Package org.apache.calcite.linq4j

Class Linq4j

java.lang.Object
org.apache.calcite.linq4j.Linq4j
public abstract class Linq4j extends Object
Utility and factory methods for Linq4j.

  • Field Details

    • DEFAULT_PROVIDER

      public static final QueryProvider DEFAULT_PROVIDER
      Query provider that simply executes a Queryable by calling its enumerator method; does not attempt optimization.

    • EMPTY_ENUMERABLE

      public static final Enumerable<?> EMPTY_ENUMERABLE

  • Method Details

    • getMethod

      public static @Nullable Method getMethod(String className, String methodName, Class... parameterTypes)

    • enumeratorIterator

      public static <T> Iterator<T> enumeratorIterator(Enumerator<T> enumerator)
      Adapter that converts an enumerator into an iterator.

      WARNING: The iterator returned by this method does not call Enumerator.close(), so it is not safe to use with an enumerator that allocates resources.

      Type Parameters:
      T - Element type
      Parameters:
      enumerator - Enumerator
      Returns:
      Iterator

    • iterableEnumerator

      public static <T> Enumerator<T> iterableEnumerator(Iterable<? extends T> iterable)
      Adapter that converts an iterable into an enumerator.
      Type Parameters:
      T - Element type
      Parameters:
      iterable - Iterable
      Returns:
      enumerator

    • asEnumerable

      public static <T> Enumerable<T> asEnumerable(List<T> list)
      Adapter that converts an List into an Enumerable.
      Type Parameters:
      T - Element type
      Parameters:
      list - List
      Returns:
      enumerable

    • asEnumerable

      public static <T> Enumerable<T> asEnumerable(Collection<T> collection)
      Adapter that converts an Collection into an Enumerable.

      It uses more efficient implementations if the iterable happens to be a List.

      Type Parameters:
      T - Element type
      Parameters:
      collection - Collection
      Returns:
      enumerable

    • asEnumerable

      public static <T> Enumerable<T> asEnumerable(Iterable<T> iterable)
      Adapter that converts an Iterable into an Enumerable.

      It uses more efficient implementations if the iterable happens to be a Collection or a List.

      Type Parameters:
      T - Element type
      Parameters:
      iterable - Iterable
      Returns:
      enumerable

    • asEnumerable

      public static <T> Enumerable<T> asEnumerable(T[] ts)
      Adapter that converts an array into an enumerable.
      Type Parameters:
      T - Element type
      Parameters:
      ts - Array
      Returns:
      enumerable

    • enumerator

      public static <V> Enumerator<V> enumerator(Collection<? extends V> values)
      Adapter that converts a collection into an enumerator.
      Type Parameters:
      V - Element type
      Parameters:
      values - Collection
      Returns:
      Enumerator over the collection

    • transform

      public static <F, E> Enumerator<E> transform(Enumerator<? extends F> enumerator, Function1<? super F,? extends E> func)
      Applies a function to each element of an Enumerator.
      Type Parameters:
      F - Backing element type
      E - Element type
      Parameters:
      enumerator - Backing enumerator
      func - Transform function
      Returns:
      Enumerator

    • cast

      public static <TSource, TResult> Enumerable<TResult> cast(Iterable<TSource> source, Class<TResult> clazz)
      Converts the elements of a given Iterable to the specified type.

      This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its RawEnumerable.enumerator() method directly or by using for (... in ...).

      Since standard Java Collection objects implement the Iterable interface, the cast method enables the standard query operators to be invoked on collections (including List and Set) by supplying the necessary type information. For example, ArrayList does not implement Enumerable<F>, but you can invoke

      Linq4j.cast(list, Integer.class)

      to convert the list of an enumerable that can be queried using the standard query operators.

      If an element cannot be cast to type <TResult>, this method will throw a ClassCastException. To obtain only those elements that can be cast to type TResult, use the ofType(java.lang.Iterable<TSource>, java.lang.Class<TResult>) method instead.

      See Also:

    • ofType

      public static <TSource, TResult> Enumerable<TResult> ofType(Iterable<TSource> source, Class<TResult> clazz)
      Returns elements of a given Iterable that are of the specified type.

      This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its RawEnumerable.enumerator() method directly or by using for (... in ...).

      The ofType method returns only those elements in source that can be cast to type TResult. To instead receive an exception if an element cannot be cast to type TResult, use cast(Iterable, Class).

      Since standard Java Collection objects implement the Iterable interface, the cast method enables the standard query operators to be invoked on collections (including List and Set) by supplying the necessary type information. For example, ArrayList does not implement Enumerable<F>, but you can invoke

      Linq4j.ofType(list, Integer.class)

      to convert the list of an enumerable that can be queried using the standard query operators.

      See Also:

    • singletonEnumerable

      public static <T> Enumerable<T> singletonEnumerable(T element)
      Returns an Enumerable that has one element.
      Type Parameters:
      T - Element type
      Returns:
      Singleton enumerable

    • singletonEnumerator

      public static <T> Enumerator<T> singletonEnumerator(T element)
      Returns an Enumerator that has one element.
      Type Parameters:
      T - Element type
      Returns:
      Singleton enumerator

    • singletonNullEnumerator

      public static <T> Enumerator<T> singletonNullEnumerator()
      Returns an Enumerator that has one null element.
      Type Parameters:
      T - Element type
      Returns:
      Singleton enumerator

    • emptyEnumerable

      public static <T> Enumerable<T> emptyEnumerable()
      Returns an Enumerable that has no elements.
      Type Parameters:
      T - Element type
      Returns:
      Empty enumerable

    • emptyEnumerator

      public static <T> Enumerator<T> emptyEnumerator()
      Returns an Enumerator that has no elements.
      Type Parameters:
      T - Element type
      Returns:
      Empty enumerator

    • concat

      public static <E> Enumerable<E> concat(List<Enumerable<E>> enumerableList)
      Concatenates two or more Enumerables to form a composite enumerable that contains the union of their elements.
      Type Parameters:
      E - Element type
      Parameters:
      enumerableList - List of enumerable objects
      Returns:
      Composite enumerator

    • product

      public static <T> Enumerator<List<T>> product(List<Enumerator<T>> enumerators)
      Returns an enumerator that is the cartesian product of the given enumerators.

      For example, given enumerator A that returns {"a", "b", "c"} and enumerator B that returns {"x", "y"}, product(List(A, B)) will return {List("a", "x"), List("a", "y"), List("b", "x"), List("b", "y"), List("c", "x"), List("c", "y")}.

      Notice that the cardinality of the result is the product of the cardinality of the inputs. The enumerators A and B have 3 and 2 elements respectively, and the result has 3 * 2 = 6 elements. This is always the case. In particular, if any of the enumerators is empty, the result is empty.

      Type Parameters:
      T - Element type
      Parameters:
      enumerators - List of enumerators
      Returns:
      Enumerator over the cartesian product

    • product

      public static <T> Iterable<List<T>> product(Iterable<? extends Iterable<T>> iterables)
      Returns the cartesian product of an iterable of iterables.

    • equals

      @Deprecated public static <T> boolean equals(T t0, T t1)
      Deprecated.
      Returns whether the arguments are equal to each other.

      Equivalent to Objects.equals(java.lang.Object, java.lang.Object) in JDK 1.7 and above.