In Kotlin there are two types of equality:
Structural equality is checked by the
== operation (and its negated counterpart
!=). By convention, an expression like
a == b is translated to:
a?.equals(b) ?: (b === null)
a is not
null, it calls the
equals(Any?) function, otherwise (i.e.
null) it checks that
b is referentially equal to
Note that there's no point in optimizing your code when comparing to
a == null will be automatically translated to
a === null.
To provide a custom equals check implementation, override the
equals(other: Any?): Boolean function. Functions with the same name and other signatures, like
equals(other: Foo), don't affect equality checks with the operators
Structural equality has nothing to do with comparison defined by the
Comparable<...> interface, so only a custom
equals(Any?) implementation may affect the behavior of the operator.
When an equality check operands are statically known to be
Double (nullable or not), the check follows the IEEE 754 Standard for Floating-Point Arithmetic.
Otherwise, the structural equality is used, which disagrees with the standard so that
NaN is equal to itself, and
-0.0 is not equal to
Referential equality is checked by the
=== operation (and its negated counterpart
a === b evaluates to true if and only if
b point to the same object. For values which are represented as primitive types at runtime (for example,
=== equality check is equivalent to the
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