Package org.apache.hadoop.fs.statistics

Class MeanStatistic

java.lang.Object
org.apache.hadoop.fs.statistics.MeanStatistic
All Implemented Interfaces:
Serializable, Cloneable
@Public @Evolving public final class MeanStatistic extends Object implements Serializable, Cloneable
A mean statistic represented as the sum and the sample count; the mean is calculated on demand.

It can be used to accrue values so as to dynamically update the mean. If so, know that there is no synchronization on the methods.

If a statistic has 0 samples then it is considered to be empty.

All 'empty' statistics are equivalent, independent of the sum value.

For non-empty statistics, sum and sample values must match for equality.

It is serializable and annotated for correct serializations with jackson2.

Thread safety. The operations to add/copy sample data, are thread safe.

  1. add(MeanStatistic)
  2. addSample(long)
  3. clear()
  4. setSamplesAndSum(long, long)
  5. set(MeanStatistic)
  6. setSamples(long) and setSum(long)

So is the mean() method. This ensures that when used to aggregated statistics, the aggregate value and sample count are set and evaluated consistently.

Other methods marked as synchronized because Findbugs overreacts to the idea that some operations to update sum and sample count are synchronized, but that things like equals are not.

See Also:

  • Constructor Details

    • MeanStatistic

      public MeanStatistic(long samples, long sum)
      Constructor, with some resilience against invalid sample counts. If the sample count is 0 or less, the sum is set to 0 and the sample count to 0.
      Parameters:
      samples - sample count.
      sum - sum value

    • MeanStatistic

      public MeanStatistic(MeanStatistic that)
      Create from another statistic.
      Parameters:
      that - source

    • MeanStatistic

      public MeanStatistic()
      Create an empty statistic.

  • Method Details

    • getSum

      public long getSum()
      Get the sum of samples.
      Returns:
      the sum

    • getSamples

      public long getSamples()
      Get the sample count.
      Returns:
      the sample count; 0 means empty

    • isEmpty

      public boolean isEmpty()
      Is a statistic empty?
      Returns:
      true if the sample count is 0

    • clear

      public void clear()
      Set the values to 0.

    • setSamplesAndSum

      public void setSamplesAndSum(long sampleCount, long newSum)
      Set the sum and samples. Synchronized.
      Parameters:
      sampleCount - new sample count.
      newSum - new sum

    • set

      public void set(MeanStatistic other)
      Set the statistic to the values of another. Synchronized.
      Parameters:
      other - the source.

    • setSum

      public void setSum(long sum)
      Set the sum.
      Parameters:
      sum - new sum

    • setSamples

      public void setSamples(long samples)
      Set the sample count. If this is less than zero, it is set to zero. This stops an ill-formed JSON entry from breaking deserialization, or get an invalid sample count into an entry.
      Parameters:
      samples - sample count.

    • mean

      public double mean()
      Get the arithmetic mean value.
      Returns:
      the mean

    • add

      public MeanStatistic add(MeanStatistic other)
      Add another MeanStatistic.
      Parameters:
      other - other value
      Returns:
      mean statistic.

    • addSample

      public void addSample(long value)
      Add a sample. Thread safe.
      Parameters:
      value - value to add to the sum

    • hashCode

      public int hashCode()
      The hash code is derived from the mean and sample count: if either is changed the statistic cannot be used as a key for hash tables/maps.
      Overrides:
      hashCode in class Object
      Returns:
      a hash value

    • equals

      public boolean equals(Object o)
      Overrides:
      equals in class Object

    • clone

      public MeanStatistic clone()
      Overrides:
      clone in class Object

    • copy

      public MeanStatistic copy()
      Create a copy of this instance.
      Returns:
      copy.

    • toString

      public String toString()
      Overrides:
      toString in class Object