org.grouplens.lenskit.cursors

Class Cursors

java.lang.Object

org.grouplens.lenskit.cursors.Cursors

public final class Cursors
extends Object

Utility methods for cursors.

Compatibility

Public

Method Summary

Modifier and Type	Method and Description
static <T> Cursor<T>	concat(Cursor<? extends T>... cursors) Concatenate cursors.
static <T> Cursor<T>	concat(Iterable<? extends Cursor<? extends T>> cursors) Concatenate cursors.
static <T> Cursor<T>	consume(int n, Cursor<T> cursor) Consume and discard the first n elements from a cursor.
static <T> Cursor<T>	empty() Create an empty cursor.
static <T> Cursor<T>	filter(Cursor<?> cursor, Class<T> type) Filter a cursor to only contain elements of type type.
static <T> Cursor<T>	filter(Cursor<T> cursor, Predicate<? super T> predicate) Filter a cursor.
static <T> ArrayList<T>	makeList(Cursor<? extends T> cursor) Read a cursor into a list, closing when it is finished.
static <T> Cursor<T>	of(T... contents) Create a cursor over a fixed set of elements.
static <T> Cursor<T>	sort(Cursor<T> cursor, Comparator<? super T> comp) Sort a cursor.
static <S,T> Cursor<T>	transform(Cursor<S> cursor, Function<? super S,? extends T> function) Transform a cursor's values.
static <T> Cursor<T>	wrap(Collection<? extends T> collection) Wrap a collection in a cursor.
static <T> Cursor<T>	wrap(Iterator<? extends T> iterator) Wrap an iterator in a cursor.

Methods inherited from class java.lang.Object

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

Method Detail

wrap

public static <T> Cursor<T> wrap(Iterator<? extends T> iterator)

Wrap an iterator in a cursor.

Type Parameters:

T - The type of data to return.

Parameters:

iterator - An iterator to wrap

Returns:

A cursor returning the elements of the iterator.

wrap

public static <T> Cursor<T> wrap(Collection<? extends T> collection)

Wrap a collection in a cursor.

Type Parameters:

T - The type of data to return.

Parameters:

collection - A collection to wrap

Returns:

A cursor returning the elements of the collection.

filter

public static <T> Cursor<T> filter(@WillCloseWhenClosed
                                   Cursor<T> cursor,
                                   Predicate<? super T> predicate)

Filter a cursor.

Type Parameters:

T - The type of cursor rows.

Parameters:

cursor - The source cursor.

predicate - A predicate indicating which rows to return.

Returns:

A cursor returning all rows for which predicate returns true.

filter

public static <T> Cursor<T> filter(@WillCloseWhenClosed
                                   Cursor<?> cursor,
                                   Class<T> type)

Filter a cursor to only contain elements of type type. Unlike filter(Cursor, Predicate) with a predicate from Predicates.instanceOf(Class), this method also transforms the cursor to be of the target type.

Type Parameters:

T - The type of value in the cursor.

Parameters:

cursor - The source cursor.

type - The type to filter.

Returns:

A cursor returning all elements in cursor which are instances of type type.

consume

public static <T> Cursor<T> consume(int n,
                                    Cursor<T> cursor)

Consume and discard the first n elements from a cursor.

Type Parameters:

T - The cursor's element type.

Parameters:

n - The number of elements to drop.

cursor - The cursor.

Returns:

The passed-in cursor, for convenience and functional-looking code. This method immediately consumes the elements, however, so cursor is modified.

transform

public static <S,T> Cursor<T> transform(@WillCloseWhenClosed
                                        Cursor<S> cursor,
                                        Function<? super S,? extends T> function)

Transform a cursor's values.

Type Parameters:

S - The type of source cursor rows

T - The type of output cursor rows

Parameters:

cursor - The source cursor

function - A function to apply to each row in the cursor.

Returns:

A new cursor iterating the results of function.

empty

public static <T> Cursor<T> empty()

Create an empty cursor.

Type Parameters:

T - The type of value in the cursor.

Returns:

An empty cursor.

makeList

public static <T> ArrayList<T> makeList(@WillClose
                                        Cursor<? extends T> cursor)

Read a cursor into a list, closing when it is finished.

Type Parameters:

T - The type of item in the cursor.

Parameters:

cursor - The cursor.

Returns:

A new list containing the elements of the cursor. The list has been allocated with a capacity of Cursor.getRowCount() if possible, but has not been trimmed.

sort

public static <T> Cursor<T> sort(@WillClose
                                 Cursor<T> cursor,
                                 Comparator<? super T> comp)

Sort a cursor. This reads the original cursor into a list, sorts it, and returns a new cursor backed by the list (after closing the original cursor).

Type Parameters:

T - The type of value in the cursor.

Parameters:

cursor - The cursor to sort.

comp - The comparator to use to sort the cursor.

Returns:

A cursor iterating over the sorted results.

of

public static <T> Cursor<T> of(T... contents)

Create a cursor over a fixed set of elements. This is mostly useful for testing.

Type Parameters:

T - The data type.

Parameters:

contents - The contents.

Returns:

The cursor.

concat

public static <T> Cursor<T> concat(Iterable<? extends Cursor<? extends T>> cursors)

Concatenate cursors. Each cursor is closed as closed as it is consumed.

Type Parameters:

T - The type of data.

Parameters:

cursors - The cursors to concatenate.

Returns:

The concatenated cursor.

concat

public static <T> Cursor<T> concat(Cursor<? extends T>... cursors)

Concatenate cursors.

See Also:

concat(Iterable)