Well, what’s wrong with a common practice of measuring elapsed time in the code like this:
long startTime = System.currentTimeMillis(); // ... long elapsedTime = System.currentTimeMillis() - startTime;
The approach is based on an intuitive assumption that the values returned by
currentTimeMillis() (in general, the current system time expressed as a number of milliseconds since midnight, January 1, 1970 UTC) grow smoothly and always are greater in the future than in the past. This is not always the case.
A simple example when this assumption fails is adjusting the system clock. On the most of the modern systems, correction of the clock is performed automatically by polling a value from the NTP server in a specific period of time. If this correction happens in the middle of your operation, the measured time interval will be wrong or even negative. Note that the typical computer clocks have an error in tens or even hundreds of milliseconds per hour that means that the correction offset will be up to few seconds if it is performed once a day.
It is safer to use the
System.nanoTime() method which does not depend on a system clock, so it’s guaranteed that its value grows smoothly from the past to the future:
long startTime = System.nanoTime(); // ... long elapsedTime = (System.nanoTime() - startTime) / 1000000; // ms
And remember that the
nanoTime values has nothing to do with any real absolute time, so don’t try to use it for the Date objects, for instance.
Null should never be returned from a method returning a collection or an array. Instead return a empty array (a static final empty array) or one of the empty collections (e.g.
Collections.EMPTY_LIST assuming the client should not be modifying the collection).
(submitted by Rand McNeely)
StringBuilder rather then
StringBuffer, unless synchronization is required.
StringBuilder is not thread safe, and therefore avoids the synchronization overhead of
(submitted by Robert J. Walker)
java.net.URL is based on a fancy rule saying that URLs of two hosts are equal as long as they are resolving to the same IP address. It is known to be incompatible with virtual hosting and should not be used.
URL.equals() would consider the URL of this blog (http://javaantipatterns.wordpress.com) to be equal to an URL of any random blog hosted on WordPress – that is obviously not true.
It is recommended to use java.net.URI objects instead.
java.net.URL class is an absolute champion in a number of implemented antipatterns of
hashCode() as well):
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.
toString() method gives you a cheap way to provide human-readable labels for objects in output. When the objects are used in GUI containers, such as
JTables, it allows to use the default models and renderers instead of writing the custom ones.
Creating new instances of immutable primitive type wrappers (such as
Number subclasses and
Booleans) wastes the memory and time needed for allocating new objects. Static
valueOf() method works much faster than a constructor and saves the memory, as it caches frequently used instances.
It is guaranteed that two
Boolean instances and
Integers between -128 and 127 to be pre-cached, thus you definitely should not use the constructor to instantiate them.