What's the difference between == and .equals in Scala?
What is the difference between ==
and .equals()
in Scala, and when to use which?
Is the implementation same as in Java?
EDIT: The re开发者_如何学Clated question talks about specific cases of AnyVal
. The more general case is Any
.
You normally use ==
, it routes to equals
, except that it treats null
s properly. Reference equality (rarely used) is eq
.
TL;DR
- Override
equals
method to compare content of each instance. This is the sameequals
method used in Java - Use
==
operator to compare, without worrying aboutnull
references - Use
eq
method to check if both arguments are EXACTLY the same reference. Recommended not to use unless you understand how this works and oftenequals
will work for what you need instead. And make sure to only use this withAnyRef
arguments, not justAny
NOTE: On the case of equals
, just as in Java, it may not return the same result if you switch the arguments eg 1.equals(BigInt(1))
will return false
where the inverse will return true
. This is because of each implementation checking only specific types. Primitive numbers dont check if the second argument is of Number
nor BigInt
types but only of other primitive types
Details
The AnyRef.equals(Any)
method is the one overridden by subclasses. A method from the Java Specification that has come over to Scala too. If used on an unboxed instance, it is boxed to call this (though hidden in Scala; more obvious in Java with int
->Integer
). The default implementation merely compares references (as in Java)
The Any.==(Any)
method compares two objects and allows either argument to be null (as if calling a static method with two instances). It compares if both are null
, then it calls the equals(Any)
method on boxed instance.
The AnyRef.eq(AnyRef)
method compares only references, that is where the instance is located in memory. There is no implicit boxing for this method.
Examples
1 equals 2
will returnfalse
, as it redirects toInteger.equals(...)
1 == 2
will returnfalse
, as it redirects toInteger.equals(...)
1 eq 2
will not compile, as it requires both arguments to be of typeAnyRef
new ArrayList() equals new ArrayList()
will returntrue
, as it checks the contentnew ArrayList() == new ArrayList()
will returntrue
, as it redirects toequals(...)
new ArrayList() eq new ArrayList()
will returnfalse
, as both arguments are different instancesfoo equals foo
will returntrue
, unlessfoo
isnull
, then will throw aNullPointerException
foo == foo
will returntrue
, even iffoo
isnull
foo eq foo
will returntrue
, since both arguments link to the same reference
==
is a final method, and calls .equals
, which is not final.
This is radically different than Java, where ==
is an operator rather than a method and strictly compares reference equality for objects.
There is an interesting difference between ==
and equals
for Float
and Double
types: They treat NaN
differently:
scala> Double.NaN == Double.NaN
res3: Boolean = false
scala> Double.NaN equals Double.NaN
res4: Boolean = true
Edit: As was pointed out in a comment - "this also happens in Java" - depends on what exactly this is:
public static void main(final String... args) {
final double unboxedNaN = Double.NaN;
final Double boxedNaN = Double.valueOf(Double.NaN);
System.out.println(unboxedNaN == unboxedNaN);
System.out.println(boxedNaN == boxedNaN);
System.out.println(boxedNaN.equals(boxedNaN));
}
This will print
false
true
true
So, the unboxedNan
yields false
when compared for equality because this is how IEEE floating point numbers define it and this should really happen in every programming language (although it somehow messes with the notion of identity).
The boxed NaN yields true for the comparison using ==
in Java as we are comparing object references.
I do not have an explanation for the equals
case, IMHO it really should behave the same as ==
on unboxed double values, but it does not.
Translated to Scala the matter is a little more complicated as Scala has unified primitive and object types into Any
and translates into the primitive double and the boxed Double as needed. Thus the scala ==
apparently boils down to a comparison of primitive NaN
values but equals
uses the one defined on boxed Double values (there is a lot of implicit conversion magic going on and there is stuff pimped onto doubles by RichDouble
).
If you really need to find out if something is actually NaN
use isNaN
:
- Java: https://docs.oracle.com/javase/7/docs/api/java/lang/Double.html#isNaN(double)
- Scala: http://www.scala-lang.org/files/archive/api/2.11.8/index.html#scala.Double@isNaN():Boolean
In Scala == first check for Null values and then calls equals method on first object
精彩评论