This class is usually preferable to {@link 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. * *

This class extends {@link Number}, but does *not* define
* methods such as {@code equals}, {@code hashCode} and {@code
* compareTo} because instances are expected to be mutated, and so are
* not useful as collection keys.
*
*

*jsr166e note: This class is targeted to be placed in
* java.util.concurrent.atomic.*
*
* @since 1.8
* @author Doug Lea
*/
public class LongAdder extends Striped64 implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
/**
* Version of plus for use in retryUpdate
*/
final long fn(long v, long x) { return v + x; }
/**
* Creates a new adder with initial sum of zero.
*/
public LongAdder() {
}
/**
* Adds the given value.
*
* @param x the value to add
*/
public void add(long x) {
Cell[] as; long b, v; int[] hc; Cell a; int n;
if ((as = cells) != null || !casBase(b = base, b + x)) {
boolean uncontended = true;
if ((hc = threadHashCode.get()) == null ||
as == null || (n = as.length) < 1 ||
(a = as[(n - 1) & hc[0]]) == null ||
!(uncontended = a.cas(v = a.value, v + x)))
retryUpdate(x, hc, uncontended);
}
}
/**
* Equivalent to {@code add(1)}.
*/
public void increment() {
add(1L);
}
/**
* Equivalent to {@code add(-1)}.
*/
public void decrement() {
add(-1L);
}
/**
* 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.
*
* @return the sum
*/
public long sum() {
long sum = base;
Cell[] as = cells;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null)
sum += a.value;
}
}
return sum;
}
/**
* 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 void reset() {
internalReset(0L);
}
/**
* Equivalent in effect to {@link #sum} followed by {@link
* #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.
*
* @return the sum
*/
public long sumThenReset() {
long sum = base;
Cell[] as = cells;
base = 0L;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null) {
sum += a.value;
a.value = 0L;
}
}
}
return sum;
}
/**
* Returns the String representation of the {@link #sum}.
* @return the String representation of the {@link #sum}
*/
public String toString() {
return Long.toString(sum());
}
/**
* Equivalent to {@link #sum}.
*
* @return the sum
*/
public long longValue() {
return sum();
}
/**
* Returns the {@link #sum} as an {@code int} after a narrowing
* primitive conversion.
*/
public int intValue() {
return (int)sum();
}
/**
* Returns the {@link #sum} as a {@code float}
* after a widening primitive conversion.
*/
public float floatValue() {
return (float)sum();
}
/**
* Returns the {@link #sum} as a {@code double} after a widening
* primitive conversion.
*/
public double doubleValue() {
return (double)sum();
}
private void writeObject(ObjectOutputStream s) throws IOException {
s.defaultWriteObject();
s.writeLong(sum());
}
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException {
s.defaultReadObject();
busy = 0;
cells = null;
base = s.readLong();
}
}