Posts Tagged ‘hashcode’
hashCode() implementations should rely only on an internal object state. Making them dependent on other objects, context of use and other external conditions conflicts with the general contracts of consistency:
“For any reference values
y, multiple invocations of
trueor consistently return
“Whenever it is invoked on the same object more than once during an execution of a Java application, the
hashCodemethod must consistently return the same integer” [*]
Violating those contracts may cause all kinds of weird and unpredictable behavior.
A known example of this antipattern is
java.net.URL implementation where they are depending on information returned by a domain name server, rather than on actual stored URL data.
java.net.URL objects used as Map keys or Set items can result in a big performance issue. Internal implementation of
hashCode() methods of java.net.URL class performs domain name resolution, so the time of their execution relies on an internet connection speed. If a computer is off-line, the time is nearly equal to a connection timeout (tens of seconds).
The same problem arises every time when URLs equality checking or hash code computation is performed.
Do not use
java.net.URL to keep URL values. Use
java.net.URI objects or the simple Strings instead.
If you override
equals() in your class, always provide a custom implementation of
hashCode() that returns the same hash code value for two or more equal objects:
a.hashCode() == b.hashCode()
This is, in fact, a general contract defined by Java API Specification. Violation of this rule (which is likely the case if
equals() method is overridden while
hashCode() is inherited from
Object) may cause numerous bugs and unexpected behaviours.
Note that the reverse rule: two or more unequal objects must have different hash codes, or
a.hashCode() != b.hashCode()
though not required, is also highly recommended to be maintained wherever possible (except for the cases when a number of class instances is known to be greater than
Integer.MAX_VALUE). Disregard of this rule (hash collisions) may reduce performance of HashMaps and Hashtables.