LongSummaryStatistics.java

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package java.util;

import java.util.function.IntConsumer;
import java.util.function.LongConsumer;
import java.util.stream.Collector;

/**
 * A state object for collecting statistics such as count, min, max, sum, and
 * average.
 *
 * <p>This class is designed to work with (though does not require)
 * {@linkplain java.util.stream streams}. For example, you can compute
 * summary statistics on a stream of longs with:
 * <pre> {@code
 * LongSummaryStatistics stats = longStream.collect(LongSummaryStatistics::new,
 *                                                  LongSummaryStatistics::accept,
 *                                                  LongSummaryStatistics::combine);
 * }</pre>
 *
 * <p>{@code LongSummaryStatistics} can be used as a
 * {@linkplain java.util.stream.Stream#collect(Collector) reduction}
 * target for a {@linkplain java.util.stream.Stream stream}. For example:
 *
 * <pre> {@code
 * LongSummaryStatistics stats = people.stream()
 *                                     .collect(Collectors.summarizingLong(Person::getAge));
 * }</pre>
 *
 * This computes, in a single pass, the count of people, as well as the minimum,
 * maximum, sum, and average of their ages.
 *
 * @implNote This implementation is not thread safe. However, it is safe to use
 * {@link java.util.stream.Collectors#summarizingLong(java.util.function.ToLongFunction)
 * Collectors.summarizingLong()} on a parallel stream, because the parallel
 * implementation of {@link java.util.stream.Stream#collect Stream.collect()}
 * provides the necessary partitioning, isolation, and merging of results for
 * safe and efficient parallel execution.
 *
 * <p>This implementation does not check for overflow of the sum.
 * @since 1.8
 */

// 对long类型的元素统计相关信息：计数、求和、均值、最小值、最大值
public class LongSummaryStatistics implements LongConsumer, IntConsumer {
    private long min = Long.MAX_VALUE;  // 最小值
    private long max = Long.MIN_VALUE;  // 最大值
    
    private long count; // 计数
    private long sum;   // 求和
    
    /**
     * Constructs an empty instance with zero count, zero sum,
     * {@code Long.MAX_VALUE} min, {@code Long.MIN_VALUE} max and zero
     * average.
     */
    public LongSummaryStatistics() {
    }
    
    /**
     * Constructs a non-empty instance with the specified {@code count},
     * {@code min}, {@code max}, and {@code sum}.
     *
     * <p>If {@code count} is zero then the remaining arguments are ignored and
     * an empty instance is constructed.
     *
     * <p>If the arguments are inconsistent then an {@code IllegalArgumentException}
     * is thrown.  The necessary consistent argument conditions are:
     * <ul>
     * <li>{@code count >= 0}</li>
     * <li>{@code min <= max}</li>
     * </ul>
     *
     * @param count the count of values
     * @param min   the minimum value
     * @param max   the maximum value
     * @param sum   the sum of all values
     *
     * @throws IllegalArgumentException if the arguments are inconsistent
     * @apiNote The enforcement of argument correctness means that the retrieved set of
     * recorded values obtained from a {@code LongSummaryStatistics} source
     * instance may not be a legal set of arguments for this constructor due to
     * arithmetic overflow of the source's recorded count of values.
     * The consistent argument conditions are not sufficient to prevent the
     * creation of an internally inconsistent instance.  An example of such a
     * state would be an instance with: {@code count} = 2, {@code min} = 1,
     * {@code max} = 2, and {@code sum} = 0.
     * @since 10
     */
    public LongSummaryStatistics(long count, long min, long max, long sum) throws IllegalArgumentException {
        if(count<0L) {
            throw new IllegalArgumentException("Negative count value");
        } else if(count>0L) {
            if(min>max)
                throw new IllegalArgumentException("Minimum greater than maximum");
            
            this.count = count;
            this.sum = sum;
            this.min = min;
            this.max = max;
        }
        // Use default field values if count == 0
    }
    
    /**
     * Returns the count of values recorded.
     *
     * @return the count of values
     */
    // 计数
    public final long getCount() {
        return count;
    }
    
    /**
     * Returns the sum of values recorded, or zero if no values have been
     * recorded.
     *
     * @return the sum of values, or zero if none
     */
    // 求和
    public final long getSum() {
        return sum;
    }
    
    /**
     * Returns the minimum value recorded, or {@code Long.MAX_VALUE} if no
     * values have been recorded.
     *
     * @return the minimum value, or {@code Long.MAX_VALUE} if none
     */
    // 最小值
    public final long getMin() {
        return min;
    }
    
    /**
     * Returns the maximum value recorded, or {@code Long.MIN_VALUE} if no
     * values have been recorded
     *
     * @return the maximum value, or {@code Long.MIN_VALUE} if none
     */
    // 最大值
    public final long getMax() {
        return max;
    }
    
    /**
     * Returns the arithmetic mean of values recorded, or zero if no values have been
     * recorded.
     *
     * @return The arithmetic mean of values, or zero if none
     */
    // 最后计算平均值
    public final double getAverage() {
        return getCount()>0 ? (double) getSum() / getCount() : 0.0d;
    }
    
    /**
     * Records a new {@code int} value into the summary information.
     *
     * @param value the input value
     */
    // 每遇到一个新的value，需要更新计数、计数、最小值、最大值的数据
    @Override
    public void accept(int value) {
        accept((long) value);
    }
    
    /**
     * Records a new {@code long} value into the summary information.
     *
     * @param value the input value
     */
    // 每遇到一个新的value，需要更新计数、计数、最小值、最大值的数据
    @Override
    public void accept(long value) {
        ++count;
        sum += value;
        min = Math.min(min, value);
        max = Math.max(max, value);
    }
    
    /**
     * Combines the state of another {@code LongSummaryStatistics} into this
     * one.
     *
     * @param other another {@code LongSummaryStatistics}
     *
     * @throws NullPointerException if {@code other} is null
     */
    // 合并两个数据源的统计信息（计数与求和相加，最小值取最小的，最大值取最大的）
    public void combine(LongSummaryStatistics other) {
        count += other.count;
        sum += other.sum;
        min = Math.min(min, other.min);
        max = Math.max(max, other.max);
    }
    
    /**
     * Returns a non-empty string representation of this object suitable for
     * debugging. The exact presentation format is unspecified and may vary
     * between implementations and versions.
     */
    @Override
    public String toString() {
        return String.format("%s{count=%d, sum=%d, min=%d, average=%f, max=%d}", this.getClass().getSimpleName(), getCount(), getSum(), getMin(), getAverage(), getMax());
    }
}