Class LongAdder
- All Implemented Interfaces:
Serializable
long
sum. When updates (method add(long)
) are contended
across threads, the set of variables may grow dynamically to reduce
contention. Method sum()
(or, equivalently, longValue()
) returns the current total combined across the
variables maintaining the sum.
This class is usually preferable to AtomicLong
when
multiple threads update a common sum that is used for purposes such
as collecting statistics, not for fine-grained synchronization
control. Under low update contention, the two classes have similar
characteristics. But under high contention, expected throughput of
this class is significantly higher, at the expense of higher space
consumption.
LongAdders can be used with a ConcurrentHashMap
to maintain a scalable
frequency map (a form of histogram or multiset). For example, to
add a count to a ConcurrentHashMap<String,LongAdder> freqs
,
initializing if not already present, you can use
freqs.computeIfAbsent(key, k -> new LongAdder()).increment();
This class extends Number
, but does not define
methods such as equals
, hashCode
and
compareTo
because instances are expected to be mutated, and so are
not useful as collection keys.
- Since:
- 1.8
- See Also:
-
Constructor Summary
-
Method Summary
Modifier and TypeMethodDescriptionvoid
add
(long x) Adds the given value.void
Equivalent toadd(-1)
.double
Returns thesum()
as adouble
after a widening primitive conversion.float
Returns thesum()
as afloat
after a widening primitive conversion.void
Equivalent toadd(1)
.int
intValue()
Returns thesum()
as anint
after a narrowing primitive conversion.long
Equivalent tosum()
.void
reset()
Resets variables maintaining the sum to zero.long
sum()
Returns the current sum.long
toString()
Returns the String representation of thesum()
.Methods declared in class java.lang.Number
byteValue, shortValue
-
Constructor Details
-
LongAdder
public LongAdder()Creates a new adder with initial sum of zero.
-
-
Method Details
-
add
public void add(long x) Adds the given value.- Parameters:
x
- the value to add
-
increment
public void increment()Equivalent toadd(1)
. -
decrement
public void decrement()Equivalent toadd(-1)
. -
sum
public long sum()Returns the current sum. The returned value is NOT an atomic snapshot; invocation in the absence of concurrent updates returns an accurate result, but concurrent updates that occur while the sum is being calculated might not be incorporated.- Returns:
- the sum
-
reset
public void reset()Resets variables maintaining the sum to zero. This method may be a useful alternative to creating a new adder, but is only effective if there are no concurrent updates. Because this method is intrinsically racy, it should only be used when it is known that no threads are concurrently updating. -
sumThenReset
public long sumThenReset()Equivalent in effect tosum()
followed byreset()
. This method may apply for example during quiescent points between multithreaded computations. If there are updates concurrent with this method, the returned value is not guaranteed to be the final value occurring before the reset.- Returns:
- the sum
-
toString
-
longValue
-
intValue
-
floatValue
public float floatValue()Returns thesum()
as afloat
after a widening primitive conversion.- Specified by:
floatValue
in classNumber
- Returns:
- the numeric value represented by this object after conversion
to type
float
.
-
doubleValue
public double doubleValue()Returns thesum()
as adouble
after a widening primitive conversion.- Specified by:
doubleValue
in classNumber
- Returns:
- the numeric value represented by this object after conversion
to type
double
.
-