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This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including
all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and
JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up
with different versions on classes on the class path).
package org.jgroups.util;
import org.jgroups.annotations.GuardedBy;
import java.util.*;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.BiConsumer;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
/**
* A store for elements (typically messages) to be retransmitted or delivered. Used on sender and receiver side,
* as a replacement for HashMap. Table should use less memory than HashMap, as HashMap.Entry has 4 fields,
* plus arrays for storage.
*
* Table maintains a matrix (an array of arrays) of elements, which are stored in the matrix by mapping
* their seqno to an index. E.g. when we have 10 rows of 1000 elements each, and first_seqno is 3000, then an element
* with seqno=5600, will be stored in the 3rd row, at index 600.
*
* Rows are removed when all elements in that row have been received.
*
* Table started out as a copy of RetransmitTable, but is synchronized and maintains its own low, hd and hr
* pointers, so it can be used as a replacement for NakReceiverWindow. The idea is to put messages into Table,
* deliver them in order of seqnos, and periodically scan over all tables in NAKACK2 to do retransmission.
*
* @author Bela Ban
* @version 3.1
*/
public class Table implements Iterable {
protected final int num_rows;
/** Must be a power of 2 for efficient modular arithmetic **/
protected final int elements_per_row;
protected final double resize_factor;
protected T[][] matrix;
/** The first seqno, at matrix[0][0] */
protected long offset;
protected int size;
/** The highest seqno purged */
protected long low;
/** The highest received seqno */
protected long hr;
/** The highest delivered (= removed) seqno */
protected long hd;
/** Time (in nanoseconds) after which a compaction should take place. 0 disables compaction */
protected long max_compaction_time=TimeUnit.NANOSECONDS.convert(DEFAULT_MAX_COMPACTION_TIME, TimeUnit.MILLISECONDS);
/** The time when the last compaction took place. If a {@link #compact()} takes place and sees that the
* last compaction is more than max_compaction_time nanoseconds ago, a compaction will take place */
protected long last_compaction_timestamp=0;
protected final Lock lock=new ReentrantLock();
protected final AtomicInteger adders=new AtomicInteger(0);
protected int num_compactions=0, num_resizes=0, num_moves=0, num_purges=0;
protected static final long DEFAULT_MAX_COMPACTION_TIME=10000; // in milliseconds
protected static final double DEFAULT_RESIZE_FACTOR=1.2;
public interface Visitor {
/**
* Iteration over the table, used by {@link Table#forEach(long,long,org.jgroups.util.Table.Visitor)}.
* @param seqno The current seqno
* @param element The element at matrix[row][column]
* @param row The current row
* @param column The current column
* @return True if we should continue the iteration, false if we should break out of the iteration
*/
boolean visit(long seqno, T element, int row, int column);
}
public Table() {
this(5, 8192, 0, DEFAULT_RESIZE_FACTOR);
}
public Table(long offset) {
this();
this.offset=this.low=this.hr=this.hd=offset;
}
public Table(int num_rows, int elements_per_row, long offset) {
this(num_rows,elements_per_row, offset, DEFAULT_RESIZE_FACTOR);
}
public Table(int num_rows, int elements_per_row, long offset, double resize_factor) {
this(num_rows,elements_per_row, offset, resize_factor, DEFAULT_MAX_COMPACTION_TIME);
}
/**
* Creates a new table
* @param num_rows the number of rows in the matrix
* @param elements_per_row the number of elements per row
* @param offset the seqno before the first seqno to be inserted. E.g. if 0 then the first seqno will be 1
* @param resize_factor teh factor with which to increase the number of rows
* @param max_compaction_time the max time in milliseconds after we attempt a compaction
*/
public Table(int num_rows, int elements_per_row, long offset, double resize_factor, long max_compaction_time) {
this.num_rows=num_rows;
this.elements_per_row=Util.getNextHigherPowerOfTwo(elements_per_row);
this.resize_factor=resize_factor;
this.max_compaction_time=TimeUnit.NANOSECONDS.convert(max_compaction_time, TimeUnit.MILLISECONDS);
this.offset=this.low=this.hr=this.hd=offset;
matrix=(T[][])new Object[num_rows][];
if(resize_factor <= 1)
throw new IllegalArgumentException("resize_factor needs to be > 1");
}
public AtomicInteger getAdders() {return adders;}
public long getOffset() {return offset;}
public int getElementsPerRow() {return elements_per_row;}
/** Returns the total capacity in the matrix */
public int capacity() {return matrix.length * elements_per_row;}
public int getNumCompactions() {return num_compactions;}
public int getNumMoves() {return num_moves;}
public int getNumResizes() {return num_resizes;}
public int getNumPurges() {return num_purges;}
/** Returns an appromximation of the number of elements in the table */
public int size() {return size;}
public boolean isEmpty() {return size <= 0;}
public long getLow() {return low;}
public long getHighestDelivered() {return hd;}
public long getHighestReceived() {return hr;}
public long getMaxCompactionTime() {return max_compaction_time;}
public void setMaxCompactionTime(long max_compaction_time) {
this.max_compaction_time=TimeUnit.NANOSECONDS.convert(max_compaction_time, TimeUnit.MILLISECONDS);
}
public int getNumRows() {return matrix.length;}
public void resetStats() {num_compactions=num_moves=num_resizes=num_purges=0;}
/** Returns the highest deliverable (= removable) seqno. This may be higher than {@link #getHighestDelivered()},
* e.g. if elements have been added but not yet removed */
public long getHighestDeliverable() {
HighestDeliverable visitor=new HighestDeliverable();
lock.lock();
try {
forEach(hd+1, hr, visitor);
long retval=visitor.getResult();
return retval == -1? hd : retval;
}
finally {
lock.unlock();
}
}
/** Returns the number of messages that can be delivered */
public int getNumDeliverable() {
NumDeliverable visitor=new NumDeliverable();
lock.lock();
try {
forEach(hd+1, hr, visitor);
return visitor.getResult();
}
finally {
lock.unlock();
}
}
/**
* Only used internally by JGroups on a state transfer. Please don't use this in application code, or you're on
* your own !
* @param seqno
*/
public void setHighestDelivered(long seqno) {
lock.lock();
try {
hd=seqno;
}
finally {
lock.unlock();
}
}
/**
* Adds an element if the element at the given index is null. Returns true if no element existed at the given index,
* else returns false and doesn't set the element.
* @param seqno
* @param element
* @return True if the element at the computed index was null, else false
*/
public boolean add(long seqno, T element) {
lock.lock();
try {
return _add(seqno, element, true, null);
}
finally {
lock.unlock();
}
}
/**
* Adds an element if the element at the given index is null. Returns true if no element existed at the given index,
* else returns false and doesn't set the element.
* @param seqno
* @param element
* @param remove_filter If not null, a filter used to remove all consecutive messages passing the filter
* @return True if the element at the computed index was null, else false
*/
public boolean add(long seqno, T element, Predicate remove_filter) {
lock.lock();
try {
return _add(seqno, element, true, remove_filter);
}
finally {
lock.unlock();
}
}
/**
* Adds elements from list to the table
* @param list
* @return True if at least 1 element was added successfully
*/
public boolean add(final List> list) {
return add(list, false);
}
/**
* Adds elements from list to the table, removes elements from list that were not added to the table
* @param list
* @return True if at least 1 element was added successfully. This guarantees that the list has at least 1 element
*/
public boolean add(final List> list, boolean remove_added_elements) {
return add(list, remove_added_elements, null);
}
/**
* Adds elements from the list to the table
* @param list The list of tuples of seqnos and elements. If remove_added_elements is true, if elements could
* not be added to the table (e.g. because they were already present or the seqno was < HD), those
* elements will be removed from list
* @param remove_added_elements If true, elements that could not be added to the table are removed from list
* @param const_value If non-null, this value should be used rather than the values of the list tuples
* @return True if at least 1 element was added successfully, false otherwise.
*/
public boolean add(final List> list, boolean remove_added_elements, T const_value) {
if(list == null || list.isEmpty())
return false;
boolean added=false;
// find the highest seqno (unfortunately, the list is not ordered by seqno)
long highest_seqno=findHighestSeqno(list);
lock.lock();
try {
if(highest_seqno != -1 && computeRow(highest_seqno) >= matrix.length)
resize(highest_seqno);
for(Iterator> it=list.iterator(); it.hasNext();) {
LongTuple tuple=it.next();
long seqno=tuple.getVal1();
T element=const_value != null? const_value : tuple.getVal2();
if(_add(seqno, element, false, null))
added=true;
else if(remove_added_elements)
it.remove();
}
return added;
}
finally {
lock.unlock();
}
}
/**
* Returns an element at seqno
* @param seqno
* @return
*/
public T get(long seqno) {
lock.lock();
try {
if(seqno - low <= 0 || seqno - hr > 0)
return null;
int row_index=computeRow(seqno);
if(row_index < 0 || row_index >= matrix.length)
return null;
T[] row=matrix[row_index];
if(row == null)
return null;
int index=computeIndex(seqno);
return index >= 0? row[index] : null;
}
finally {
lock.unlock();
}
}
/**
* To be used only for testing; doesn't do any index or sanity checks
* @param seqno
* @return
*/
public T _get(long seqno) {
lock.lock();
try {
int row_index=computeRow(seqno);
if(row_index < 0 || row_index >= matrix.length)
return null;
T[] row=matrix[row_index];
if(row == null)
return null;
int index=computeIndex(seqno);
return index >= 0? row[index] : null;
}
finally {
lock.unlock();
}
}
public T remove() {
return remove(true);
}
/** Removes the next non-null element and nulls the index if nullify=true */
public T remove(boolean nullify) {
lock.lock();
try {
int row_index=computeRow(hd+1);
if(row_index < 0 || row_index >= matrix.length)
return null;
T[] row=matrix[row_index];
if(row == null)
return null;
int index=computeIndex(hd+1);
if(index < 0)
return null;
T existing_element=row[index];
if(existing_element != null) {
hd++;
size=Math.max(size-1, 0); // cannot be < 0 (well that would be a bug, but let's have this 2nd line of defense !)
if(nullify) {
row[index]=null;
if(hd - low > 0)
low=hd;
}
}
return existing_element;
}
finally {
lock.unlock();
}
}
public List removeMany(boolean nullify, int max_results) {
return removeMany(nullify, max_results, null);
}
public List removeMany(boolean nullify, int max_results, Predicate filter) {
return removeMany(nullify, max_results, filter, LinkedList::new, LinkedList::add);
}
/**
* Removes elements from the table and adds them to the result created by result_creator. Between 0 and max_results
* elements are removed. If no elements were removed, processing will be set to true while the table lock is held.
* @param nullify if true, the x,y location of the removed element in the matrix will be nulled
* @param max_results the max number of results to be returned, even if more elements would be removable
* @param filter a filter which accepts (or rejects) elements into the result. If null, all elements will be accepted
* @param result_creator a supplier required to create the result, e.g. ArrayList::new
* @param accumulator an accumulator accepting the result and an element, e.g. ArrayList::add
* @param the type of the result
* @return the result
*/
public R removeMany(boolean nullify, int max_results, Predicate filter,
Supplier result_creator, BiConsumer accumulator) {
lock.lock();
try {
Remover remover=new Remover<>(nullify, max_results, filter, result_creator, accumulator);
forEach(hd+1, hr, remover);
return remover.getResult();
}
finally {
lock.unlock();
}
}
/**
* Removes all elements less than or equal to seqno from the table. Does this by nulling entire rows in the matrix
* and nulling all elements < index(seqno) of the first row that cannot be removed
* @param seqno
*/
public void purge(long seqno) {
purge(seqno, false);
}
/**
* Removes all elements less than or equal to seqno from the table. Does this by nulling entire rows in the matrix
* and nulling all elements < index(seqno) of the first row that cannot be removed.
* @param seqno All elements <= seqno will be nulled
* @param force If true, we only ensure that seqno <= hr, but don't care about hd, and set hd=low=seqno.
*/
public void purge(long seqno, boolean force) {
lock.lock();
try {
if(seqno - low <= 0)
return;
if(force) {
if(seqno - hr > 0)
seqno=hr;
}
else {
if(seqno - hd > 0) // we cannot be higher than the highest removed seqno
seqno=hd;
}
int start_row=computeRow(low), end_row=computeRow(seqno);
if(start_row < 0) start_row=0;
if(end_row < 0)
return;
for(int i=start_row; i < end_row; i++) // Null all rows which can be fully removed
matrix[i]=null;
if(matrix[end_row] != null) {
int index=computeIndex(seqno);
for(int i=0; i <= index; i++) // null all elements up to and including seqno in the given row
matrix[end_row][i]=null;
}
if(seqno - low > 0)
low=seqno;
if(force) {
if(seqno - hd > 0)
low=hd=seqno;
size=computeSize();
}
num_purges++;
if(max_compaction_time <= 0) // see if compaction should be triggered
return;
long current_time=System.nanoTime();
if(last_compaction_timestamp > 0) {
if(current_time - last_compaction_timestamp >= max_compaction_time) {
_compact();
last_compaction_timestamp=current_time;
}
}
else // the first time we don't do a compaction
last_compaction_timestamp=current_time;
}
finally {
lock.unlock();
}
}
public void compact() {
lock.lock();
try {
_compact();
}
finally {
lock.unlock();
}
}
/**
* Iterates over the matrix with range [from .. to] (including from and to), and calls
* {@link Visitor#visit(long,Object,int,int)}. If the visit() method returns false, the iteration is terminated.
*
* This method must be called with the lock held
* @param from The starting seqno
* @param to The ending seqno, the range is [from .. to] including from and to
* @param visitor An instance of Visitor
*/
@GuardedBy("lock")
public void forEach(long from, long to, Visitor visitor) {
if(from - to > 0) // same as if(from > to), but prevents long overflow
return;
int row=computeRow(from), column=computeIndex(from);
int distance=(int)(to - from +1);
T[] current_row=row+1 > matrix.length? null : matrix[row];
for(int i=0; i < distance; i++) {
T element=current_row == null? null : current_row[column];
if(!visitor.visit(from, element, row, column))
break;
from++;
if(++column >= elements_per_row) {
column=0;
row++;
current_row=row+1 > matrix.length? null : matrix[row];
}
}
}
public Iterator iterator() {
return new TableIterator();
}
public Iterator iterator(long from, long to) {
return new TableIterator(from, to);
}
public Stream stream() {
Spliterator sp=Spliterators.spliterator(iterator(), size(), 0);
return StreamSupport.stream(sp, false);
}
public Stream stream(long from, long to) {
Spliterator sp=Spliterators.spliterator(iterator(from, to), size(), 0);
return StreamSupport.stream(sp, false);
}
protected boolean _add(long seqno, T element, boolean check_if_resize_needed, Predicate remove_filter) {
if(seqno - hd <= 0)
return false;
int row_index=computeRow(seqno);
if(check_if_resize_needed && row_index >= matrix.length) {
resize(seqno);
row_index=computeRow(seqno);
}
T[] row=getRow(row_index);
int index=computeIndex(seqno);
T existing_element=row[index];
if(existing_element == null) {
row[index]=element;
size++;
if(seqno - hr > 0)
hr=seqno;
if(remove_filter != null && seqno-hd > 0) {
forEach(hd + 1, hr,
(seq, msg, r, c) -> {
if(msg == null || !remove_filter.test(msg))
return false;
if(seq - hd > 0)
hd=seq;
size=Math.max(size-1, 0); // cannot be < 0 (well that would be a bug, but let's have this 2nd line of defense !)
return true;
});
}
return true;
}
return false;
}
// list must not be null or empty
protected long findHighestSeqno(List> list) {
long seqno=-1;
for(LongTuple tuple: list) {
long val=tuple.getVal1();
if(val - seqno > 0)
seqno=val;
}
return seqno;
}
/** Moves rows down the matrix, by removing purged rows. If resizing to accommodate seqno is still needed, computes
* a new size. Then either moves existing rows down, or copies them into a new array (if resizing took place).
* The lock must be held by the caller of resize(). */
@GuardedBy("lock")
protected void resize(long seqno) {
int num_rows_to_purge=computeRow(low);
int row_index=computeRow(seqno) - num_rows_to_purge;
if(row_index < 0)
return;
int new_size=Math.max(row_index +1, matrix.length);
if(new_size > matrix.length) {
T[][] new_matrix=(T[][])new Object[new_size][];
System.arraycopy(matrix, num_rows_to_purge, new_matrix, 0, matrix.length - num_rows_to_purge);
matrix=new_matrix;
num_resizes++;
}
else if(num_rows_to_purge > 0) {
move(num_rows_to_purge);
}
offset+=(num_rows_to_purge * elements_per_row);
}
/** Moves contents of matrix num_rows down. Avoids a System.arraycopy(). Caller must hold the lock. */
@GuardedBy("lock")
protected void move(int num_rows) {
if(num_rows <= 0 || num_rows > matrix.length)
return;
int target_index=0;
for(int i=num_rows; i < matrix.length; i++)
matrix[target_index++]=matrix[i];
for(int i=matrix.length - num_rows; i < matrix.length; i++)
matrix[i]=null;
num_moves++;
}
/**
* Moves the contents of matrix down by the number of purged rows and resizes the matrix accordingly. The
* capacity of the matrix should be size * resize_factor. Caller must hold the lock.
*/
@GuardedBy("lock")
protected void _compact() {
// This is the range we need to copy into the new matrix (including from and to)
int from=computeRow(low), to=computeRow(hr);
int range=to - from +1; // e.g. from=3, to=5, new_size has to be [3 .. 5] (=3)
int new_size=(int)Math.max( (double)range * resize_factor, (double) range +1 );
new_size=Math.max(new_size, num_rows); // don't fall below the initial size defined
if(new_size < matrix.length) {
T[][] new_matrix=(T[][])new Object[new_size][];
System.arraycopy(matrix, from, new_matrix, 0, range);
matrix=new_matrix;
offset+=from * elements_per_row;
num_compactions++;
}
}
/** Iterate from low to hr and add up non-null values. Caller must hold the lock. */
@GuardedBy("lock")
public int computeSize() {
return (int)stream().filter(Objects::nonNull).count();
}
/** Returns the number of null elements in the range [hd+1 .. hr-1] excluding hd and hr */
public int getNumMissing() {
lock.lock();
try {
return (int)(hr - hd - size);
}
finally {
lock.unlock();
}
}
/**
* Returns a list of missing (= null) elements
* @return A SeqnoList of missing messages, or null if no messages are missing
*/
public SeqnoList getMissing() {
return getMissing(0);
}
/**
* Returns a list of missing messages
* @param max_msgs If > 0, the max number of missing messages to be returned (oldest first), else no limit
* @return A SeqnoList of missing messages, or null if no messages are missing
*/
public SeqnoList getMissing(int max_msgs) {
lock.lock();
try {
if(size == 0)
return null;
long start_seqno=getHighestDeliverable() +1;
int capacity=(int)(hr - start_seqno);
int max_size=max_msgs > 0? Math.min(max_msgs, capacity) : capacity;
if(max_size <= 0)
return null;
Missing missing=new Missing(start_seqno, max_size);
long to=max_size > 0? Math.min(start_seqno + max_size-1, hr-1) : hr-1;
forEach(start_seqno, to, missing);
return missing.getMissingElements();
}
finally {
lock.unlock();
}
}
public long[] getDigest() {
lock.lock();
try {
return new long[]{hd,hr};
}
finally {
lock.unlock();
}
}
public String toString() {
return String.format("[%d | %d | %d] (%d elements, %d missing)", low, hd, hr, size(), getNumMissing());
}
/** Dumps the seqnos in the table as a list */
public String dump() {
lock.lock();
try {
return stream(low, hr).filter(Objects::nonNull).map(Object::toString)
.collect(Collectors.joining(", "));
}
finally {
lock.unlock();
}
}
/**
* Returns a row. Creates a new row and inserts it at index if the row at index doesn't exist
* @param index
* @return A row
*/
@GuardedBy("lock")
protected T[] getRow(int index) {
T[] row=matrix[index];
if(row == null) {
row=(T[])new Object[elements_per_row];
matrix[index]=row;
}
return row;
}
/** Computes and returns the row index for seqno. The caller must hold the lock. */
// Note that seqno-offset is never > Integer.MAX_VALUE and thus doesn't overflow into a negative long,
// as offset is always adjusted in resize() or compact(). Even if it was negative, callers of computeRow() will
// ignore the result or throw an ArrayIndexOutOfBound exception
@GuardedBy("lock")
protected int computeRow(long seqno) {
int diff=(int)(seqno-offset);
if(diff < 0) return diff;
return diff / elements_per_row;
}
/** Computes and returns the index within a row for seqno */
// Note that seqno-offset is never > Integer.MAX_VALUE and thus doesn't overflow into a negative long,
// as offset is always adjusted in resize() or compact(). Even if it was negative, callers of computeIndex() will
// ignore the result or throw an ArrayIndexOutOfBound exception
@GuardedBy("lock")
protected int computeIndex(long seqno) {
int diff=(int)(seqno - offset);
if(diff < 0)
return diff;
return diff & (elements_per_row - 1); // same as mod, but (apparently, I'm told) more efficient
}
/**
* Iterates through all elements of the matrix. The range (from-to) can be defined, default is [hd+1 .. hr] (incl hr).
* Matrix compactions and resizings will lead to undefined results, as this iterator doesn't maintain a separate ref
* of the matrix, so it is best to run this with the lock held. This iterator is also used by {@link #stream()}
*/
protected class TableIterator implements Iterator {
protected int row, column;
protected T[] current_row;
protected long from;
protected final long to;
protected TableIterator() {
this(hd+1, hr);
}
protected TableIterator(final long from, final long to) {
//if(from - to > 0) // same as if(from > to), but prevents long overflow
// throw new IllegalArgumentException(String.format("range [%d .. %d] invalid", from, to));
this.from=from;
this.to=to;
row=computeRow(from);
column=computeIndex(from);
current_row=row+1 > matrix.length? null : matrix[row];
}
public boolean hasNext() {
return to - from >= 0;
}
public T next() {
if(row > matrix.length)
throw new NoSuchElementException(String.format("row (%d) is > matrix.length (%d)", row, matrix.length));
T element=current_row == null? null : current_row[column];
from++;
if(++column >= elements_per_row) {
column=0;
row++;
current_row=row+1 > matrix.length? null : matrix[row];
}
return element;
}
}
protected class Remover implements Visitor {
protected final boolean nullify;
protected final int max_results;
protected int num_results;
protected final Predicate filter;
protected R result;
protected Supplier result_creator;
protected BiConsumer result_accumulator;
public Remover(boolean nullify, int max_results, Predicate filter, Supplier creator, BiConsumer accumulator) {
this.nullify=nullify;
this.max_results=max_results;
this.filter=filter;
this.result_creator=creator;
this.result_accumulator=accumulator;
}
public R getResult() {return result;}
@GuardedBy("lock")
public boolean visit(long seqno, T element, int row, int column) {
if(element != null) {
if(filter == null || filter.test(element)) {
if(result == null)
result=result_creator.get();
result_accumulator.accept(result, element);
num_results++;
}
if(seqno - hd > 0)
hd=seqno;
size=Math.max(size-1, 0); // cannot be < 0 (well that would be a bug, but let's have this 2nd line of defense !)
if(nullify) {
matrix[row][column]=null;
// if we're nulling the last element of a row, null the row as well
if(column == elements_per_row-1)
matrix[row]=null;
if(seqno - low > 0)
low=seqno;
}
return max_results == 0 || num_results < max_results;
}
return false;
}
}
protected class Missing implements Visitor {
protected final SeqnoList missing_elements;
protected final int max_num_msgs;
protected int num_msgs;
protected Missing(long start, int max_number_of_msgs) {
missing_elements=new SeqnoList(max_number_of_msgs, start);
this.max_num_msgs=max_number_of_msgs;
}
protected SeqnoList getMissingElements() {return missing_elements;}
public boolean visit(long seqno, T element, int row, int column) {
if(element == null) {
if(++num_msgs > max_num_msgs)
return false;
missing_elements.add(seqno);
}
return true;
}
}
protected class HighestDeliverable implements Visitor {
protected long highest_deliverable=-1;
public long getResult() {return highest_deliverable;}
public boolean visit(long seqno, T element, int row, int column) {
if(element == null)
return false;
highest_deliverable=seqno;
return true;
}
}
protected class NumDeliverable implements Visitor {
protected int num_deliverable=0;
public int getResult() {return num_deliverable;}
public boolean visit(long seqno, T element, int row, int column) {
if(element == null)
return false;
num_deliverable++;
return true;
}
}
}