org.grouplens.lenskit.cursors

Class GroupingCursor<C,E>

java.lang.Object

org.grouplens.lenskit.cursors.AbstractCursor<C>

org.grouplens.lenskit.cursors.GroupingCursor<C,E>

Type Parameters:

E - The type of item in the wrapped cursor.

C - The type of aggregate to be returned by this cursor.

All Implemented Interfaces:

Closeable, AutoCloseable, Iterable<C>, Cursor<C>

public abstract class GroupingCursor<C,E>
extends AbstractCursor<C>

Base class for cursors that group the output of another cursor.

Since:

2.1

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	GroupingCursor(Cursor<? extends E> base)

Method Summary

Modifier and Type	Method and Description
protected abstract void	clearGroup() Clear the accumulated group.
void	close() No-op implementation of the Cursor.close() method.
protected abstract C	finishGroup() Finish the current group and return it.
protected abstract boolean	handleItem(E item) Handle an item from the base cursor.
boolean	hasNext() Query whether the cursor has any more items.
C	next() Fetch the next item from the cursor.

Methods inherited from class org.grouplens.lenskit.cursors.AbstractCursor

fast, fastNext, getRowCount, iterator

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.lang.Iterable

forEach, spliterator

Constructor Detail

GroupingCursor

protected GroupingCursor(Cursor<? extends E> base)

Method Detail

hasNext

public boolean hasNext()

Description copied from interface: Cursor

Query whether the cursor has any more items. If the cursor or underlying source has been closed, this may return even if the end has not been reached.

Returns:

true if there remains another item to fetch.

next

@Nonnull
public C next()

Description copied from interface: Cursor

Fetch the next item from the cursor.

Returns:

The next item in the cursor.

close

public void close()

Description copied from class: AbstractCursor

No-op implementation of the Cursor.close() method.

Specified by:

close in interface Closeable

Specified by:

close in interface AutoCloseable

Specified by:

close in interface Cursor<C>

Overrides:

close in class AbstractCursor<C>

clearGroup

protected abstract void clearGroup()

Clear the accumulated group. Called by close().

handleItem

protected abstract boolean handleItem(E item)

Handle an item from the base cursor. Each item of the base cursor is passed to this method. The method builds up the underlying aggregate.

Parameters:

item - The item to handle.

Returns:

true if the item has been accepted and added to the group; false if it cannot be added to the current group. If it cannot be added to the current group, the cursor will call finishGroup() to finish the group, and then call this method again with item (at which point it is required to return true).

finishGroup

@Nonnull
protected abstract C finishGroup()

Finish the current group and return it. After this method has been called, #handleItem(E) should add items to a new group. This method will not be called unless #handleItem(E) has been called at least once since the last group was finished.

Returns:

The group that has been finished.