LongAdder

public class LongAdder extends Number
implements Serializable

One or more variables that together maintain an initially zero 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.

Public Constructor Summary

LongAdder() Creates a new adder with initial sum of zero.

Public Method Summary

void	add(long x) Adds the given value.
void	decrement() Equivalent to add(-1).
double	doubleValue() Returns the sum() as a double after a widening primitive conversion.
float	floatValue() Returns the sum() as a float after a widening primitive conversion.
void	increment() Equivalent to add(1).
int	intValue() Returns the sum() as an int after a narrowing primitive conversion.
long	longValue() Equivalent to sum().
void	reset() Resets variables maintaining the sum to zero.
long	sum() Returns the current sum.
long	sumThenReset() Equivalent in effect to sum() followed by reset().
String	toString() Returns the String representation of the sum().

Inherited Method Summary

From class java.lang.Number

byte	byteValue() Returns the value of the specified number as a byte, which may involve rounding or truncation.
abstract double	doubleValue() Returns the value of the specified number as a double, which may involve rounding.
abstract float	floatValue() Returns the value of the specified number as a float, which may involve rounding.
abstract int	intValue() Returns the value of the specified number as an int, which may involve rounding or truncation.
abstract long	longValue() Returns the value of the specified number as a long, which may involve rounding or truncation.
short	shortValue() Returns the value of the specified number as a short, which may involve rounding or truncation.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Constructors

public LongAdder ()

Creates a new adder with initial sum of zero.

Public Methods

public void add (long x)

Adds the given value.

Parameters

x	the value to add

public void decrement ()

Equivalent to add(-1).

public double doubleValue ()

Returns the sum() as a double after a widening primitive conversion.

Returns

• the numeric value represented by this object after conversion to type double.

public float floatValue ()

Returns the sum() as a float after a widening primitive conversion.

Returns

• the numeric value represented by this object after conversion to type float.

public void increment ()

Equivalent to add(1).

public int intValue ()

Returns the sum() as an int after a narrowing primitive conversion.

Returns

• the numeric value represented by this object after conversion to type int.

public long longValue ()

Equivalent to sum().

Returns

• the sum

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.

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

public long sumThenReset ()

Equivalent in effect to sum() followed by reset(). 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

public String toString ()

Returns the String representation of the sum().

Returns

• the String representation of the sum()