A parallel of equality in 2 worlds (Ruby and Java)

Here is a short presentation of equality methods in Ruby and Java:

Ruby Java
Object#equal? ==
Object#eql? Object.equals
Object#== Object.equals
Object#=== N/A

So, in both Ruby and Java we have an instance reference equality or identity (Object#equal?, respectively the == operator).

Another similarity is for objects used as keys in maps (hashes). If you want to customize their behavior, in both Ruby and Java will have to override Object#eql? and Object#hash, respectively Object.equals() and Object.hashCode(). And that would be it with the similarities.

In Ruby world, Object#eql? and Object?== are named equivalence. I haven’t been able to find out the reasons for having both Object#eql? and Object?== and the only example I have found is the following:

2.eql? 2.0 => false
2 == 2.0 => true

Ruby has another equality method Object#=== and this one is used only in case statements.


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5 responses to “A parallel of equality in 2 worlds (Ruby and Java)

  1. mfp

    By the way, ri is pretty informative too:
    ———————————————————— Object#eql?
    obj == other => true or false
    obj.equal?(other) => true or false
    obj.eql?(other) => true or false
    Equality—At the +Object+ level, +==+ returns +true+ only if _obj_
    and _other_ are the same object. Typically, this method is
    overridden in descendent classes to provide class-specific meaning.

    Unlike +==+, the +equal?+ method should never be overridden by
    subclasses: it is used to determine object identity (that is,
    +a.equal?(b)+ iff +a+ is the same object as +b+).

    The +eql?+ method returns +true+ if _obj_ and _anObject_ have the
    same value. Used by +Hash+ to test members for equality. For
    objects of class +Object+, +eql?+ is synonymous with +==+.
    Subclasses normally continue this tradition, but there are
    exceptions. +Numeric+ types, for example, perform type conversion
    across +==+, but not across +eql?+, so:

    1 == 1.0 #=> true
    1.eql? 1.0 #=> false

  2. mfp

    Oops, ignore my comments — you obviously know that.
    I replied before reading your posting in full, my apologies.

  3. the_mindstorm

    No problem! I just got a little worried that maybe my post vanished away completely [blink/].


    .w( the_mindstorm )p.


  4. SatishTalim

    Would you know of any site that compares Java and Ruby – where once can know the differences at a glance?

  5. mirosul at gmail.com

    Please give me an idea of reporting tool for ruby. I made a software and I need a reporting tool, besides text or html. I saw a post of yours with some pdf stuff.
    Thank you,

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