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POWhy == operator and equals() behave differently for values of AnyVal in Scala
 

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  1. POWhy == operator and equals() behave differently for values of AnyVal in Scala
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    <p>In the scaladoc of <code>scala.Any</code>, the operator <code>==</code> (or, method <code>==</code>) is explained:</p> <blockquote> <p>The expression <code>x == that</code> is equivalent to <code>if (x eq null) that eq null else x.equals(that)</code> <a href="http://www.scala-lang.org/api/current/#scala.Any">http://www.scala-lang.org/api/current/#scala.Any</a></p> </blockquote> <p>For objects of subclasses of <code>AnyRef</code>, I can understand it easily, and I didn't see any strange things.</p> <p>However, for values of <code>AnyVal</code>, (I mean <code>Int</code>, <code>Double</code>, <code>Long</code>, and so on,) the above definition is somewhat tricky (<code>1 eq null</code>? This does not compile if we do not convert <code>1</code> to java.lang.Integer). Also, <code>==</code> and <code>equals()</code> behave differently.</p> <p>I'll give some examples.</p> <pre> scala> 1 == 1 res0: Boolean = true scala> 1 == 1.0 res1: Boolean = true scala> 1 == 1.2 res2: Boolean = false scala> 2 == BigInt(2) res3: Boolean = true scala> 2.0 == BigInt(2) res4: Boolean = true scala> 2 == BigInt(3) res5: Boolean = false </pre> <p>So far, nothing is strange. But if we do the same things with <code>equals()</code> methods,</p> <pre> scala> 1 equals 1 res7: Boolean = true scala> 1 equals 1.0 res8: Boolean = false scala> 1 equals 1.2 res9: Boolean = false scala> 2 equals BigInt(2) res10: Boolean = false scala> 2.0 equals BigInt(2) res11: Boolean = false scala> 2 equals BigInt(3) res12: Boolean = false </pre> <p>So if the types are different, equals() always returns false, whereas == tests if they represent the same value if they are converted to the same type.</p> <p>In the case of subclass of <code>AnyRef</code>, methods <code>==</code> and <code>equals()</code> return the same.</p> <pre> scala> BigInt(2) == 2 res25: Boolean = true scala> BigInt(2) == 2.0 res26: Boolean = true scala> BigInt(3) == 2 res27: Boolean = false scala> BigInt(2) equals 2 res28: Boolean = true scala> BigInt(2) equals 2.0 res29: Boolean = true scala> BigInt(3) equals 2 res30: Boolean = false </pre> <p>So, why methods <code>==</code> and <code>equals()</code> are diffrent for <code>AnyVal</code>?</p> <p>I'm using Scala version 2.10.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_25).</p> <p><strong>EDIT 1</strong><br/> I saw that == cannot be overriden directly, as it is defined as a final method in class Any according to <a href="http://rads.stackoverflow.com/amzn/click/0981531644">Programming in Scala, 2nd Edition</a>.</p> <p><strong>EDIT 2</strong><br/> Although there is an answer, my question remains. I will leave this question open.<br/> <br/> What correspond to <code>scala.Int</code> and <code>scala.Long</code> in Java are Java's primitive types <code>int</code> and <code>long</code>.<br/> In Java, <code>java.lang.Integer</code> and <code>java.lang.Long</code> are classes, so their variables are references, which can have <code>null</code>. That means, they are like <code>AnyRef</code> in Scala. Not <code>AnyVal</code>.<br/> Scala's <code>AnyVal</code> - <code>scala.Int</code> and <code>scala.Long</code> cannot have <code>null</code> values, neither can Java's <code>int</code> and <code>long</code>.<br/> Also, <code>java.lang.Integer</code>'s <code>==</code> in Java is for reference equality (same as <code>eq</code> in Scala).<br/> What you get using <code>java.lang.Integer</code> in Scala REPL will be quite different from what you get with it in pure Java Project with .java source file in this respect.<br/> <br/> However, what I could get from using classes of primitive types in Java was: (THIS IS JAVA)</p> <pre><code>class Main { public static void main(String[] args) { System.out.println(String.valueOf(new java.lang.Integer(1).equals(1))); System.out.println(String.valueOf(new java.lang.Integer(1).equals(1L))); System.out.println(String.valueOf(new java.lang.Integer(1).equals(1.0))); System.out.println(String.valueOf(new java.lang.Integer(1).equals(new java.lang.Integer(1)))); System.out.println(String.valueOf(new java.lang.Integer(1).equals(new java.lang.Long(1)))); } } </code></pre> <p>output:<br/><pre> true false false true false</pre> Yes, they behave similar to scala AnyVal's <code>equals()</code>. But, then, why does this happen?</p> <p>Does Scala's <code>AnyVal</code>'s <code>==</code> correspond to <code>==</code> of Java's primitive type<br/> and does Scala's AnyVal's <code>equals()</code> correspond to <code>equals()</code> of Java's class types?<br/> What about equality tests with BigInt? There is no corresponding primitive type in Java.<br/> The question remains...</p> <p><strong>EDIT 3</strong><br/> I could find some information from scaladoc. (<a href="http://www.scala-lang.org/api/current/index.html#scala.Int">http://www.scala-lang.org/api/current/index.html#scala.Int</a>)<br/> The <em>Implicit information</em> from the item of <em>Shadowed Implicit Value Members</em>,<br/> I could find <code>==</code> was overloaded for <code>Char</code>, <code>Short</code>, <code>Float</code>, and ...,<br/> and <code>==</code> will call implicit conversions <code>int2double</code>, <code>int2float</code>, or <code>int2long</code>.<br/> Whereas <code>equals()</code> is only defined for <code>Any</code>, and it will call implicit conversion <code>int2Integer</code>.<br/> That is, <code>Int.equals()</code> will be the same as <code>java.lang.Integer.equals()</code>.<br/> <br/> One question remains:<br/> Why <code>==</code> of <code>AnyVal</code> is overloaded, and <code>equals()</code> of <code>AnyVal</code> is not overloaded?</p>
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    1. COI am sorry but I am confused after the whole post. Can you please specify in the end of what is the exact question?
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    2. CO@Jatin The method `==` and `equals()` are the same in `AnyRef` values in Scala. I think they should be the same for `AnyVal` values, too. In fact, they are different. However, I couldn't find anything about this while I was studying Scala. So, why `==` and `equals()` are not the same for `AnyVal`? Is there some specification about this?
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