java.util.Spliterators#AbstractSpliterator ( )源码实例Demo

private static <T> Spliterator<T> takeWhile(Spliterator<T> splitr, Predicate<? super T> predicate) {
    return new Spliterators.AbstractSpliterator<T>(splitr.estimateSize(), 0) {
        boolean stillGoing = true;

        @Override
        public boolean tryAdvance(Consumer<? super T> consumer) {
            if (stillGoing) {
                boolean hadNext = splitr.tryAdvance(elem -> {
                    if (predicate.test(elem)) {
                        consumer.accept(elem);
                    } else {
                        stillGoing = false;
                    }
                });
                return hadNext && stillGoing;
            }
            return false;
        }
    };
}
 

/**
 * Returns an infinite sequential ordered {@code Stream} produced by iterative
 * application of a function {@code f} to an initial element {@code seed},
 * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
 * {@code f(f(seed))}, etc.
 *
 * <p>The first element (position {@code 0}) in the {@code Stream} will be
 * the provided {@code seed}.  For {@code n > 0}, the element at position
 * {@code n}, will be the result of applying the function {@code f} to the
 * element at position {@code n - 1}.
 *
 * <p>The action of applying {@code f} for one element
 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
 * the action of applying {@code f} for subsequent elements.  For any given
 * element the action may be performed in whatever thread the library
 * chooses.
 *
 * @param <T> the type of stream elements
 * @param seed the initial element
 * @param f a function to be applied to the previous element to produce
 *          a new element
 * @return a new sequential {@code Stream}
 */
public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
    Objects.requireNonNull(f);
    Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
           Spliterator.ORDERED | Spliterator.IMMUTABLE) {
        T prev;
        boolean started;

        @Override
        public boolean tryAdvance(Consumer<? super T> action) {
            Objects.requireNonNull(action);
            T t;
            if (started)
                t = f.apply(prev);
            else {
                t = seed;
                started = true;
            }
            action.accept(prev = t);
            return true;
        }
    };
    return StreamSupport.stream(spliterator, false);
}
 

/**
 * Returns an infinite sequential ordered {@code Stream} produced by iterative
 * application of a function {@code f} to an initial element {@code seed},
 * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
 * {@code f(f(seed))}, etc.
 *
 * <p>The first element (position {@code 0}) in the {@code Stream} will be
 * the provided {@code seed}.  For {@code n > 0}, the element at position
 * {@code n}, will be the result of applying the function {@code f} to the
 * element at position {@code n - 1}.
 *
 * <p>The action of applying {@code f} for one element
 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
 * the action of applying {@code f} for subsequent elements.  For any given
 * element the action may be performed in whatever thread the library
 * chooses.
 *
 * @param <T> the type of stream elements
 * @param seed the initial element
 * @param f a function to be applied to the previous element to produce
 *          a new element
 * @return a new sequential {@code Stream}
 */
public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
    Objects.requireNonNull(f);
    Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
           Spliterator.ORDERED | Spliterator.IMMUTABLE) {
        T prev;
        boolean started;

        @Override
        public boolean tryAdvance(Consumer<? super T> action) {
            Objects.requireNonNull(action);
            T t;
            if (started)
                t = f.apply(prev);
            else {
                t = seed;
                started = true;
            }
            action.accept(prev = t);
            return true;
        }
    };
    return StreamSupport.stream(spliterator, false);
}
 

/**
 * Wrap the JDBC {@link ResultSet} as a stream object. Note that the stream has to be consumed within
 * the boundaries of the statement execution
 * @param rs
 * @return
 */
public Stream<Row> wrap(final ResultSet rs) throws SQLException {
    ResultSetMetaData md = rs.getMetaData();
    final String[] columnNames = new String[md.getColumnCount()];
    for (int i=0; i<columnNames.length; i++) {
        columnNames[i] = md.getColumnName(i+1);
    }
    
    Spliterator<Row> s = new Spliterators.AbstractSpliterator<Row>(Long.MAX_VALUE, Spliterator.NONNULL | Spliterator.IMMUTABLE) {

        @Override
        public boolean tryAdvance(Consumer<? super Row> action) {
            try {
                if (!rs.next()) {
                    return false;
                }
                
                // wrap the current result record as a Row which is then handed
                // off to the stream
                action.accept(new Row(rs, columnNames));
                return true;
            }
            catch (SQLException x) {
                // ouch
                logger.log(Level.SEVERE, x.getMessage(), x);
                return false;
            }
        }
    };
    
    // Create a sequential stream of the rows from the result set
    return StreamSupport.stream(s, false);
}
 

static <T> Spliterator<T> emptySpliteratorWithExactSize(long exactSize) {
    return new Spliterators.AbstractSpliterator<T>(0, Spliterator.SIZED) {

        @Override
        public long getExactSizeIfKnown() {
            return exactSize;
        }

        @Override
        public boolean tryAdvance(Consumer<? super T> ignored) {
            return false;
        }
    };
}