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.

Reference:

• The Ruby Way
• Programming Ruby- The Pragmatic Programmer’s Guide 2nd Ed (known also as the ~PickAxe book)
• Object#==
• Object#===
• Object#eql?
• Object#equal?
• Object.equals
• comparing objects
• which method do I have to override?
• hash and ==
• identity
• equivalence relation

category: Java, Ruby, Identity, Equivalence

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