesn't always make sense, sometimes the objects cannot be logically ordered.
Also note that equals always exists, since it's defined in Object class.

It's a good practice to always haven equals method when you have compareTo, and they should be consistent.

Also you might want to have a look at this question.

edited May 23 '17 at 10:31

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answered Jan 17 '14 at 14:30

Maroun

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score 0 · Answer 3 · edited May 23 '17 at 10:25

0

It is not interchangeable for Strings.

x.equals((String)null)  returns false

Code 2

x.compareTo((String)null) == 0 // throws a NullPointerException

See here:Java Strings: compareTo() vs. equals()

edited May 23 '17 at 10:25

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answered Jan 17 '14 at 15:01

Sitansu

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Object comparison using equality & comparable interface

3 Answers3