Class 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(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.

Constructors

LongAdder

public LongAdder()

Creates a new adder with initial sum of zero.

Methods

add

public void add(long x)

Adds the given value.

Parameters:

x - the value to add

increment

public void increment()

Equivalent to add(1).

decrement

public void decrement()

Equivalent to add(-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 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

toString

public String toString()

Returns the String representation of the sum().

Overrides:

toString in class Object

Returns:

the String representation of the sum()

longValue

public long longValue()

Equivalent to sum().

Specified by:

longValue in class Number

Returns:

the sum

intValue

public int intValue()

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

Specified by:

intValue in class Number

Returns:

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

floatValue

public float floatValue()

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

Specified by:

floatValue in class Number

Returns:

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

doubleValue

public double doubleValue()

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

Specified by:

doubleValue in class Number

Returns:

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